Top end head

[Joeymf]

Anybody know what this came from and can I ride it like this I smoothed it out a bit but maybe it did that from using the wrong cheap 15 dollar piston I bought I need to smooth it out a lil more but what about the inside

[billie_morini]

Joey,

the mechanical damage is caused by detonation ("pre-ignition"). Detonation is the most common 2-stroke engine damage. Your pboto shows a textbook example.

Two things cause it and you can have one or both.
1) Fuel octane (too low)
2) Compression (too high)

No, you cannot run an engine with that head the way it is. More mechanical damage will occur and at an increasing rate. Do it long enough and, "boom!" You can destroy top end, bottom end, and even crankcase. This is not theoretical or exaggeration. It's 1st-hand information obtained through road racing.

There are two solutions and additional note.
1) Use higher octane fuel (without ethanol)
2) Decrease compression (fatter head gasket or non-milled head)

Note: Make sure correct spark plug range is used.

billie

[KDXGarage]

How long did you have the bike before all this?

Did your piston shatter?

What octane gasoline were you using?

I would not use that at all.

https://www.ebay.com/itm/116302719860

[Joeymf]

It wasn’t too much compression bc it was a bad head gasket and also I used 87 and had this bike for about 3 years now it barley happened this last time after I put a cheap piston in it and I think it wasn’t meant to even go in my bike but also I don’t like my water pump was working either bc the radiator is bad so I’m replacing everything now especially the bottom end

[SS109]

That head is toast and cannot be reused. Well, technically, it could be saved but that involves welding and then reshaping the chamber which the cost is just not worth it.

Now, it might not be because of the piston itself but rather possibly an air leak(s) after doing the top end. A leak down test is always a good idea after doing a top end.

Also, I freakin don't care what anyone says but I'll only run 91 octane or above in any bike I own. It's worth that little bit more in cost as extra insurance against detonation damage like you have.

[KDXGarage]

All those cavities are the pennies you saved by not buying premium. If you get it rebuilt, use premium.

[Joeymf]

How do you do a leak test

[Molly's 70]

Lots of info on YT for 2 stroke leak down testing.

[Joeymf]

Is that for bottom end or top or do you have to do both

[Joeymf]

How can I check a leak with the crank case already split or would it be fine if I just rebuild and put it back together

[Joeymf]

I’m doing a full engine rebuild so I don’t see why their will be any leaks but ima still do a leak down test

[Joeymf]

Will this work

[billie_morini]

Joeymf: probably. By the way, here's all we needed in 1975 (Thank you, Dirt Bike magazine). Oh, we pressurized the engine with a bicycle pump.

20240911_115942.jpg (2.21 MiB) Viewed 2616 times

[Joeymf]

So cheap back then

[Molly's 70]

That plug set looks ok. Watch lots of YT videos on 2 stroke leak down testing. You'll get it figured out.

[Joeymf]

Ok I’ve been but what psi should I put in my air compressor

[KDXGarage]

not over 7 (seven)

[billie_morini]

Yup, 6 to 7 psi. More pressure then this can damage seals.

Then watch gauge reading decrease. It is acceptable to see a drop of 1 psi per minute. Less than 1 psi per minute is okay. More than 1 psi per minute indicates something leaks too much.

[billie_morini]

Joey, check the range on the gauge you intend to use. Notice the range in the magazine page photo above. It's 0 to 15 psi. The range in the product photo in your post is 0 to 86 psi. This range is too large and it will not provide the accuracy you will need.

[pumpguy]

Earlier in this thread, and the video, talks about leakage due to porous castings.

Locktite offers a product they recommend for sealing porous castings, their #290 wicking grade thread locker. There's quite a bit of information on this on Loctite's website.

Its important the surfaces be degreased and they recommend using their Primer T for prep. This primer smells like acetone, and I've used just acetone with great success; the bond is very quick.

While I haven't used Loctite 290 for sealing MC cases, I have used it for sealing porosity in iron pump castings so it might be something worth looking into.