Using R80 heads on 1000cc jugs-and update on DonW

[Major Softie]

What kind of "wedge?" I'm not sure what the results will be if the wedge gets bigger into the combustion chamber (might be almost like having no squish band, might work really well), but if it gets bigger approaching the cylinder wall, that doesn't sound good, and I can see how that could happen with a smaller diameter band on the head and a larger piston dome.

[DonW]

Hey Major, as you surmise, with a smaller head, the "wedge" is bigger at the cylinder. I am concerned, but might try it anyway. The "wedge" never gets close to closing, but there might be an area that never lights off during combustion.

Don

[Major Softie]

yeah, my guess is that it would be better if the top edges of the pistons were turned to be parallel with the squish band of the heads, but who knows how much difference it would make.

[Garnet]

I supose it depends weather this is a permenant swap, or are you just running these heads till the R100 heads are rebuilt?

If this is permenant, I would definatly turn the pistons so the outer lip or squish band is parallel to the head like this:
http://moragafalconers.org/bmw_pistons/ ... milled.htm

It is not realy a matter of clearnace as much as combustion efficiancy (power and fuel economy) IMO.

[George Ryals]

http://www.garrettwade.com/5-1-2-in-tra ... /10K20.01/

DonW, buy yourself one of these. I suggest the 5 1/2" one. With your piston locked at TDC, gauge the profile across the top of the piston and cylinder. The other edge of the tool takes the shape of the piston/cylinder and can be then held to the head to see exactly the relationship of piston/head. Don't forget the gasket.

Here's another one.

http://www.leevalley.com/us/wood/page.a ... 58&p=32523

[Motorhead]

How did it work? do you like the early effects?

[DonW]

Decided not to do it, did a valve job on the R100 heads instead. When I put modeling clay in the jug, tightened things up, and turned the engine through a cycle, the results showed a "wedge" of trapped area between the outside edge of the 800cc head, and the 1000cc jug. With the piston at TDC, there is an area that would probably never ignite. It could be fixed with machining of the head and/or the piston, but it was simpler to just use my other heads.

Don

[Roy Gavin]

In the BMW instructions for fitting the 336 Cam BMW nominate 90 thou squish clearance.

In Kevin Cameron's article - King of the Boomers - Udo Gietl tells you why you need 90 thou - anything less than 70 thou and his pistons were hitting the head.
Now that was on a a race bike running to 8000 RPM, but I feel that 40 thou less carbon build up is asking for trouble.

[DonW]

Update on this project. Last night and this morning I competed the re-assembly of the engine. New rings, fresh valve job, con rods and pistons checked and balanced, everything clean and nice. Went back together easily and quickly.

Warmed the oil pan with a heat gun until the oil temp was about 120 degrees F, spun the engine with no plugs until I had oil pressure, put the plugs in, and put my gear on. Hit the starter, engine instantly fired up and I road off on a 40 mile break in ride. Engine was smooth and ran well, idled great, and after the first 10 miles or so got more "powerful".

Whoopeee!

Don

[Ross]

Well done on the build and breakin ride mate. You must be so happy.