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Surprise 812 Yoshi Pistons - but will they work for me
Yoshi823:
When I was in the market for a bigger piston kit for my CB750F2 I rode down to Dixon Racing in Godalming and spoke to the man himself. He didn't have any 812cc (64mm) piston kits in stock at the time so I bought the 823cc (64.5mm) kit instead. This was just a straight overbore from the 61mm standard bore that gives that engine the 736mm size. It is my understanding that 836mm (65mm) is the maximum that the sleeves will take before needing punching out & replacing with thicker ones.
I had the barrels bored to size then honed to clearance by a colleague that had a boring bar and a stone hone. As far as I can recollect (this was back in 1981) the hone to clearance was standard Honda specs as the pistons are cast, not forged, which have a different spec.
I have some spare piston rings that are surplus to requirements that I am willing to sell. I knew I kept them for a reason.
PM me if you are interested.
rem1859:
--- Quote from: Yoshi823 on August 04, 2022, 01:42:31 PM ---I have some spare piston rings that are surplus to requirements that I am willing to sell. I knew I kept them for a reason.
PM me if you are interested.
--- End quote ---
A couple of people have mentioned they might be 823 pistons because of the skirt size, I'd be surprised because they are still marked 812, did your 823 pistons have markings on?
It might be worth me trying the 823 rings.
Thanks, I've sent you a PM
rem1859:
A thread on this issue has been running on the US forum with a lot more activity. I am adding this here just in case somebody reads this thread without seeing the other. Sorry if this upsets anybody, but I have found so many threads that peter out without any final information, and I don't want to do the same.
I have measured the bores and pistons twice now, over 12 hours apart. Making sure the micrometer was zeroed against it's test piece both times, and very carefully setting up the bore gauge. I have had pretty consistent readings both times, so I am reasonably confident of my measuring.
So now I have the following measurements to consider.
Cylinder 1 2 3 4
Bore 64.46 64.45 64.45 64.45 Measured 30mm down on the thrust axis
Piston 64.41 64.40 64.39 64.40 Measured 10mm below pin on thrust axis
Difference 0.05 0.05 0.06 0.05 Bore - Piston
They aren't as bad as I first thought, although I had been using a vernier to measure without the benefit of a bore gauge.
I think I can take the following actions.
Hone the bores lightly, the marks are not nearly as deep as I first thought and I think will remove quite easily.
Measure again.
If the bores clean up and the difference (clearance) is still 0.07mm or below (which I have seen as a maximum on a thread by an authoritative contributor to this thread)
Or
If the clearance goes above 0.07mm look for sleeving or new barrels and a fresh bore.
Buy some 823 (64.5mm) rings.
Build the engine, happy days.
rem1859:
I did have to get new barrels in the end. I have put this on the other SOHC forum, so I thought I should give an update here.
It has been a long time (ish) since I have added to this thread but I am now able to carry on with this engine. I hate threads that run out without any conclusion.
Eventually after honing and cleaning the bores they were too big, so I found another set of barrels and had them bored.
I also found the right rings, apparently NOS Yoshi 823 ones from a big supplier.
So I can now get on and continue looking at the rest of this engine.
I have put a lot more info about this in a YouTube video https://youtu.be/YcIc5S9Z47g on the Spanner Rash channel.
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