Surprise 812 Yoshi Pistons - but will they work for me

[rem1859]

Thanks for some useful information.

I wasn't looking for a big bore, and was genuinely surprised to find these pistons in the engine.  The PO of the engine made quite a few mistakes in building this engine, but who am I to criticise!

I was, and secretly still am, hoping to find rings and get barrels bored to suit, but I am not willing to throw money into a pit.

It seems a shame that what appear to be a lightly used set of pistons are destined to be desk art!

Not to worry.

[Oddjob]

Sell them. Someone will buy them, let them have the problems and use the money to buy a decent set of barrels.

[philward]

Have you considered going for a Cruzinimage 836 piston kit? I have been running one on my K2 (3000 miles) and my cr750 rep (few hundred miles only). Standard compression and quality is recommended on this forum. Cheap too - ebay link -https://www.ebay.co.uk/itm/165430559105?hash=item26846e0181:g:W4gAAOSwibph5MJM
Engineers on here will rightly say its boring standard liners to limit but if you are not racing, it should be fine (especially with the limited mileage our type of bikes get)
Much improved bottom end torque over standard is a bonus.
You wouldn't need to buy another set of barrels AND new piston/rings/head gasket
Just a suggestion

[rem1859]

The last two comments may be the best road ahead.
Sell the ones I've got, and go for a 836 set.

I will definately consider that if I reach a dead end with the rings for my existing pistons.

[philward]

Maybe Andy at https://good-bits.co.uk/ may be able to point you in the right direction with regard to sourcing Yoshi rings - he races Honda 750's

[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]

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.

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.