Penton Owners Group

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

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

Larry P

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

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

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

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

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.

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

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

I'm pretty sure you can get those from wiseco

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

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

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

Larry P

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

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?

The motor hasn't been sitting still but hasn't run in years. I would squirt some Marvel Mystery Oil in the spark plug hole and kick it over every so often. I just think it's worn out and needing to be freshened up. I bought the bike "sight unseen" years ago and have no knowledge of it's history or maintenance. I'm just now getting around to bringing it back to life.


Steve Minor