SOHC.co.uk Forums > CB750
Surprise 812 Yoshi Pistons - but will they work for me
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.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version