HopFrog's 1980 KDX175 Build Log

Discussion specific to the various air-cooled KDX models sold in the USA
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HopFrog175
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


Thanks for the tips on mixing oils and to watch out for the new volume in the fork tube from the smaller spring - I was overlooking this!

With everything in place, I started fork assembly. First, a little extra swabbing for the right tube which had evidentially ingested a bit of water over the years of sitting.


Final swabbing
_click image to zoom_
kdx175_00_fork_assembly.jpg
kdx175_00_fork_assembly.jpg (230.09 KiB) Viewed 24701 times


Fit the seals in followed by the (original) snap rings. I could not find new ones anywhere but it would be an easy replacement if the need came up.


Fork seal
kdx175_01_fork_assembly.jpg
kdx175_01_fork_assembly.jpg (181.92 KiB) Viewed 24701 times
kdx175_02_fork_assembly.jpg
kdx175_02_fork_assembly.jpg (196.29 KiB) Viewed 24701 times


The initial test fit (without oil or springs) looks good.


Forks mounted
kdx175_03_fork_assembly.jpg
kdx175_03_fork_assembly.jpg (233.33 KiB) Viewed 24701 times


One problem - the darn new fork boots are too short! They simply do not extend sufficiently to accommodate the 9-1/2" travel. And they do not really slide over the lowers far enough. I have the originals but they are too far gone, one has a hole and the other is in three pieces. Fortunately, this is just a test fit of everything so it will be easy to put on better boots once I find some.


New fork boot is too short
kdx175_04_fork_assembly.jpg
kdx175_04_fork_assembly.jpg (294.77 KiB) Viewed 24701 times


Well, at least all of the cleaning and work is done. I will pick up some 10wt fork oil to mix with the 5wt and then fill. Thinking about it, it would seem that I want the same height which should now take a greater volume of oil than specified in the manual. Also, the manual specifies the fork oil height with the forks fully extended, springs removed. No problem except this is a good length, around 21 inches. After I am happy that I have this height, I will try fully compressing the forks and remeasuring to establish a new short height. It seems like this new height would be closer to 11-12 inches with the forks compressed - easier to measure/adjust in the future. Does this sound right?
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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

Trying to determine a height with collapsed forks might be the ticket.

Daystar used to make fork boots in different sizes.


EDIT (later add on) these maybe?? https://www.daystarweb.com/product/fork ... -collapsed
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »

Daystar looks like the ticket, order sent - thanks!
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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

You're welcome.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


Moving on to the carb now that the forks are virtually finished. It has been somewhat protected in a box but still needed of a bit of cleaning.


Stock carb for the KDX175
_click image to zoom_
kdx175_00_carb.jpg
kdx175_00_carb.jpg (293.59 KiB) Viewed 24685 times


The inside looks better than I would have thought.


Float bowl removed
kdx175_01_carb.jpg
kdx175_01_carb.jpg (170.68 KiB) Viewed 24685 times


Here are all of the parts. I think that most will be reusable except for the float bowl gasket. All of the metal parts get dunked.


Carb blown apart
kdx175_02_carb.jpg
kdx175_02_carb.jpg (217.27 KiB) Viewed 24685 times


Everything came out nice and clean.


Dunked and washed
kdx175_03_carb.jpg
kdx175_03_carb.jpg (318.64 KiB) Viewed 24685 times


Looking at the slide, though, it does not look too good. The chrome (?) plating is flaking off.


Deteriorated slide
kdx175_04_carb.jpg
kdx175_04_carb.jpg (169.08 KiB) Viewed 24685 times

It slides ok with no binding but may be a bit looser. I will have to evaluate my (limited :grin: ) options. I guess I could run it as is and see how good/bad the carburetion actually is. All of the passages blow clean and the rest looks ok so I will probably give it try.
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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

I think I would have to find the garbage can for that slide.

Maybe sudco or jetsrus has a #2 slide as the diagram shows.

