shaved heads

[Hawkins]

Cylinder heads of course. Anyone here consider shaving their heads to boost the compression ratio? With higher octane gas and/or EFI, it should be possible to do without inducing knock. Of course there are other considerations (piston valve clearance, maybe higher cyl pressures to open ex valves against, etc.). Just curious. I know some of the muscle car dudes do it with their engines.

[tw1980]

I seem to remember seeing some spirited discussions about that.I couldn't find the posts but I'm pretty sure they're in there somewhere.

[trbig]

You can shave the heads SLIGHTLY, but there's not much room to do so. This applies even more so on the later heads with the larger valves. Even a stock head has already had some "Meat" removed from where the bottom exhaust studs are, and there's very little room between the valve and the edge already. This head has the larger exhaust valves and look how close it is.. as well as the bottom exhaust stud. This head was shaved ever so slightly just to make sure the surface was true.




Here's a stock '78 head just to show you that the material is already close to the exhaust studs, but notice there's much more room by the valves to do some shaving.






The earlier heads had a much smaller valve relief space in the head than the '80 and later heads, so the simplest way to boost up your compression would be to use an early model head over later model stock pistons... which are much higher domed than the 78-79 pistons, and even higher domed than the new Wiseco pistons for the big bore kits.


Tod

[The Lorax]

I know nothing about shaving the heads on the XS however, this would be a question for that "Dan Hodges" guy, he seems to know all about horsepower gains and such from near all engine mods. his posts both fascinate and befuddle me. You might consider sending him a PM, he'll give you the answers you're looking for, and maybe some you're not. have a nice day and ride safe

[TADracer]

Yes I remember this post. Spirited at the very least.

You can safely take off about .010" which will give you maybe 1/2 point in compression ratio. Go more than .012" and you may start having valve/piston/cam chain concerns.

Before you go milling your head (which is not reversible) try to find a thinner head gasket. The stock gasket (compressed) should be about .055" thick. If you can find a .045" thick head gasket (such as a copper one) you will get the same result and that is reversible.

Some people say copper gaskets will leak oil on a street machine but others say that if they are installed correctly with the correct sealants they will perform very well on a motor with both aluminum cylinder head and block. I do not know this to be a fact but it may be worth a try before you go cutting your head up.

[TADracer]

trbig, have you ever tried a thinner gasket? I have been thinking about this and I know you have done alot in this area.

[trbig]

No, I didn't know they made a thinner one... although, I may have done this inadvertantly! lol. I've re-used head gaskets over and over as long as they don't de-laminate. Maybe they were getting a touch thinner each time?


Tod

[b.walker5]

Before you go milling your head (which is not reversible) try to find a thinner head gasket. The stock gasket (compressed) should be about .055" thick. If you can find a .045" thick head gasket (such as a copper one) you will get the same result and that is reversible.

Some people say copper gaskets will leak oil on a street machine but others say that if they are installed correctly with the correct sealants they will perform very well on a motor with both aluminum cylinder head and block. I do not know this to be a fact but it may be worth a try before you go cutting your head up.

It's a little primitive but quite acceptable to get a piece of 1.2mm copper sheet and make your own. Cut your pattern, anneal the copper (heat it dull red with a blow torch), let it cool (dont quench with water or oil), use a good head sealant (I dont use RTV) and it'll work fine. Factory copper ones often have a steel edge around the cylinder holes for reinforcing, but I've made plenty over the years for V8's and such and never had a problem without the steel bit.

[TADracer]

Thats kind of what I was thinking. The gasket is not really that complicated and I do have a few sheet metal tools. And since I have my motor out right now I can do the checking for piston to valve clear very easily with light light valve springs. Besides, we have no pressurized oil passages through the gasket or any coolant passages to worry about. Seems like a simple way to pick up a bit of compression ratio.

[DiverRay]

I did a search a few years back, and there was a company that would do copper head and base gaskets. I think they are in the Los Angeles area, but I don't remember. It's been a computer or two ago, as I was looking to get thicker base gaskets to LOWER compression a little. Turbo chargers work better if you keep them spinning. I'm still not there, but I am SLOWLY collecting parts for the "hot rod".

[trbig]

I'm still not there, but I am SLOWLY collecting parts for the "hot rod".

....And by the time he finaly gets "'round to it" he's gonna be so dang old that he'll be scared to ride it..


Tod

[DiverRay]

....And by the time he finaly gets "'round to it" he's gonna be so dang old that he'll be scared to ride it..

Yeah, but I've at least got two other bikes I can ride, and good weather almost year round to ride in! But on the flip side, if I DIDN'T live in California, I would probably have the bike done already!
But then, I need to find another job so I CAN afford the parts and machining.....

[trbig]

I need to find another job so I CAN afford the parts and machining.....

Pffft! It's just money, Ray! You can't take it with you, you know? Think about how much extra TIME you have to do it right now..

Can't ya sell off a kidney or sumthin'?


Tod