72 125/6A Piston Questions

[KJDonovan]

Greetings,

I'm just starting my winter project, ripping down my wife's 72 Six-Day and restoring it and have a couple of piston questions.  First off, prior to ripping it down it had some noticeable piston slap when it idled.  When riding it, it was not that noticeable, only at a idle.  Anyway, I pulled the head and jug and the cylinder walls look perfect, the piston on the other had is a mess.  The top of it is all scored up around the edges almost all the way down to the first ring grove in a couple of places, so it needs to be replaced.  The size stamped on the top it is 54.43.  What I need to know is the 1st over piston or original?  I am also wondering about the L-Ring on the top grove, its facing up, is this correct?  Lastly, seeing I am hoping to get away without boring the jug what size piston should I replace this one with, the same or something else? Any advice that can be provided would be helpful.

Thanks,

Kevin


Kevin J. Donovan
Foster, Rhode Island
72 Jack Piner (My Ride)
72 Six Day (Wifes Ride)

[Ron]

Hi Kevin,
To do it right you'll need to measure the bore in the cylinder.
Measure the bore at the top, mid and bottom, to get an accurate measurement.
Then you can decided on the proper over size piston you'll need.
I'd also try to determine the cause of the piston damage.
My first impression is somebody let a lot of dirt get into the engine.
Which could mean the crankcase may have dirt in it also, not a good thing for the crank bearing and mains.
If the motor is out of the bike you may be able to flush some debris out, but for longevity it should be and stay as clean as possible.
Good Luck,
RonW

[Larry Perkins]

Kevin,

Measure like Ron says to be sure but probably 54.75 will clean things up.  Al B. has some of these. 54.0 was standard and there are different pistons so that piston you have is a full first over.  There were some 54.25 pistons also so you could say it was second over.  depends on how you look at it and the terminology used at the time.

I would also split the bottom and do all new mains and seals and do the rod while I was at it or if you don't feel confident in that send it to Doug Wilford and have the Master do it.  Many times I see someone do the top and make it fresh and because of the dirt and wear situation that Ron has described the fresh strong topend does in the now weaker bottomend in short time and both have to be done anyway.  It is more expensive to do both but the way we ride these today you will most likely never do it again in your lifetime.  Well at least in mine.  While you are at it put in a J.P. Morgan shift key and if Doug is doing it have him adjust the shifting as again he is the Wiz.  Best of luck with your project.

Larry P

[KJDonovan]

Larry / Ron,

Thanks for the info. I measured the jug and at the top and bottom its 54.50 in the center about 54.60.  Based on this I'm thinking the 54.75 with a light honing.  Does this sound safe?  I'm trying to avoid doing a bore because I don't know anyone local that is reputable.  When I was restoring my Jack Piner, I originally had a guy bore the jug and he did a lousy job.  I ended up re-sleeving it in the end.  Just don't want to go down that road again.  Also, I do plan on splitting the cases and replacing all the bearing and seals as well.  I have all winter to do this project, so I want to do it right.

Thanks,

KD


Kevin J. Donovan
Foster, Rhode Island
72 Jack Piner (My Ride)
72 Six Day (Wifes Ride)