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So it turns out the right-hand cylinder head is corroded to a point where repairing it would be cost-prohibitive. I've found a head with a matching part number, so that's one problem solved. The next issue is that there is pitting within the cylinder that I can actually feel, so I'm thinking I will have to have the cylinder re-bored to the 1st, potentially 2nd over-size. The current outside diameter of the both pistons is 81.960mm, and the cylinder bore is 82.000mm.

Please correct me and offer guidance as required: the first over-size should be 82.500mm, but I can't find a figure for the second over-size. Furthermore, if I have the cylinders re-bored (I plan to do both), does this mean I need to replace the pistons too, or do the over-size rings meet this purpose?

I very much appreciate any other advice or guidance you can offer on this subject.

You'll need the pistons. If you find 2nd oversize pistons, they'll be 83mm.

Tell your machinist you want .0015" piston-to-bore clearance.

If you just bore it and reuse the pistons, they'll have .0215 or more piston-to-bore clearance and they'll be really noisy and won't seal.

Check out availability and price of parts first. Over-size pistons are expensive (at least here in the UK, take a look at e.g motobins prices), and over-size piston rings even more so, compared to standard sized ones. And I've seen pitting so deep that boring to over-size spec wasn't enough to remove it.

Have you thought of taking it to 800 cc? Then you can aim at standard size, and find cheaper parts. Even second-hand pistons, from the guys moving up to 1000 cc - and the choice of lower compression pistons as well, if your engine currently suffer froms pinging.

Last year I was able to swing the 1st over rebore and Pistons for my 75/5. The pistons were a NOS set with pins and rings from eBay and were affordable by today's prices (was able to watch and wait for a set and got lucky).
The rebore was by a local shop with history and expertise. So far so good. I included one of the Pistons with the jugs at drop off to avoid any confusion. They weren't offended and said they appreciated it.

To avoid this damage, always store a bike with the pistons at TDC. They usually come to rest part way down and the damage is above the piston top ring. They breath moisture due to heating and cooling while in storage and it combines with the carbon on the piston crown. Then it falls off and that water will rust the cylinder.

Better to have this happen with the piston at TEC.

I would probably go to 800 cc. It is OK to bore just one side, but the pistons must be balanced to equal weight. We did it when necessary with no resulting issues that I know of.

.......I would probably go to 800 cc. It is OK to bore just one side, but the pistons must be balanced to equal weight. We did it when necessary with no resulting issues that I know of........


Amazing. You really did that?

robert wrote:.......I would probably go to 800 cc. It is OK to bore just one side, but the pistons must be balanced to equal weight. We did it when necessary with no resulting issues that I know of........


Amazing. You really did that?

Duane is sounding more like me every day.

robert wrote:.......I would probably go to 800 cc. It is OK to bore just one side, but the pistons must be balanced to equal weight. We did it when necessary with no resulting issues that I know of........


Amazing. You really did that?

We bored one side to the next oversize, not to 800 cc as could be inferred by my poor writing.

Is that now clear? Sorry bout that.

OK gents. Forgive my ignorance, but can someone please tell me if the cylinders + pistons from an R75/6 are a direct swap onto a /5?

Kriegmeister wrote:OK gents. Forgive my ignorance, but can someone please tell me if the cylinders + pistons from an R75/6 are a direct swap onto a /5?

Only those cylinders from a '74 or '75 /6. The engine case opening on the /5 is 97mm...it was increased to 99mm for the '76 models.

Kurt in S.A.