Compression question

[Steve Minor]

Did a compression check on a 76 125 KTM today....exhaust pipe on, carb attached to airbox....throttle wide open. Gauge registered 80psi  three times. My question is what is the lowest compression a motor can have and still crank and run?

Thanks as always


Steve Minor

[Larry Perkins]

The least to get them to kickstart and run decent is 119-120psi.

Larry P

[Steve Minor]

I was afraid of that. I had the head off a few months ago and it seems like the piston had 60 stamped on top. Can I get one more rebore or is it sleeve time?

Thanks Larry, I can always count on your knowledge...



Steve Minor

[454MRW]

Here's a good price on a . 080 Wiseco on eBay.
https://www.ebay.com/itm/KTM-125-Penton-125-Wiseco-Piston-Kit-080-Oversize-404P8-462P8-LC-CLOSEOUT/174018559125?hash=item2884507c95:g:yhsAAOSw8gVX9ppE

Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Penton
1976 MC5 400
1978 KTM 78 GS6 250
L78-79 MX6 175-250 KTM's
1976-78 125-400 RM's
2007 CR125R Honda
1977 MC250 Maico
2017 KTM Freeride 250R

[454MRW]

And another one even cheaper.
 https://www.ebay.com/itm/VINTAGE-WISECO-PISTON-404P8-or-462P8-KTM-PENTON-125cc-REED-VALVE-THRU-1982/184160886480?hash=item2ae0d81ed0:g:TCUAAOSwP~leO1l-
You might want to double-check the part numbers because this one says Reed valve, not sure why it doesn't look like a reed valve piston.
Michael R. Winter
I enjoy rebuilding and appreciating Pentons!
1974 250 HS Penton
1976 MC5 400
1978 KTM 78 GS6 250
L78-79 MX6 175-250 KTM's
1976-78 125-400 RM's
2007 CR125R Honda
1977 MC250 Maico
2017 KTM Freeride 250R

[Steve Minor]

Those are great prices. I'll pull the head off tomorrow and double check the piston that's in it now.

Thanks Mike

Steve Minor

[t20sl]

I would tear down and measure piston/cylinder/ring end gaps. May only need a set of rings / or piston/rings of same size.  Course never hurts to have an oversize waiting.

[Larry Perkins]

There are two types of Wiseco pistons, forged and cast.  I would reccomend against the cast as they swell up worse and have to be setup with more clearance.

Larry P

[Steve Minor]

Pulled the piston today...measured 54.20 and the piston had 020 and an arrow stamped on the top. It also had 397 and a fancy W on the bottom inside. I'm taking this to be a Wiseco 397 piston for a 20 overbore, as the stock bore is 54mm. The piston and bore looks pretty good so my question where can I find a set of rings and a needle bearing for the top rod connection?

Steve Minor

[Dale Sonnenschein]

I'm pretty sure you can get those from wiseco

[KJDonovan]

Steve,

You may want to check with Jake Fischer on the rings - I recall he had some piston's for the early KTM 125 motors, so I am guessing he has rings too.


Thanks,

Kevin

Kevin J. Donovan
Foster, Rhode Island
72 Jack Piner
72 Six Day
72 Berkshire
74 Hare Scrambler

[Steve Minor]

Thank you Dale and Kevin. I'll check Wiseco and Jake, but I might just opt for a rebore and new piston assembly. That way I'll know the top end is fresh.

Steve Minor

[Larry Perkins]

I have a couple of NOS piston kites for KTM 125 motor.  What size do you need?

Larry P

[Steve Minor]

The piston in it now is 54.20 but if I'm going to replace it, it only makes sense to have it rebored one size up. I don't know any reliable machine shops in my area so I'll need to send it out. I was thinking I'd contact Gary Ellis to see if he could have it bored and fit a new piston.

Thanks

Steve Minor

[Carl Hill]

Question: Had the machine been sitting for a long time? I have seen 2 stroke motors that have been sitting get "dry" and show low compression. When you dump an ounce of oil I the cylinder it jumps up. Have you checked ring end gap and cylinder to piston clearance?