R90/6 with R100 Jugs and Pistons?

[jjwithers]

I have a R90/6 block and bottom end. I have the heads too.
It is a large spigot block.
I also have some R100/7 Pistons and Cylinders and the cylinders fit nicely into the block.

I'm hoping to make a hot rod, fast bike out of this R90.

Any concerns or issues I should be aware of before I use those R100 Pistons/Cyls on the R90 block and heads ?

I imagine I'll have to fuss with the carbs to get the jetting right.

-Josh

[George Ryals]

Make sure that the squish area of the pistons you have are compatible with the squish area of the heads you have.

[jjwithers]

I have no idea how to know for sure...
If it is a circumference thing of matching the head to the piston, i can probably see it, but if its the domed valve area, I'm not sure how I can figure that out.
would R100 heads be safer/better?

[Kurt in S.A.]

The specs seem to be pretty close for an R90/6 and R100/7. The /7 has a slightly higher stock compression ratio. At some point, /7 heads had bigger valves for better breathing...probably not the stock /7 but more the RS or S models.

I'd be inclined to keep the 1000cc stuff together. Save the R90 head for another build later.

Kurt in S.A.

[jjwithers]

My wallet says run the R90 heads... as I don't have R100 Heads... But I agree. Just as long as the R90 Bottom end, crank, rods, can handle the R100 top end.

[tsa]

Make sure that the squish area of the pistons you have are compatible with the squish area of the heads you have.

+ 1 on that.

This means, if the R100 pistons have "flat shoulders", as opposed to a domed top that extends all the way to the edge, you probably need heads that have a 94 mm wide opening for the pistons. If fully domed pistons, they will most likely clear the heads. I currently run R100RS pistons under standard R80 heads with no problems. Although the squish band geometry isn't optimal, the performance is OK.

If you want quick acceleration rather than high top speed, you may consider getting a 32:10 final drive for it. And a lightened flywheel, which allows the R100 pistons to do the job they were designed for without struggling too much.

[Beemerboff]

Why dont you bolt it up with a few thick strips of 120 thou plastigauge on the head and find out what clearance you have.
Guys who were racing and revving these old things right out claim that they getting head piston contact at 70 thou.
Some race tuners only work with late new blocks and some internet experts claim 25 thou works great for them with the old ones but few seem to be a willing to nominate a safe number.
FWIW on Brit iron we set the deck height at zero and let the 16 gauge copper gasket set the squish so 66,66 thou worked for us.
I have just read that at least one of the vendors of the kits now set the squish the same way and supply a special aluminum head gasket the required thickness.

[Wobbly]

I agree with the above. We use modeling clay in our shop. Simply assemble it and allow the piston to pass TDC one time. Then dis-assemble and check the thickness.

The "squish band" is the 3/8" of the piston crown closest to the cylinder wall.

[jjwithers]

I found a really cheap set of R100 heads. I'll need to inspect them... but hopefully valves are OK and I'll be good to go.

Has anybody ever ported heads on BMW's for better airflow? I'm just curious... because I learned to do it on a 2stroke engine and wondering if it's a benefit to airhead heads as well.

[Kurt in S.A.]

I would think that smoothing the intake so the flow turbulence is minimized should help with combustion. I don't know the effort involved but my guess is that the measurable effects would be fairly small. Couldn't hurt if you have time on your hands.

Kurt in S.A.