Those floats look so different than the later floats.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


The KDX175 has limited options when it comes to the carb but I did some digging and have a few different ways to go. This is probably common knowledge but is new to me and I will list it out for any future KDX175ers.

Going from cheapest, I could rebuild my original carb as many or all of the parts are currently available.

Next, it seems like the basic carb design is one variant of the Mikuni VM Series round slide single carburettor which has been around for quite a while and a modern-day Mikuni non-OEM variant is still in production. The KDX175 came with an OEM variant called the VM34SS (as opposed to the VM34 Aluminum) and has some features that are unique. Additionally, as detailed here VM38-SS - VM38-9 Comparison, although OEM carbs differ from new after-market carbs, they have been successfully used with a little work. So, a new Mikuni non-OEM VM34 is a possibility.

Another possibility is a modded OEM carb that RB Designs creates. Although offered for the KDX200/220, it seems like one may work on the 175, A query to the proprietor (Ron Black) would be needed; he is reportedly winding down his operation and may, or may not, have time/interest to put into a carb for the 175.

Finally, there is Lectron Fuel Systems and their well known carb. From their site, it looks like they can send one configured for the 175 but querying ahead of time is needed to verify.

Price-wise, it would be under $50 to refurb my original carb, ~$120 for a new VM34, ~$400+ for Ron Black's carb (if available) and ~$500+ for the Lectron.

So, to start with, I think I will replace my slide and try my old carb. Later, if needed, move to one of the more pricey options.

I will post a chart of my findings in the next post.


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Last edited by HopFrog175 on 02:06 am Apr 06 2021, edited 1 time in total.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »

KDX175 Carb Options

Apr 5, 2021

1980 KDX175 Carburetor Options
====================================================================================


1 - Replacement Carburetor
--------------------------

                        Dennis Kirk*       RB Designs**        Lectron***
                        ------------       ------------        ----------

OEM Replacement Carb    $105.88            ---                 ---
VM34-275
Tuning required
CA 2.0 slide required

Modded OEM Carb         ---                 $400+              ---
Tuning required

Replacement Carb        ---                 ---                $500+
Tuning required



* Dennis Kirk
      - check for matching of KDX175 specs on jets and slide
      - https://www.denniskirk.com/mikuni/34mm- ... 165145.sku


** Check availability for KDX175 (available for KDX200/220)
      - https://rb-designs.com/rb_designs_llc_009.htm


*** Lectron Fuel Systems
      - check for availability for KDX175
      - https://www.lectronfuelsystems.com/sbm/ ... i/kdx.html




2 - Common Replacement Parts
----------------------------

                         ebay*                Jets R Us**       Sudco***
                         -----                -----------       --------

slide VM34/110 CA 2.0    $27.00 - $80.00      $40.50            $40.50
(003.100 VM34/110 2.0)
https://www.jetsrus.com/individual_part ... 00_su.html

Needle and Seat VM38/11  $20.00               $31.95            $32.00
(002.200VM38/11 3.5 N/V)
https://www.jetsrus.com/individual_part ... 00_su.html

Float Bowl Gasket         $7.00               $5.45             $5.45
(002.100 999-631-011 GAS)
https://www.jetsrus.com/individual_part ... 00_su.html

Carb Kit MK-606           ---                 $29.00            $29.00
(only some parts may fit - CHECK FIRST!)
(needle/seat do not fit)

Carb Kit generic         $11.00               ---               ---
(only some parts may fit)
(needle/seat usually do not fit)

Additional VM34 Parts may be listed here:
https://www.jetsrus.com/carburetor/carb ... parts.html
http://www.nichecycle.com/ncs/parts-fin ... -list.html


* Ebay price and availability vary


** Jets R Us - additional VM34 parts
      - https://www.jetsrus.com/carburetor/carb ... parts.html


*** Sudco price list
      - http://www.sudco.com/images/Sudco%20Price%20List.pdf
      - Buy through a distributor
        http://www.sudco.com/usa_distributors.html

