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R50 Top-end refresh

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dardaris2725
Posts: 55
Joined: Fri Oct 27, 2006 3:00 am

R50 Top-end refresh

Post by dardaris2725 »

After discovering this past season that exhaust valve lash settings on my 1959 R50 are tightening up in as little as several hundred miles, I've pulled it apart to see what's up. Also a compression test yielded 100psi on both cylinders.
So both exhaust seats are pitted and the guides are a little loose. I will get those attended to.
BUT what I am really writing about is the pistons, rings, and cylinders. The problem is that they look great! I went thru this bike 25 years ago; Using the Many-in-one tool I serviced the slingers, went thru the motor replacing bearing, seals, etc. At the time the pistons and cylinders looked good, clearance was well within specs., and I decided to hone and put new rings in. Now, 20 years and 15,000 miles later the pistons, cylinders, and rings still look good. I can still see traces of the cross-hatching. The piston clearance is .003", the ring end gaps are within specs., in the .010" - .016" range. One of the oil rings is however, way out at .030"+
The conundrum I have, is that these are the original 5 (FIVE) ring pistons, standard size. The cylinders measure 2.677" (68mm) and the pistons 2.674" (67.92mm). The bore is the 68mm and the pistons are less than 68mm.
Should I replace these with "modern" three ring pistons? Is there anything to warrant this?
The pistons I see on Benchmark's site list 68mm as std. size. Is this the new standard size, meaning the bore is over 68mm?
Also, on the Benchmark site, every other piston listed says Kolben Schmidt (KS) EXCEPT the std. size R50-R50/2 piston. Can this be true?
I'd like to hear what others think about replacing my still good, 5 ring pistons. Thanks.
Damon Dardaris

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caker
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Re: R50 Top-end refresh

Post by caker »

Hi Damon,
The pistons I see on Benchmark's site list 68mm as std. size. Is this the new standard size, meaning the bore is over 68mm?
My understanding is, for example: 68mm pistons are for a 68mm bore, and are actually 67.92 (or whatever it is) and includes the "proper" piston to cylinder clearances.

-Chris

Daves79x
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Re: R50 Top-end refresh

Post by Daves79x »

The piston/cylinder clearance of .003 inch (.07mm) is fine for the R50. Specs say you can wear to .004 inch but that'll be a little noisy. My R50 was only clearanced to .0025, sort of me and my machinist's oversight. I broke it in very carefully, and have over 2000 miles on it now without problems. It is one of the most quiet engines you'll hear, but I would never clearance another one that tight. If the cylinder is not out-of-spec for total clearance and ovality and taper, a fresh hone and new rings are probably all you need, if you can still find rings for those pistons.

Dave
Dave

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