Shocking Discovery in my Honda CB750 Cafe Racer Engine | Yoshimura Big Bore Piston Motor | 08
In this Honda CB750 Cafe Racer engine episode I assess the pistons and barrels as well as looking for what could be causing the oil leaks on my CB750 Cafe Racer project bike.
Once I had spent more time looking at the barrels before removing them I started to find some really worrying things. There were definite vertical marks on the bores, and some of them seemed to be a little more than just a mark!
When I pulled the barrels, I found that the engine is fitted with Yoshimura 812 pistons, and although they do have some marks here and there overall they look completely usable. I felt that this was a really good sign that some money had been spent on this engine in the past.
Piston Ring Pliers: ebay.us/aFb6mS
Vernier Calliper: ebay.us/9iXBSW
Rubber Mallet: ebay.us/T7qw80
Unfortunately my excitement was short lived. When I measured the pistons, and bores, then checked a couple of the piston rings I very quickly realised that the bores were too big for the pistons. I also found that some of the vertical marks in the bores were scratches that could be felt with your finger nail.
Although I am not using a bore gauge, I can tell using my vernier that the tops of the bores are 64.3mm or more. It is likely that this is the tightest part and that the bore is a little wider lower down. The pistons are 63.95mm, so the bore should be around 64.05mm.
It looks like these bores have probably been honed quite a bit to clean them up, making them very oversize. Near enough to the next size up if you were reboring them one size, ie for 64.25mm pistons. I would have to hone them to try to remove the marks and scratches, so the bores would be getting bigger still.
I wondered what was causing the marks and the scratches until I checked the piston ring end gaps. They should be 0.2mm to 0.4mm, I actually found them to be nearly 2mm! It would be the ends of the piston rings marking and scratching the bores due to very big gaps.
So I now have a piston and bore combination that can not be made to work! And I so wanted to use the Yoshi pistons. I now think I have 4 main options:
1. Find a new (second hand) barrel and get it bored to suit the Yoshi pistons, having also sourced new rings.
2. Have these barrels, which are in great condition externally, resleeved and bored to suit the Yoshi pistons, not forgetting the rings again.
3. Find a working combination barrel and pistons, even if they are 750cc and use them.
4. Get a Wiseco 836 kit and bore these barrels up to suit.
I haven’t decided which route to go yet, a lot depends on finding the right parts.
I originally looked at this bike for parts, then felt it was too nice to break, now I seem to be into a big bore project. Talk about mission creep!
I also found that the head gasket that had been used was too small for the bore. It was for 61mm bores instead of a 64mm. I’m not sure what was going on there.
I then gave some time to consider the potential reasons for the oil leak from the head. It looks like virtually every possible cause had been built in apart from there were no badly sealed pucks. :)
Briefly, here is what I found:
- The knock pins are around 1mm too long which means they press against the head maybe stopping it seating down correctly.
- The non standard head gasket would be a thickness that wouldn’t match the oil way o rings. It looks like the MLS head gasket would give a 0.4mm clearance with the o rings that were fitted.
- This is an F Series Head, some of the head nuts should be cap nuts with copper washers to stop oil going down the threads. All of the nuts are of a standard type, there were some copper washers, but they weren’t all on the correct studs!
There are probably a few other things that I have missed but I am getting a little dispirited now.
The final thing to add to my concerns so far, was that I found some swarf in the head. Even though the engine hasn’t been run for more than a few seconds. I noticed that the cam tower fixing holes had been helicoiled, so I am hoping that they just did a mucky job. But I am not holding my breath. To be fair the oil and filter when I removed them didn’t show any signs of debris, but I am going to check the oil pump as well now.
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