Surprise 812 Yoshi Pistons - but will they work for me

[rem1859]

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

[Bryanj]

0.1 mm piston to bore clearance is too big i think, thats 4 thou

[Trigger]

As Bryan has already stated, it has been over bored. Quite a common problem when a machine shop does not know the silicone content of the alloy piston or the spec for piston to bore clearance. 
Your measurement of 64.4 needs to more .400 to be accurate and every bore will measure with a difference, also you need to measure the skirt of the piston for a true measurement   

If it has Yoshi pistons it should have the oversize Yoshi liners. If it has standard Honda bore with that much over boring, there is a big risk of the liner's cracking.

[rem1859]

I have seen so much conflicting information about piston clearance.  I was left with the impression that .1mm would be at the top end of the useable range for 64mm pistons.  Going off the .0015" for each inch of bore that I have seen mentioned. 

I want to have a working engine, not one for racing, or even for long distance, I was thinking that the combination I have might work.  I am going to buy some better measuring equipment, but it doesn't really matter if I can't work out what piston rings to buy. 

I would consider a different barrel to have bored correctly, or resleeving, but I can't find any solid information that I could use to get this done for these piston.  I was surprised to find that the flanks of the pistons were 64.4 for supposedly 64mm pistons.  The pistons have 812 PopY on one side and Yoshimura in Japanese on the other.

The engine has a 410 head, and I think the barrels are for the same F series model, but I am not sure.

[Trigger]

Is this a CB750 F2 SOHC engine ?

If you want it for road use, just get a standard set of barrels and a new piston kit at 0.50 oversize, get it rebored with a one thou piston to bore clearance 

[Bryanj]

That clearance is NOT for Hondas, you only have to look in the proper Honda manual for piston to bore clearance to know its wrong

[rem1859]

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.

[Nurse Julie]

Aftermarket pistons, ie Cruzinimage should have a piston to bore clearance of 1 thou inch, ie 0.0254 mm after rebore.
OEM Honda pistons have less than this clearance
.8 thou inch, 0.0203 mm piston to bore clearance after re bore.

[Trigger]

A race Yoshi kit came originally with a set of sleeves, pistons, rings, head gasket and base gasket . You would have to bore out the barrels for the liners to fit, you would also need to bore out the top crank case as, the OD of the liners were bigger. Once the liners are it the bores it would have to be skimmed as the liners would sit proud of the surface.
The piston to bore clearance was done at the factory as, it all come as a race kit.

I have thousands of liners in the workshop but, you are not telling us what engine it is ?

[rem1859]

Thanks to everybody that has contributed to this thread, but I don't have any answers to my original questions.  I know that the clearance is too big (but by how much?  Would it really not work at 0.1mm), that I should probably go back to standard  pistons and bore (but it would be shame to retire the pistons).  My questions were, with a couple of follow on points:-

What rings should I buy? Where?

If I was to buy another barrel or sleeves, what would be the recommended fitment by Yoshi? Where in the UK could I get the machine work done.

The engine number starts B750GE so it is an F series engine. 

I mentioned earlier that the head is a 410 so the F series with the bigger valves (I know somebody is going to tell me that F series should have different Yoshi pistons), but I suspect that the compression would be back down to around standard assuming these are the 10.25/1 pistons.

I didn't actually set off to even consider the big bore route until I found the pistons hiding in my engine.

[Trigger]

With those numbers the engine is a CB 750F2, engine came black from the factory  Completely different from the CB750F1.
You seem to be hooked on these Yoshi pistons which, i guess have been over bored and 0.1000 is miles out even for yoshi pistons.

Are the liners Yoshi ?
Have the head studs been up graded to take the high compression ? Maybe this is why the head is leaking in the first instance.
Does it have Yoshi alloy con rods ?
Is the crank Yoshi and has it been Dynamic balanced ?
If all these answers are NO then, someone has been playing at being a engineer 

[rem1859]

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.

[Oddjob]

I can understand you thinking that. However experience on this site says that a lot of engineers get the clearance size wrong for Hondas, they either refuse to accept the right clearance and insist it will seize or just plain ignore you and bore to what they think is acceptable.

I suspect you have fallen into this category, the PO has gotten it wrong by the look of it.

Experience also shows that the Yoshi 812 kit wasn't the best idea in the world anyway, it sounded good in practice but didn't really warrant the cost involved or the problems they could create.

Best bet, buy a set of decent second hand barrels and go back to standard, it doesn't sound as cool standing by the bike telling people it's got Yoshi pistons etc but the reliability of the OE system far outweighs any cool factor IMO.

[Bryanj]

Yoshi were designed as race kit with a limited life and i for one would never use second hand race parts.
From memory, and yes i built a Yoshi 500 many years ago, the life of their big kits when used on road was not high

[Trigger]

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 have 28 years experience with Honda SOHC engines 
The 812 pistons or rings have not been available for years. I had a Henry Abe big bore engine in for a rebuild once and that was the same, rings not available. This is a problem with any big bore kits, they do not hold stock once they have changed to another kit or the business has closed down.

Wiseco 65mm 836cc big bore kit is a option but, at about £700 + P&P +import tax + the machine work needed. And you will need a set of APE upgraded head studs. That is a very expensive option.

 

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