squish clearance?

[bernardofeio]

Hello

I will have to take the heads out of the bike and Iwant to adjust the squish band clearance.

What is the Squish clearance recommendation to a BMW r100gs engine?

thks

 

[Beemerboff]

This has just been done to death over on ADV riders old school.

BMW recommend 80 thou /2 mm minimum, but that isnt going to give much squish.

Udo Gietl built a 100 hp 10,000 RPM racer, and found that the piston started hitting the head at anything less than 70 thou clearance.

50 thou was the minimum on the 8000 RPM British racing 500cc singles.

The road bikes went fine with zero plus the thickness of the copper head gasket, which was 60 thou.

With 80 thou and a bit carbon build up on the stock 8.2 to one pistons in my R100GS I was getting piston to head contact.

But I ring the Fokker's neck.

The usual Yank, err, experts, claim to run 30 thou with no problems. On race bikes.

So, you have to make up your own mind who to believe.

But, as some of the guys who claim to run 30 thou also claim that a compressed head gasket is 52 thou thick and the ones I have measured are actually 28 thou, I dont give much credence to their claims.

I would go for 30 thou + / - 25 thou, with the preference for plus 25.

But just dont say I told you.

There are 18 mm thick copper head gaskets around on Ebay, for the same price as OEM, so they are a free way reduce the squish a bit.

Or just have 20 thou milled from the head - according to someone who should know that can be done to most stock engines without any problems.

 

[bernardofeio]

Thanks for your very complete answer!

To avoid misunderstandings: 80 thou = 0.080 inches = 0.002 m = 2 mm right?

Some more questions:

The squish band in the a r100 engine head is 0º?
I currently have wossner pistons and MAHLE barrels (moto Israel 10.5:1 Kit) and the band on the pistons is 0º. I’m asking this because if the head is not 0º (flat) I will have to rework the head shape which is not what I want.

(Richard Moore pistons also have 0º squish band)

copper head gasket = 60 thou = 1.50mm?? Where can I find them?

What is the thickness of the stock head gasket?

Before this conversation I was planning to go to something like 1mm (0.040´´) but from your comments it seems a bit too low. Something around 1.4mm (0.055´´) should be more reasonable?...

 

[Beemerboff]

Yes 2mm is 80 thou.

The old British copper head gaskets were 16 gauge sheet which is approximately 60 thou, but I dont think anyone has ever made BMW gaskets that thick , as the are sold as a cheap way of increasing the compression ratio.

They ones I have are 18 thou thick, around 10 thou thinner than a compressed stock gasket which is around 28 thou.

What squish you run will depend on what sort of revs you do, and how hot the engine gets, and how how long the engine will go between strip downs, and how accurate your measurements are.

The main aim of reducing the clearance is to reduce the amount of dead air, or unburnt fuel,at the perimiter of the cylinder, the extra turbulence is a minor secondary benefit and it value is considerably overstated- on a head with decent swirl/ turbulence there is almost no benefit.

So, IHMO reducing the clearance to something around 50/55 thou will eliminate the dead air and give a reasonable margin of safety, something less may be possible but but it adds risk for little if any gain.

And remember I was getting contact with the factory clearance which is usually 120 thou, which is as close as the factory want to go to their 80 thou minimum.

 

[bernardofeio]

hello

thanks again

were can I find head gaskets with 18 thou?


I will probaly take the heads out next week

 

[Beemerboff]

San Jose BMW list copper gaskets, but the don't give the thickness.

The might tell you if you asked.

Or they might not.

They are funny that way.

I have seen them on US Ebay, but not recently. Probably not since SJ startedn listing them.

If you get them Permatex Ultra Gray 3294 or Loctite 601 have been suggested as suitable for use with copper gaskets, more for oil than compression.

 

[bernardofeio]

hello

the copper gaskets that they sell are 0.030 thick. (0,75mm)