R51/3 pistons

[moyald]

I'm doing the top end on a 1951 R51/3 (smoking, poor compression). Current cylinders maxed out & I will be reconditioning a used set which will probably come in at 2nd oversize. Current pistons are 5 ring. I wonder if there are any opinions as to optimum pistons - brand, number of rings etc. What was the advantage of the 5 ring setup in the first place? Thanks in advance for your opinions/input. David M

[Micha]

David good morning,
I apologize for not answering your questions directly, but I would love to enlighten you on secondary issues, which to me where no less important when I was at the same point as you, at the time.
First of all - slingers of course. I do not know when you last tested them but it may be the right time to do so.
Second thing: again, I do not know how many kilometers you traveled with the bike, but keep in mind that sometimes, not always, it will be difficult for the lower engine (ie crankshaft, etc.) which is in one worn condition or another, to withstand the new load that new fresh pistons will load. In terms of compression, etc.
Regarding the number of piston rings - do not see this as an advantage and it is doubtful whether there is a suitable piston out there with more than five rings. I just remember that I bought mine from Mark in Swiss, standard KS pistons and here in the workshop in Israel new sleeves were implanted (was on 3rd o.s.).
It is very important for the workshop master to be informed that this is an air-cooled engine from the 1950s using 40 weight oil, so that he can adjust the correct honing after the re-bore.
Of course, the run-in you will make to the engine afterwards is very important too.
I hope someone here will be more helpful to you regarding those specific issues you raised.
Take care and good luck.

[moyald]

Thanks Michael; not looking for more than 5 rings (that'd be crazy), but wonder what the advantage of 5 rings vs. 3 rings might be. Your points about the bottom end are well taken. It has not been that many miles since the engine was apart. The cylinders were not honed properly when it was assembled, so it smokes and has bad compression. You must have the only /3 in Israel! David

[Flx48]

Hi David-
I'll offer up my take on our pistons/rings.

Piston rings have three jobs; contain compression and combustion, remove heat from piston to cylinder, and control oil loss.

The advantage of the old five ring pistons was that they worked as best they could with the technology of the day.

And the technology of the day was way different from the technology of today.

Piston clearance had to be huge (by today's standards) because of piston expansion, and rings just had a harder time doing all three of their jobs; materials and technology were quite different just seventy short years ago.

Like many things, the basic concepts remain the same as back then, but materials and technology have moved way forward.
(remember cars in the 50s and 60s? we teenagers were driving cars with 60k-70k miles on them, that was because they were clapped out and dirt cheap; today some cars are getting their first oil change at that mileage)

Today's pistons and rings (like the three ring KS pistons available from most of our parts suppliers) can be installed with a much tighter fit in the bore due to less piston expansion, and the rings also are able to do a better at all three jobs with better material and design, along with modern cylinder bore prep.

If optimum in this case is recreating how the bike left the factory then the five ring pistons are the answer; if optimum means having an engine that performs as best it can these days, then the modern three ring pistons/ bore prep will not disappoint.
Best-
George