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I have a 1974 R75/6 and parts from a 1978 R100/7. Will the bottom end of the R75 drive the top end of the R100? These parts are just sitting around and I'd like to use them. The R100 cylinders need to be bored out. They suffered some damage from it's bottom end blowing up due to an obstruction of oil flow... oooops!
Anyhow, any information regarding this topic will be greatly appreciated.

-John

Just a welcome here to the forum, John.
Am sure you'll hear from the learned side of the board
soon......

Were your /6 a bit newer, then it would be fairly easy. Stick your cylinders up to the holes in the block. They won't quite fit. That just requires a bit of machining.

Others have done it and seem to feel that it works out OK.

I am aware that I will have to mil off 1mm at the base. I guess what I am wonder is if there are any unforeseen problems with this modification.

Thanks

You don't have to "mill" anything off, you have to bore the cases up to the same size as the spigot on the jugs.
It is more work but a better solution.
What's even a better solution is boring your R75 jugs out to R80 size and pick up some R80 pistons. (get the pistons first)
Now THAT is a sweet running engine, especially if you dual plug it. Talk about smooth and quiet and good mileage, AND you don't need to change the final drive and the speedo

An R90 top fits nicely onto an R75/5 bottom. (That's what I was riding when it went into the "wet-cat" mode.)
I'd check the measurements for the block boring job and the cost...then consider if someone would make a swap if the numbers don't come out OK for you.
We assume you have the pushrods, etc for the R100 top end? An R100 top is a bit taller (longer) than an R75 top.

Jean wrote:An R90 top fits nicely onto an R75/5 bottom. (That's what I was riding when it went into the "wet-cat" mode.)
I'd check the measurements for the block boring job and the cost...then consider if someone would make a swap if the numbers don't come out OK for you.
We assume you have the pushrods, etc for the R100 top end? An R100 top is a bit taller (longer) than an R75 top.

Where do you get your info?
It's wrong.
All the post 1969 airheads have the same stroke EXCEPT the R65 and R45.
The R90 top end will slip right on if you use the correct years, ie, 1974 and 1975.

These are 97mm cylinders bored out to take standard R100 pistons.




The spigots are pretty slim, just about 1.5mm thick. I wouldn't want to bore them out and loose another quarter mm.

When Motoren Israel and Siebenrock make extra large displacement cylinders (up to 98+mm) they remove the cylinder foot completely and just bolt them to the block
. I asked them about this, and they said it works just fine this way. No issues. Maybe it's because they're using longer rods......but the slipper pistons they use are chopped down so they have the rings in the same area. Just wondering if simply cutting off the base would work for putting R100 (or larger) jugs on early 97mm blocks.

wirewrkr, I said the R90 would fit right up.
I questioned the R100 parts. Mine (I have both an R75/5 and a '78 R100s) measures longer(Taller) on the outside, so I thought it would be good to check. My factory data sheet shows the stroke on the R75/7 as being the same as the R100, likewise the cam dimensions.
I don't have any factory data on a '74 /6 unfortunately, but assumptions can get expensive.
Both Haynes and Clymer imply the con-rod journals, etc. are the same but measurements prove it.

Going further, I found the clutch spring is stouter on the R100/7 (and s) than on the R75/7 STOCK clutch. It's the same diameter but made of thicker material and has a higher pressure rating. Probably not a problem unless you were going racing.