R100S top end

[Mal S7]

I am spending my Easter Sunday fitting a R100S top end to my R100/7. Its a nice religious way to spend the day I think, Gott im Himmel!

Exhaust valve seats have little chips out of them, on the inner edge of the taper. They don't cut right across and I managed to lap the valves OK. Just wondering what might cause that. The engine eaten something hard?

S pistons appear to be the original items. My Haynes manual reckons they would be 9.5:1 compression, as opposed to the 9:1 ratio of the /7. The /7 though is on second oversize, so they could be anything. I haven't got the /7 heads off yet, but is there an easy way of telling? Will it be obvious from the crown?

It would be nice if I could just swap heads.

cheers and happy easter
Mal

[Deleted User 62]

Mal, in a discussion awhile back, it was noted that the S pistons have higher domes and have more "defined" valve pockets. Sorry, the forum won't let me add a picture, but you can see more here: http://boxerworks.com/forum2/viewtopic. ... 211#p50195

[Mal S7]

Thanks Tim. I took the heads off the /7 and yes I can see the difference on the pistons, the S appears to be domed a couple mm higher, its the deeper valve pockets that give it away.

[John Falconer]

Actually the R100S came with at least two different piston configurations, maybe three or four.
* 1977 to early 1978 - 9.5-1 - with a pronounced peripheral squish band
* 1978 (later) on - 9.5-1 - with a hemispherical dome that extends to the periphery of the piston

And by now, many of these bikes have been retrofitted with other pistons, perhaps most commonly with the flat-topped 8.5-1 pistons that became the standard in later BMW bikes as leaded gas was phased out.

The 1977 /7 had 9.0-1 pistons with a nice squish band but also, if I recall correctly, the smaller inlet valves, so be careful to check both pocket depth and radius.

I recently picked up the files showing some of the different pistons (which I used to host at my old home) so I'll put those back up on a server in this home, maybe later today.

[Mal S7]

Great! What is a "pronounced peripheral squish band" ?

[dougie]

Great! What is a "pronounced peripheral squish band" ?

"per-if-err-all"

[Major Softie]

Great! What is a "pronounced peripheral squish band" ?

"per-if-err-all"

nyuck nyuck

A flat band around the edge of the piston to "squish" the mixture against a similar flat band on the head. If the dome goes all the way to the edge of the piston, then it has no "squish band."

[Mal S7]

Oh brilliant!
I have purr-iff-err-al squish band.
What's it do?

[Major Softie]

squishes

[Major Softie]

Oh, and increased combustion chamber turbulence. This gives a more complete burn, and helps prevent detonation.

It is supposed to promote better heat transfer to the head as well, but I've never understood how that worked.