Head Skimming

[AirCooledHeaven]

Hi

I just took the head and barrels off my GS1000G as I can no longer tolerate the oil leak from the front of the base gasket. Now it's apart and I'm trying to get the remains of the head gasket away from the head I can see there is like a dimpled effect that appears to go into the metal of the head.

I decided it's probably better to have the head skimmed rather than trying to get the mess off and then spending days with wet and dry paper. So then I started thinking, if I'm having it skimmed why not take a bit more off and raise the compression a bit? Got to help performance hasn't it?

Does anyone know how far I can go and how much I can take off using the standard pistons and valves before I get pre-ignition problems, or worse still, valves hitting the pistons?

Thanks

[Dave8338]

I wouldn't go over 10:1 on the compression or your going to run into an issue fuel wise, always having to add octane booster with 92 octane or better fuel. As far as the piston to valve clearance, that is specific to each engine and that needs to be determined in relation to having the head planed. If you're going that far, what the heck, pull the pistons and have them fly cut, than you'll be good to go with whatever you decide. \\/

[49er]

I wouldn't go over 10:1 on the compression or your going to run into an issue fuel wise, always having to add octane booster with 92 octane or better fuel. As far as the piston to valve clearance, that is specific to each engine and that needs to be determined in relation to having the head planed. If you're going that far, what the heck, pull the pistons and have them fly cut, than you'll be good to go with whatever you decide. \\/

Radical skimming of the head will reduce the effective squish area and compromise some of your cooling. You will also need to cut your valve pockets deeper into your pistons. If your pistons are similar in design to the 850's, you will be risking crown failure at 10-1 or higher. The crown thickness is minimal with the stock pistons.
A clean up of 0.005in -0 .010in off your head will not cause valve clearance issues, provided you keep the cams stock and retain the stock head gasket.

[Guest]

Can we have a pic?
If you get the head machined you'll have to get slotted cam sprockets and degree in your cams.

[SqDancerLynn1]

You will have to degree the cams also. I WOULD NOT take any more off than needed to clean it up

[AirCooledHeaven]

Ok forgive me if I'm missing the point, but why would I need to change the valve timing if I skim the head? I can see that I might need to retard the ignition but not the valve timing ??

[koolaid_kid]

You are not exactly changing the valve timing, you are bringing it back to the correct settings. When you mill (skim) the head, it changes the relationship between the chain and the gears on the cams (the gears are now closer to the crankshaft). When you elongate the slots in the gears on the cams, it allows you to bring the cam timing back to the correct settings. This is called degreeing the cams.
Obviously, you can do as you wish. However, were it my bike, I would take a large piece of glass, put some valve lapping compound on it (PepBoys carries it, the package has a coarse and fine tub included), and grind the head and gasket down by hand. This would prevent any other issues from cropping up.

[Big T]

Hi

I just took the head and barrels off my GS1000G as I can no longer tolerate the oil leak from the front of the base gasket. Now it's apart and I'm trying to get the remains of the head gasket away from the head I can see there is like a dimpled effect that appears to go into the metal of the head.

I think you're going too far here. If the problem is that you can't remove the old head gasket, have you tried some gasket remover to soften it up?

[Footy.]

I agree with 49er, in that a std 5 to 10 thou clean up skim will cause no issues.

As an engine reconditioner i do such work all the time. I have never experienced any skim related problems in "ANY" engine within such a tolerence. Often the engine makers will try to tell you skimming is not possible, and you must buy a new cylinder head! But that is bulls**t in my experience.

Within this clean up tolerence, the effect upon cam timing, is to slightly retard this. What this does is to slightly "tilt" the power curve further up the rev range.

Again within the clean up 5 to ten thou skim range, you are not going to notice any difference in general running because of this.

S Foote..

[bwringer]

I've rebuilt two GS850 engines, and I've had to get the heads lightly machined on both to clean up the gasket mating surfaces. The original gasket becomes part of and actually damages the surface of the aluminum, and they can't be separated any other way.

Any good machine shop should have no trouble doing this. Modern car engines have very stringent requirements for finishing the cylinder head surfaces -- many need to be polished to a near mirror finish. On a GS, a surface with a little "tooth" to it will be fine.

They don't even need to get deep enough to remove every trace of the dimpled pattern -- the critical areas are the piston sealing rings and where the oil passages and o-rings are in the corners.

It also saves a lot of time and frustration to have the block finished this way, too. Since the cylinder liners are steel, they'll have to be careful about setting up the machine they use so the cutter doesn't skip or gouge where the steel and aluminum meet.

[AirCooledHeaven]

You are not exactly changing the valve timing, you are bringing it back to the correct settings. When you mill (skim) the head, it changes the relationship between the chain and the gears on the cams (the gears are now closer to the crankshaft). When you elongate the slots in the gears on the cams, it allows you to bring the cam timing back to the correct settings. This is called degreeing the cams.
Obviously, you can do as you wish. However, were it my bike, I would take a large piece of glass, put some valve lapping compound on it (PepBoys carries it, the package has a coarse and fine tub included), and grind the head and gasket down by hand. This would prevent any other issues from cropping up.

OK, now I got it. Thanks.

[AirCooledHeaven]

Thanks for all your input. I decided I'll just get it skimmed the minimum amount to clean up the critical gasket mating areas.

Cheers

Mark