1976 CB550F1 (what have I let myself in for) Project

[Bryanj]

You say 0.6 and 0.05 so total of 0.65 and if that is mm its over 20 thou or way to much.
OR  are you measuring at crown not skirt.
Only reason i can think of for piston to be at angle is either bad piston machining or bored at an angle

[Laverda Dave]

Have you tried swapping the oil rings around between the pistons? This will tell you if it's the piston ring gap not having enough depth or if it's the actual rings at fault. The 250RS piston I had a similar issue with was a pattern part from DSS. I can't remember the actual make but it was pattern but not IMD or Cruzinimage. I remember it was in a blue and white box though.

I meant to say ring grove depth not gap! Although interesting to see if the ring gap is larger or smaller if fitted to another piston but in the same bore.

[eight0]

Just tried swapping 3 and 4 and the issue with centering doesn't go away.
Yes I mean I measured the crown above the top ring not the body, just to show how much is was pressed on one side.

[eight0]

I think I'm going to have to split the cases and double check the rods are on the correct way around. I really didn't want to have to do do it, but at this point I have to rule it out.

[eight0]

Have you tried swapping the oil rings around between the pistons? This will tell you if it's the piston ring gap not having enough depth or if it's the actual rings at fault. The 250RS piston I had a similar issue with was a pattern part from DSS. I can't remember the actual make but it was pattern but not IMD or Cruzinimage. I remember it was in a blue and white box though.

I meant to say ring grove depth not gap! Although interesting to see if the ring gap is larger or smaller if fitted to another piston but in the same bore.

Yeah I tried different rings on the same piston and no difference. It doesn't seem to matter which combination of pistons or rings I try, the oil rings jam the pistons up.

[Laverda Dave]

That's exactly the same problem I had. I didn't know it at the time but the top oil control ring had a very slight bend on it only seen with a magnifying glass. I put this down to damage caused as a result of trying to push the piston in the bore. DSS wouldn't sell me a replacement set of rings as they were in a piston kit so I had to buy the whole set just for one ring! Strangely enough I had no issues with the new set, fitted great. I think you are going about the investigation in a thorough way by eliminating all possible causes even if it's long winded and a pia.

[eight0]

I put one of the Conrods from my other crank on number 4 and it was fine. Took a closer look at the one I took off and... Spot the problem? Well, bugger.

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[eight0]

Can I use my original rods?

The new crank and old rods make Brown, green green green all of which I have.

[Nurse Julie]

I put one of the Conrods from my other crank on number 4 and it was fine. Took a closer look at the one I took off and... Spot the problem? Well, bugger.

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Aaaah, the Banana con rod 😁😁😁 Rods usually bend in pairs, so it #1 rod bent as well?

[eight0]

Yes! Which is why the middle two pistons were centered out outside weren't.

Think it's safer to use all 4 of my old rods?

[hairygit]

If you have shells for them, yes. Where did you get the crank and rods from?

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[eight0]

Crank and rods were from DK.

[hairygit]

Call them and tell them what you found, and mention you're documenting the build on a bike forum. They seem to be pretty good at sorting problems, and may offer a partial refund or better.

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[Laverda Dave]

Well done, looks like you have solved the problem👍. This is a great thread.

[eight0]

Thanks guys.
Well there's still the oil ring problem. On the bench the pistons were still too tight, but I'll get back to that once I've fixed the bottom end.