====================================================================================



The carb of the 1980 KDX175 is the Mikuni VM34SS. When fitting a modern-day Mikuni non-OEM variant VM34 carb, be sure to get one configured as close to the original VM34SS as possible which, in this case, appears to the the Mikuni Aluminum Spigot 34mm Mount Type Carb - VM34-168. Still, most of the jets, needle and slide are pretty far off and will need to be replaced with new (or used) components of the proper specification. It may be possible to transfer over jets, needles, etc if you have the old VM34SS. (If you have the old one, might as well try rebuilding first!)

Specs from the manual:
Main Jet ............. 137.5R
Air Jet .............. (2.5)
Jet Needle ........... 6DH7-3
Needle Jet ........... Q-4/8
Cutaway .............. 2.0
Pilot Jet ............ 40
Air Screw (turns out). 1-3/4
Service Fuel Level ... 9 +/- 1mm

Specs of the modern-day Mikuni non-OEM variant VM34-268:
3.3 needle valves
Carburetor Size .................. 34.00 mm
Pilot Jet ........................ 35 (VM22/210)
Needle ........................... 6DH4
Needle Jet ....................... Q-2 (159 Series)
Main Jet ......................... 260 (4/042)
Air Jet .......................... 2.0
Slide Cutaway .................... 2.5 (VM34/110)
Needle Valve ..................... 3.3 (VM34/39)
Spigot Mounting Outside Diameter . 40
Features ......................... Clamp-On Type Mounting

If replacing parts on your original VM34SS consult the Sudco Catalog. One part specific to the original KDX175 carb is the needle and seat. The main uniqueness is that the seat has 10mm threads. They are metal and probably last forever but, if you need on, this is the one to get ($32):

Needle and Seat VM38/11
(002.200VM38/11 3.5 N/V)
KDX175 Carb Needle and Seat


VM38/11
_click image to zoom_
kdx175_01_carb_needle_jet.jpg
kdx175_01_carb_needle_jet.jpg (11.57 KiB) Viewed 24670 times


A few other common parts are listed in the table located at the top of this post. If your carb is in good shape, you may be able to clean and use it with a new float bowl gasket and be set.

Carb kits for the VM34SS really do not exist as far as I can tell although some are close, From Mikuni, the MK-606 looks like the closest match - and it is really not that close. Some parts might fit but the needle seat is wrong and will not fit. There are a lot of carb kits labeled VM34 but I did not find a single one with the VM38/11 needle seat for the KDX175. So, if you do buy a kit, at best, you are paying for a gasket, o-rings and maybe a jet or some adjusting screws.

Finally here is a blow up of my actual 1980 KDX175 VM34SS carb. Some parts and hoses are missing or not visible but it gives an idea of parts count and position. The numbers are listed under the image and correspond to the Mikuni numbers from the Sudco catalog. The numbers missing from the pic correspond to parts that my carb does not have.


VM34SS Carb
kdx175_00_carb.jpg
kdx175_00_carb.jpg (1.75 MiB) Viewed 24670 times
 1 - Rubber Cap, Throttle Cable
 2 - “A” Cable Adjuster
 3 - Locknut, Cable Adjuster
 4 - Top, Mixing Chamber
 5 - Gasket, Top Mixing Chamber
 6 - Spring, Throttle Valve
 7 - Plate, Spring Seat
 8 - Clip, Needle Positioning
 9 - Jet Needle
10 - Throttle Valve (Slide)
11 - Needle Jets
12 - Rubber, Starter Plunger
13 - Fitting, Starter Plunger
14 - Lever, Starter
15 - Spring Plate, Starter Lever
16 - Starter Plunger Spring
17 - Starter Plunger
18 - Mixing Chamber Body
19 - Float Bowl Gasket
20 - Float Bowl Baffle Plate
21 - Pilot Jet
22 - Float Arm Hinge Pin
23 - Float Arm
24 - Float - Two basic types of float system: Independent (Type A) and Twin (Type B).
25 - Float Bowl
26 - Main Jet Plug Washer
27 - Main Jet Plug (Drain Plug)
28 - Air Jet
29 - Air Adjusting Screw
30 - Air Adjusting Screw Spring
31 - Idle Adjusting Screw Spring
32 - Idle Adjusting Screw
33 - Needle Valve Washer
34 - Needle Valve Set (Needle Valve & Seat Assembly)
35 - Main Jet Washer
36 - Main Jet
37 - Anchor Plate
38 - Float Bowl Screw
39 - Main Jet Extender
40 - Needle Jet Setter
41 - Needle Jet Setter O-Ring
42 - Banjo Bolt Washer
43 - Banjo Bolt

