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CB750 / Surprise 812 Yoshi Pistons - but will they work for me
« on: July 31, 2022, 08:25:46 PM »
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
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