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750 K2 Strip & Rebuild - help please
royhall:
--- Quote from: Lobo on May 29, 2022, 10:56:34 PM ---An interesting last Friday with Lindsay, who is rebuilding my K2 engine.
The first job was new cushs into the Primary drive sprocket component, which had to be disassembled due riveted. Within there are 12 cylindrical rubber cushs; at 50yo they were rock hard and if deformed slightly (using a pliers) simply split and stayed that way. We stuck ‘em in a bag, and when I put it down on the bench there was a solid clunk like it was a bag of marbles.
Initially 2x new needle roller bearings were pressed into the shaft. Next the 12 new cushs went in to the lower sprocket, with upper sprocket then pushed in place. (needed a press to coax this - very tight). Then finally 6 new Allen screws, into the holes pre tapped and countersunk, using thread locker of course. All now good as new.
--- End quote ---
Interesting write up on the cush rubbers. I wonder what makes the rubbers rock hard on one rebuild but still soft and pliable on another, after all they are all a similar age. I was going to do mine, but after a good prod through the holes they feel as good as new. Maybe the type of oil used or mileage? Anyway I wasn't complaining as it's a pig to do, just wondering.
Sesman:
Yes, shock rubbers…..🤔. When I stripped my 77 550F2, I found the primary chain to be approximately 60-70% worn (that’s a dimensionless estimate based on comparison with new), but remarkably the primary rubbers were comparable with new in condition. I replaced them anyway. The motor showed absolutely no signs off being dismantled in the past. So primary rubbers seem to be something of a mystery?
Sesman:
……though I believe others on this forum find that the ‘norm’ is to find them as ‘hard as marbles’.
Lobo:
My guess is (my) old rubber cushs had little useful deformation ability left given what I saw; but given their loadings so uniformly applied I’d agree they might have gone on forever - with little real indication to any rider. (tho’ time will tell on this one).
As the first cush came out in my hand, there was little obviously wrong with it…. and I stated so. Lindsay got a pair of pliers and squeezed it… it split and deformed - as any old rubber would. When a new one was in my mitt the difference was then very apparent; rubbery rubber versus hardened.
There was a choice for me…. at extra cost of course, but I figured we were in so deep it was better to go the extra mile. The bike is 50yo, the cushs have experienced numerous heat, compression / decompression cycles, and like the rest of us, must be getting a bit tired and stiff!
In my case, no regrets having seen the old pieces….
Lobo:
It seems Roy & I pretty much neck in neck… though it’s certainly no race at the pace I work.
Engine now complete, and very pretty. Head completely reworked, with valve guides re-lined, new seats, valves worked. Bores taken to 0.5 o/s, with new Cruzinimage pistons.
New chains, seals, bearings, primary cushs, cam tensioner / followers, and obviously painted & baked. Old clutch refitted as in gc.
Starter motor now with new brushes, positive wire…and painted.
Frame and all black components powder-coated.
Various bits now Cobalt-Nickel plated, which is a pain as needs to be done at 50-60C for an hour, and best preceded with a copper ‘flash’ plate (15 mins / 50C). My garage looks like something out of breaking bad. The toolkit freshened up…
Carbs and wheels next.
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