A - Starter Rod Circlip
B - Starter Rod Plastic Slider

hf175w.jpg
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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

This is an incredible thread for KDX175 owners. I am hereby declaring this thread a sticky.
soletitbewritten.jpg
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Re: HopFrog's 1980 KDX175 Build Log

Post by Goofaroo »

Yep. A lot of detailed info. This should definitely be a sticky.

HopFrog, your carb looks to be in really good overall condition. Just my $.02 but the slide doesn’t look all that bad. Obviously I haven’t had it in my hand and had a good look at it but judging by the pics it looks serviceable. As long as it slides easily without binding it will probably work just fine. It looks like you have already tidied it up. I would have checked it for burrs and given it a light wet sanding with some 1000 grit to clean it up and it’s probably good to go.

As far as the needle and seat it could probably be made serviceable as well but you may have already found a kit that has the gaskets you need as well as a new needle and seat.

You’re being very thorough with this build. It’s all in the details. I did a very thorough mechanical rebuild of my 87 model several years ago. It was spendy and time consuming but well worth the effort. It has been a very fun bike and utterly reliable.

You’re doing a fantastic job and I’m watching with great interest. I’m really looking forward to seeing the results.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


Thanks for the kind words and sticky :grin: I've gotten to the point where I noticed doing things right is the way to do 'em. I am benefiting a lot from others, too, and am happy to contribute back a little.

I am planning to run the KDx175 indefinitely, maintaining as needed - and the less maintenance the better (plenty of projects waiting in the wings).

On the carb slide, a NOS turned up on ebay (listed under part number only) for $27. Hopefully, that will complete the carb.

The float needle valve is all metal (no rubber tipped needle). Is it reasonable to lap the needle to the seat with some valve grinding compound? Or, is it even needed? I want to avoid a leaking carb if possible.


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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

no rubber tip on part 34?

How does it seal?? Is the rubber tip completely destroyed by old gasoline??
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


Yep, I had the same doubt on needing rubber and, as far as I can tell though, part 34 (needle) never had any rubber on the tip.


KDX175 Needle before Cleaning
_click image to zoom_
20210406_kdx175_needle.jpg
20210406_kdx175_needle.jpg (85.07 KiB) Viewed 24642 times


When I was looking through the Sudco catalog, I caught another clue: needle valve assemblies are described as metal tip or viton tip. Here is a pic of the full line of the Mikuni Needle Valve Assemblies (KDX175 uses the VM38/11):


Mikuni Needle Valves
20210406_kdx175_metal_tip.jpg
20210406_kdx175_metal_tip.jpg (166.64 KiB) Viewed 24642 times


(Come to think of it, I was dealing with a 90-yr old carb some time ago and I do not remember the needle having a rubber tip - of course, it just had to leak out the full tank of gas just after but that was due to a leaky brass float.)

So I was thinking that if I lap the metal (brass) needle in the seat with a bit of fine valve grinding compound, I might get a better seal. If that leaks or seeps I can try something finer and, if all fails, I can order a new needle valve assembly.

Of course, maybe it is fine like it is - I could just run it and then try lapping if it fails to seal consistently.

