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Neils 1978 CB550 F2 Project - Newbie Let Loose
Neilw:
--- Quote from: Oddjob on February 11, 2026, 11:27:09 PM ---If you use a used ring and it's within spec then yes stay standard. Change the pistons though. However you'll normally find it will be out of spec, then you're left with wondering if it's the rings which are worn or the bores or both. Only with a set of new rings can you be certain. I did this test on my NOS barrels and you'd expect them to all be at the bottom of the spec range but in fact they were around the middle. Wasn't best pleased by that but what can you do. If you were local I'd pop round with a NOS set of standard rings and we could see what they measure up at. However check the bores first, with that amount of damage on the pistons I'd expect some on the bores as well. Maybe post some pics of the bores.
--- End quote ---
Here are some pics of the bores. To the touch, they feel smooth, but one cylinder has a slight vertical indentation about an inch long, about halfway down.
I used the second ring from each piston to check the gaps at the top and bottom of each cylinder (since I broke the top ones), which should indicate wear.
* Top 1- 0.55mm, 2 - 0.63mm, 3 - 0.65mm, 4 - 0.35mm
* Bottom 1 - 0.70mm, 2 - 0.75mm, 3 - 0.70mm, 4 - 0.55mm
Looking at the book, these are all outside the standard range of 0.15mm to 0.35mm, assuming I'm reading it correctly.
BTW, Thank you all very much in helping me sort out what to do.
mickwinf:
I would go with a rebore to .5 oversize then you know its right. I have used IMD pistons on a 550 with rebore done by Trigger and all seemed well. I think they do rebores as well, worth asking.
mickwinf:
just had a look and IMD are £167 per set plus P&P also do rebores from £35 per bore.
Murf:
I am not sure your readings mean a great deal because it sounds like you used a different ring in each bore, the rings may have different degrees of wear or even gaped slightly different when first used.
At The very top of the bore, top 1 to 3mm, there should be virtually no wear. Use one ring for all bores, take the reading at the top most edge, then about 3 readings down the length of the bore. The variation between the reading at the top and those further down the bore will indicate bore wear. As the same ring is used in each bore an indication of difference between the bores can also be seen.
All that said, having seen the photos of your bores, plus the fact you need new pistons anyway, I would just go for a rebore.
The piston to bore clearance are very small 0005 to 0008 thou as I recall, there is plenty of past posts about this on the site, so Get someone that knows about boring these engines to do the rebore if possible. If not, then tell your machine shop what you want and don’t be dissuaded when they try to tell you the clearance are too small. Check this out further though.
Neilw:
Update.
I've sent the cylinders off to IMD Pistons, as they do Rebores and supply the pistons, so rather than faffing around, i get a complete service, and a discount on the pistons.
While I'm waiting for their return, I've been cleaning up the cylinder head, have removed the valves for cleaning/lapping, and removed the bottom end from the frame so I can clean and prepare it for painting. I've decided not to strip the paint from the block, but give it a good clean and rub down.
The frame is in good condition with some small rust spots, which I'll rub down and paint. I don't think a full powder coat is needed.
I'm coming across bits and pieces that are missing or broken, and on the whole i'm finding parts, but some things like the seat, grab rail, and front and rear mudguards are almost impossible to find or so expensive it's worth keeping the rusted ones I have!
I'm starting to take apart some of the electronics and wonder how i clean them up. Do i disassemble the connections, clean and reassemble or do i just clean the surfaces? Pic is an example of one i have to do.
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