Modifing heads.

[Garnet]

Does anyone know of a shop in the PNW or Canada that is set up for this kind of work?
http://www.motoren-israel.com/popup_ima ... 41/imgID=0

I have a set of 1000cc cylinders and squishband pistons that I would like to mate to my R75 heads.

[Kurt in S.A.]

Check with Shails in Vancouver.

Kurt in S.A.

[mattcfish]

I've got an idea about that, that I've been milling around for awhile. Instead of machining a squish band into the head, do the opposite, and have the squish band milled off. Then, by shimming the base of the cylinder by the same depth as the desired band, you created a squish band within the cylinder with the same compression ratio (or greater if you don't shim as deep). That's what I was thinking of doing with my 90/6.
Somebody please tell me why that won't work.

[Garnet]

On my pistons, that look like these:
http://moragafalconers.org/bmw_pistons/1978%209.5-1.htm
the piston dome comes very close to the gasket surface of the head leaving me with a wedge shapped squish band like this:


What I will probably do is turn the piston down slightly so it has a bigger squish band sorta like this: http://moragafalconers.org/bmw_pistons/ ... milled.htm

But I still need to round off the edge of the gasket surface of the head (sorry no pic), that is now in the combustion chamber, to avoid a hot spot.

In the end I hope it will look something like this:
http://moragafalconers.org/bmw_pistons/ ... arance.htm

Make any sence?

[John Falconer]

If you have pistons just like the ones you point to, and the clay you are holding is oriented so that the part you hold is towards the center line of the cylinder, then it looks like you have the '77 head and piston combo. If true, then I'd just run it as is (or if you want to fiddle just a bit you could reduce the thickness of the squish band a little but be careful - you don't want any piston to head connections! John

[Garnet]

Yes John, I believe that the used pistons that I have are from a 78 (according to seller) and looking at this comparison:
http://moragafalconers.org/bmw_pistons/ ... 0sides.htm
And you are correct that the (wax) mold is being held by the combustion chamber side.

But I am useing R75 heads. The mold confirms that I have clearance, but I am concerned that the shape of the squish band is wedge shaped the wrong way( less clearance in the centre than at the cylinder wall). I believe this would be inefficient, robbing power and gas mileage.

I got stuck at this point last winter and decided to look for used R100 heads, but every time the subject of big jugs on small heads comes up I get very interested in drggin all my bits out of storage and trying to make a pigs ear out of a silk purse.

[Max Headroom]

I've been fortunate recently to acquire a pair of slightly used Venolia forged 1050cc pistons, along with a pair of '77 squishband heads. The heads are a bit odd, in that they are set up for 40mm headers but only have 42mm inlet valves, not 44mm. They have a few other issues too, but nothing insurmountable. I've already got a pair of shot-peened rods and 336 cam for this particular project.

Next step is to get a pair of cylinder liners and bore a pair of R100 barrels to suit. From what I've read, there's no point bothering with running the Venolias directly in R100 barrels due to the lack of necessary thickness for adequate support.

[George Ryals]

Max, unless you are building an all out race engine, I would stay with the 42 mm intakes to keep the intake runner velocity high. With the 336 cam the engine will be much happier with higher intake velocity and maybe 10 to 1 mechanical compression ratio.

[Max Headroom]

Max, unless you are building an all out race engine, I would stay with the 42 mm intakes to keep the intake runner velocity high. With the 336 cam the engine will be much happier with higher intake velocity and maybe 10 to 1 mechanical compression ratio.

I was thinking along the same lines, George. The plan is to build a road-legal track-day bike, so ultimate power is not the primary goal. Main priority is reliable fun.

[John Falconer]

OK - now that I look more closely I see what you say - same problem one runs into with the flat heads and tapered pistons (like '77 heads and later pistons). And you're right - you don't want the taper enlarging towards the edge. Solution - cutting metal carefully.