Or, actually, I just noticed - the VM28/511 has identical dimensions and has a viton tip. Methinks I found a better solution :grin:

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Re: HopFrog's 1980 KDX175 Build Log

Post by KDXGarage »

I'll bet one crisp dollar that the black ring around that valve is all that is left of a rubber tip.
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Re: HopFrog's 1980 KDX175 Build Log

Post by Goofaroo »

That needle might clean up and work fine as long as it isn’t pitted. I have had good results for years with Brasso brass polish. For the seat I cut a few q-tips in half and chuck it up in a drill with some Brasso on it. Stick in in the seat and spin it against the seat. As you pull it out work it against the sides as well. Then repeat. Lastly hit it with a dry q-tip and then clean it with carb cleaner. Same for the needle but chuck it up in the drill and use some Brasso on a clean cloth and polish the needle against the cloth. About 90% of the time it’ll bring them to useable condition.

You’re obviously very particular in your work. A good way to clean all of your brass and other things is Barkeeper’s Friend. It’ll make your carb brass look great. Use the powder version and hold your jets, emulsion tubes, and seat in the palm of your hand. Sprinkle some Barkeepers Friend on it and a bit of water. Scrub the parts against your palm with a toothbrush and then rinse with water.

It also works on the carb body and other things. For spokes cut some strips out of an old t-shirt and dip them in a paste of Barkeepers Friend. Loop it around the spokes and work it up and down and it’ll make them look great.

I assume you know how to bench test the needle and seat by attaching a fuel line to the fuel inlet. Blow through it with the carb upright and the turn it upside down and it should hold pressure.

Needles can be tricky sometimes. Don’t be discouraged if you bench test it and then when you put it on the bike and it leaks. It happens. Sometimes even new ones will do this. Even after a thorough cleaning a particle can cause the needle to leak. Bounce the bike around a bit with fuel in the bowl and see if it stops. If not go ride it on a rough surface or a few small jumps. It’ll usually dislodge whatever is causing it not to seal. I’ve had many bikes in my shop that I have fixed by just riding it around my property. lol

Edit: Your needle never had a rubber tip. It just has some goo on it.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


Great info, thanks! I will try the q-tip with Brasso procedure and follow it up with lapping the needle/seat mating surface a bit. It seems like lapping, at the least, won't hurt and I am curious to find out as I run into old carbs periodically.

Also, it looked like the VM28/511 needle/seat would be a direct replacement for the original VM38/11 but then I noticed that the VM28/511 is rated 2.5 compared to VM38/11's 3.3. I am not sure what the numbers indicate but larger numbers go with gravity feed and smaller numbers with fuel pump feed. I will stick with the original for now.

Barkeeper's Friend is new to me (I must be hanging out in the wrong places :-) but I will definitely get a hold of some.

Speaking of bench testing, I will do a crankcase leakdown test after I put new crank seals in. Looking around, I just stumbled across the Klemm Vintage 2 Cycle site and their write up on leakdown testing - plus, transmission leakdown testing :hmm: They mention it in context of rotary valve engines but I might try it just for fun.

They also advocate a “Dry Top End Assembly” break in :shock: This is: assemble the piston/rings/cylinder without any oil and then make your break in runs. The rational is that lubing the top-end during assembly introduces excess oil that, in turn, leads to a glaze on the cylinder that takes a while to wear off resulting in prolonged break-in periods.

Interesting but I think I will try it some other time primarily because I am not in a hurry and I want to avoid risking damage to the Nikasil.

(Klemm Vintage has encapsulated 2-stroke experience, knowledge and anecdotes into a handful of subjects such as Gas, Oil, Break-in, Detonation, etc - worth a read even just for the historical context)


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Re: HopFrog's 1980 KDX175 Build Log

Post by Goofaroo »

If you don’t have any brass polish you can pick it up at most hardware stores and some grocery stores. Same with Barkeeper’s Friend. It comes in liquid and powder. You want the powder. I do happen to own a bar but I actually discovered Barkeeper’s Friend about 20 years ago when I stole my wife’s can from under the kitchen sink. It did a great job on a model aircraft engine I was restoring. It and Brasso have become staples in my shop.

