Topic: cb500/550 big end and main bearing clearances

[DrPat1959]

I have a cb500K0 engine that has been built from a box of bits and several breaker 550 engine. The crankshaft is a 550, as are the conrods, although the cases are original 500. The bike was shipped into Australia in 1970, although the K0 wasn't shipped to the US until 1970. I want to keep the original engine number and frame number together, which is just my personal preference. Anyway, I have been through an exhaustive process of measuring the crank journals and case case journals with a quality mic and an inside mic and matched the best bearing shells from all those that came off the 3 engines. I have managed to achieve 0.025mm to 0.038mm with plastigauge (flexigauge actually), except 1 big end and 1 crank pin, which are around 0.05mm to 0.7mm. I read in the Honda workshop manual from 1977 that the serviceable clearances are 0.08mm, but I have read on this forum that the maximum clearance for the cb750 is only 0.05mm. I think I can move a couple of shells and get the larger clearances down to around 0.05mm, which will obviously mean the 0.25mm ones will be a little larger, maybe 0.038mm. I have also read here that these smaller capacity engines prefer a slightly wider gap, but that was a few years ago. It will be on club plates and only ridden a couple of times a month. It should also be noted that it is quite a bit hotter here in Australia, so our air cooled engines tend to run much hotter that in North America. What do small capacity enthusiasts think about my situation. New shells are very difficult to source here as well.

Patrick

[jaytee-nz]

Welcome aboard Patrick.
I've restored quite a few 500 and 550's and the clearance measurements (with plastigauge) you are getting will be fine. As long as you are within Mr Honda's tolerances all will be good, unless they are badly scored. However, I've put bearings back in that have a few minor scratches and had no issues.
BryanJ from the UK might chime in here - he has a wealth of knowledge with these bikes, as does Dave500 from Oz.

[bryanj]

You got there first,
The main bearing case codes and con rod size codes will be marked and will still be accurate, more so than a bore gauge.
Pin sizes need to be measured very accurately and not to be rude , most people are not accurate enough with micrometers.
Shells are still available but not cheap for full sets and if you search the uk forum there is a firm in Japan (not yamiya) that still lists all the sizes.
Without going to my books i am not sure of the Honda tolerances BUT i would be dubious of using used shells unless absolutely perfect with no matkings at all.
On an asside the K0 was modified on the gearchange, clutch and clutch cover, again the bulletin is available in alladins cave on the uk site, and this is really necessary for a decent running bike.
As the enine is in bits a new primary chain and damper rubbers in theprimary is a must as worn chains eat the main oil gallery, also slightly thicker O rings on the head gasket plus extra care when fitting cam cover, many 550 part fit except gearbox, gearchange, clutch etc.
Hope that helps.

I have fitted a full set of tightest shells before and run in carefully for 1,000 miles

[Oddjob]

Impex have some shells Bryan but no black or yellow main shells. TBH you can still find green and brown but Impex prices are competitive so may be worthwhile ordering from them, depends on postage and import duties I suppose.

Pat whilst you can use plastigauge to determine whether the crank clearance is still in spec it's really only possible to check the wear on the crank with a new set of shells, otherwise you're normally just measuring how worn the shell is, not the crank. Shells are sort of sacrificial and should in theory wear before the crank does.

What Bryan meant by the tightest shells is more accurate to say the biggest shell, which is black. You can fit a full set of black shells, all 10, and as Bryan says run the engine in very carefully and they should be fine. To me that would depend on whether any of the original shells were yellow, which is the thinnest, TBH the difference between each shell size is really small so maybe I'm being too careful but personally I'd be wary of doing that if any were originally yellows. On the back of the lower crankcase  are 5 letters stamped into the metal, those are the original sizes of the crank journals, as determined by Honda. If any of those letters is an A then it's possible you had a set of yellows fitted as standard, possible, not certain. If any of them are the letter C then it's possible you already have a set of black shells fitted, again, possible not certain.

I have some black main shells, they are quite hard to find now, almost impossible at times. If you need some PM and I'll discuss it. I also have some yellows but I doubt you'd need some of those.