Honda-SOHC

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.

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.

So, it seems the advice I am being given is :- Give Up!  Throw the pistons away!  This is different to my experience on similar forums in the past, where people made suggestions about how to make things work.

I am still no further on with the original questions. 

What rings should I buy?

If I was to buy another barrel or sleeves, what would be the recommended fitment by Yoshimura?

Maybe nobody on the forum has any real experience of fitting secondhand Yoshi pistons so can't help.  I am feeling a little discouraged at the moment.

I will probably go down the standard (ish) barrel route.  But equally if I can find rings I would like to use the Yoshi pistons, with new barrels or resleeved if necessary.

I was hoping that somebody could tell me more about the Yoshi 812 piston sizing, clearances etc maybe even have a copy of the original Yoshi fitment instructions.  Looking in the Honda manual hasn't actually been that helpful for aftermarket pistons.  In that, going off the serviceable limits for standard pistons, bore 61.1mm and piston 60.85mm it seems to imply a .25mm clearance (which I know is nonsense) but for a rebore it gives a minimum of .01 - 0.04mm. I can't see any maximum mentioned so .1mm doesn't seem so outrageous on larger pistons.

Like I said originally there is a lot of conflicting information, but nothing authoritative!  It's easy to give up and follow the standard route, but not as interesting.

It looks like somebody has had these barrels bored to work with these pistons originally, they wouldn't have accidentally got a set of barrels bored to 64.5mm round, no taper etc.  Which really surprised me when I used the bore gauge. They may have been working to Yoshi figures, unlikely, but I don't know and I don't seem to be able to find any information to be sure.

The fact that the piston rings were totally wrong seems to have been a major faux pas, which may actually cause the show stopper with the marked bores.

I am going to try to get some better measuring equipment to make sure that it isn't my inaccurate measurement that is the problem.  I am confident that all of the bores are the same dimensions using my bore gauge but less confident that I am measuring them accurately to 64.5.

I started from a positon of thinking the barrels were scrap, so I haven't really got a lot to lose.  I have been looking for another set of barrels, or a source of a set of sleeves (in the UK) at a realistic price, but without some idea of getting rings I am not going to gamble the money.

I bought a project bike that had been paritally Cafe Racered!  The PO told me they had bought the engine complete but never had it running.  It appeared to be in quite good cosmetic condition, but had been left to stand for 5 years, so some corrosion had started.  I did get it to start without too much trouble, but it leaked a lot of oil coming from the front of the cylinder head, so I knew I had to strip it to investigate.

Cutting a long story short when I stripped the top end I found that it had been recently rebuilt, but not well!  And best of all it had 812 Yoshimura pistons.  BUT.  The bores are a little marked, with a couple of scratches, which seemed to be caused by undersized piston rings.  When I checked the ring gaps they were nearly 2mm instead of the 0.2 to 0.4mm they should be, I think this was causing the marks.

When I measured the tops of the bores, and the pistons before removing them I found that they were 64.4mm and 63.95 respectively which I felt was going to mean the bores where too big for the pistons, hence the big ring gaps.

However now I have removed the pistons and measured the bores using a dial gauge, I have found some interesting things which are confusing me. 

The bores actually are very well sized at 64.5mm, being very consistently sized top to bottom and in both directions across.  Which I thought meant the bores would be too big for the pistons.  But when I measured the pistons I found that they were 64.4mm across the thrust sides just below where the pin goes through.  So with .1mm clearance I think the pistons should be OK. 

The measurement across the head of the piston above the rings is 63.95mm which really threw me.  Along with the massive ring gaps.

I wonder if anybody else has any experience of Yoshi pistons?  I expected them to measure around 63.95 to give 64mm bore, so the extra .45mm is a bit of a surprise.  I don't have appropriate piston rings, so I am trying to work out what to buy.  I suspect that the rings that were in it were standard CB350 ones, but have no way of telling. 

Has anybody any experience of these pistons that can give me some advice?  I am thinking of buying oversize CB350 rings but don't know if this would work.

I will try to put a video of what I have found out so that you can see the issues I am having with this engine.  I really want to try to get it as a runner this summer, but without rings I wont be able to do that.


Update:
For those who are interested, I have a series of videos following this CB750 project bike on Youtube here: https://youtube.com/playlist?list=PLZCsrCcblKI_sOuZow0mNHIz0wBBN3Wr_

And here's the video where I discover the pistons: https://youtu.be/LJYek5UFTps