You’re doing a great job on the bike! I just wish you would hurry up and finish it so we can see the results.
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Re: HopFrog's 1980 KDX175 Build Log

Post by Goofaroo »

I forgot to mention another grocery store item to keep around your shop. Peanut oil does a good job of rehydrating plastic and rubber. Any plastic or rubber bits you have laying around for later installation will benefit from an occasional coat of peanut oil while they are waiting to be installed.

In particular it might give your shock seal a better chance of success. While you have it apart you could apply some peanut oil to the seal and the shaft and work the shaft in and out. Repeat every few days for a while. Also you could store it upright and leave a puddle of oil on top of the seal. No guarantees but it certainly won’t hurt and will definitely give your shock seal a better chance to succeed.

If you’re planning to reuse the intake and airbox boots they will certainly benefit from the same treatment. Also the airbox itself. If you’re planning to reuse the the fuel tank you could start coating it as well. Just keep coating it until it quits drinking it in. It may not bring the color back but it will rehydrate the tank and won’t make it any more difficult for you to scrape, sand, and heat it back to the original color.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »

This is a new one for me, thanks, makes sense and worth a try! I definitely will try soaking the top of the shock and seal. The shock is a weak point - if it leaks and turns into a pogo stick, I will have to dive into the rabbit hole of shocks and possible modern replacements of the KDX175.
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Re: HopFrog's 1980 KDX175 Build Log

Post by HopFrog175 »


I am waiting for parts for a number of loose ends (fork boots, etc) but need to keep moving so on to the engine. I expect this to be fairly cut-n-dry because I went through it last century but never ran it. I am paranoid about keeping seals up to date after losing an engine to a crank seal leak in the past so all seals get replaced. Additionally, I will keep an eye out for anything that I let slide as good enough back when I was thinking I was going to be passing this bike on to a relative.

I pulled the cylinder and clutch cover a while back just to make sure that things looked as I remembered. I also took the opportunity to test fit replacement socket-head screws I rounded up (I will compile an engine screw list and sources in a later post).


Time for new seals
_click image to zoom_
kdx175_00_disassembly.jpg
kdx175_00_disassembly.jpg (253.28 KiB) Viewed 24510 times


I tried the strap wrench on the kick-start spring and it worked surprisingly well.


Disassembly
kdx175_01_disassembly.jpg
kdx175_01_disassembly.jpg (312.44 KiB) Viewed 24510 times


For some reason, the crank gear was stubborn. After pulling off the shift mechanism, the stator-side case pulled right off.


Pulling apart
kdx175_02_disassembly.jpg
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Pressing out the crank
kdx175_03_disassembly.jpg
kdx175_03_disassembly.jpg (260.88 KiB) Viewed 24510 times


When I put this together, last century, I was keen to try a new Loctite sealer. Now labeled "518 Flange Sealant", it really held up, still relatively soft and pliable, looked fresh. Actually, it stuck a little too well on the clutch cover I had a heck of a time breaking the seal. I expect this sealer to work well on the KDX as I have had success elsewhere.


The sealer lasted
kdx175_04_disassembly.jpg
kdx175_04_disassembly.jpg (309.73 KiB) Viewed 24510 times

Loctite 518 (tech info) is still available so I will be using it again on assembly. (There are some variants such as 510 and 574 for high temp and other applications.) Cure time is about a day but you can speed it up to about an hour or two with an optional activator LOCTITE SF 7649 (tech info). Klean'N'Prime is the retail branding of the activator if you look for it outside of industrial suppliers.

I have seen it locally at Grainger and online through Summit Racing and Amazon:


After a little cleanup, I will be ready to put the new seals in.
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