Low compression but rings, pistons, and cylinder look good

[Devon007]

Hi Guys,

1981 KDX 175 rider here. New to me, been riding it about 18 months on trails very happily. Performance started dwindling so I checked compression before the job and it was at 60, whereas service limit is 120-180. Performance seems to particularly dwindle when it is hot.

First time doing a top end, so please forgive my ignorance.

I opened it up and the rings, pistons, and cylinder look really good.

Top and bottom gasket were worn, but seems unlikely that they would be wholly responsible for the compression loss.

My plan is to replace the rings and gaskets and put this thing back together. Anything else I should check compression-wise before reassembly? I am new to wrenching, as I mentioned.

Thanks!

[Molly's 70]

I would also measure your piston & cylinder for service limits.

[doakley]

Definitely measure piston and cylinder (see manual). Be sure to use new gaskets and I would put in new top end bearings, pin and clips. Actually, by the time you do all this you aren't saving a lot so I'd probably go with a new piston too, but that's just me.
Good luck!

[KDXGarage]

Measure and measure again.

If it was me, I would just replace the piston as well. Wiseco used to make a kit. I don't know if it came with the small end bearing.

[Devon007]

I would also measure your piston & cylinder for service limits.

Thanks. I guess the micrometer will tell the story even if my eyes don't spot anything.

[KDXGarage]

If you can tell metal fatigue with eyesight, please change your user name to Clark Kent.

Thanks.

[doakley]

I think what everyone is saying is something like, "Darned if I would go to the expense of tearing down the top end and not replace a piston of unknown age or condition."

[KDXGarage]

Check the main bearings and seals also.

[Devon007]

Thanks all. Took to shop today they said they would measure it for free. After measurement, they said "cyclinder needs to be done, and obviously replace the piston". So much for the evaluation by Clark Kent. So the cylinder is on its way to Millenium Tech. Millenium Tech said worst case not more than $300.

[KDXGarage]

Oh well. Thanks for helping out the ol' gal. Keep fresh rings and pistons in there for many more miles and smiles.

[KDXGarage]

Hopefully, pretty soon you will have one even better than this one I used to have:

DSCN2436.JPG (3.61 MiB) Viewed 8067 times

[Devon007]

cost from Millennium technologies for honing the cylinder: $98. Nice!

[KDXGarage]

I guess you have a cylinder that at some point got sleeved?

[Devon007]

I guess you have a cylinder that at some point got sleeved?

Correct. Millenium said it had been sleeved.

[KDXGarage]

Wow. What a bunch of talk over nothing to do with what you really have. :-)

[Devon007]

Honed cylinder and new piston due to arrive Monday via UPS!

[Devon007]

Any tips for getting the piston back in the cyclinder? Man those rings are tight! Millenium bored the cyclinder to 67mm, and sent me the correct new Wiseco piston and ring set. Lubricated all with oil and lined up rings with the notches but still really tough to get them past the lip of the cyclinder.

[Molly's 70]

I just use my fingers to squeeze them together & work the cylinder on.

[KDXGarage]

me, too

I pinch about 90 degrees around from the opening.

[KDXGarage]

put just the rings in and measure ring end gap

It shows in the manual about how far down / up is the correct spot I think