Minimum piston to head & piston to valve clearance?

[mattcfish]

I did the modeling clay test to try and figure out what my R90 head clearances are with the Siebenrock 1000cc pistons (modified by Siebenrock to fit small chamber heads). I'm using a later 336 cam so valve clearance was also very important. Squish band is 1.45mm piston to head. Intake and exhaust valves both have a clearance of 3.12mm after I had the intake pockets deepened and radiused.
The specs from the famous BMW tech sheet on 336 cams http://moragafalconers.org/BMW_336_Cam/ ... 0Draft.pdf states that a minimum clearance of 2mm is required at the squish band and a minimum of 2.7mm is required at the valves.
Further reading suggest that much tighter piston to head clearances, slightly under 1mm are acceptable.
According to my calculations I only need to mill off about .4 to .44 millimeters from the heads to acheive the desired 10:1 compression. If I do that the valve clearnce will be right on but the squish band clearance will be a tight 1.05mm.
Should I mill or is 1.45mm piston to head clearance already too tight?

[Garnet]

I can't comment from first hand experiance other to look at this http://moragafalconers.org/bmw_pistons/ ... arance.htm

I have also read the 2.0mm spec so I just don't know.

[George Ryals]

If you are building a motor to run on the street for many, many miles(carbon build-up) then stay with the 2 mm squish dimension. If you are building a motor that will be torn down every few hundred miles(race motor), go for the ablolute minumum on the squish band.

[mattcfish]

Anybody know the minimum clearance for heat expansion etc.? I'll put them on and see how she does. I know the heads have been skimmed at least twice during rebuilds. Seems like Siebenrock would have built the pistons to have the minimum 2mm clearance (I'm at 1.45mm). I probably already have higher than 9.5:1 compression. I should cc them to find out for sure.....but I really want my bike back ASAP! I'll take a feeler gauge to the squish band area and see if it's any tighter than one of my spare unmolested heads.
I've read that you really want 10:1 or higher with the 336 cam. Anybody run one with less?
I dual plugged my first head last night. My welded up jig setup that holds the head at the precise angle while guiding the drill press worked like a charm. I hate to brag but, it looks like a pro machine shop did it. I've got to make a few modification to make the process faster, but I'm happy with the design so far. I'm trying 10mm plugs in the bottom, just to preserve as much metal as possible. We'll see how well the threads hold up.

[Garnet]

We'll see how well the threads hold up.

Thread degradation is a real problem around here.

How thick are the stock aluminum base gaskets? .5mm??

I'd start off with a tight squish band clearnace and then pop the heads off after break in and have a very close look. If there is a problem you can add some clearnace with another base gasket. Easier would be to find some high compresion head gaskets, then you could go back to stock head gaskets to gain clearnace if needed.

The more compresion, the better your low end throttle responce will be. Adding a longer duration cam lowers efective compresion at lower rpms, which is why some people don't like a "cammed" engine. It takes power from the bottom and moves it up the RPM scale. Raising compresion helps keep the "off cam" performance.

[mattcfish]

We'll see how well the threads hold up.

Thread degradation is a real problem around here.

How thick are the stock aluminum base gaskets? .5mm??

I'd start off with a tight squish band clearnace and then pop the heads off after break in and have a very close look. If there is a problem you can add some clearnace with another base gasket. Easier would be to find some high compresion head gaskets, then you could go back to stock head gaskets to gain clearnace if needed.

The more compresion, the better your low end throttle responce will be. Adding a longer duration cam lowers efective compresion at lower rpms, which is why some people don't like a "cammed" engine. It takes power from the bottom and moves it up the RPM scale. Raising compresion helps keep the "off cam" performance.

There was somebody selling different thickness copper head gaskets on eBay a while back. Haven't seen them for awhile.
The Siebenrock cylinders are designed to be run without a base shim, but there's really no reason why you couldn't use one. So, if I did mill the heads too far I could bring them back to spec that way. I think I'll just put them on after I've finished dual plugging. I've got the 6 degree advanced 336 cam (I'm running with the ignition hole blocked off and a crank mounted ignition instead). The advanced cam is supposed to be better for the low and midrange. We'll see.

Anybody know the stock cc's for an R90/6 head? I've got a spare set of heads I plan to build and slap on next year. I think the R100 heads are around 58cc's if I remember correctly.

[Garnet]

High comp head gaskets: http://cgi.ebay.com/ebaymotors/BMW-R100 ... 3a65268ac9

[mattcfish]

High comp head gaskets: http://cgi.ebay.com/ebaymotors/BMW-R100 ... 3a65268ac9

I wish those gaskets weren't so notorious for leaking oil on your feet. He states that a stock compressed gasket is .035" (.88mm). My stock R90 ones are no more than .60mm (I measured them a dozen times). His .40mm ones wouldn't be much of an improvement if my measurements are correct.

[Garnet]

Come on Matt, yur supposed to believe everything you read on the interweb.

[mattcfish]

Come on Matt, yur supposed to believe everything you read on the interweb.

My problem is, I don't believe anything.