Hi, Iam working on my 27 Coupe that has been in storage for over 35 years. I think I have a handle on getting the motor running. Ive got the pistons un stuck and will pull them out to check the rings. Have done a valve job. I now will clean everything up and get it back together. My head has been welded in the past. I have another head but it needs to be decked as the surface is pitted around the water jackets. So how much can I take off? I would like a bit more compression. I dont want go to far so can anyone shed some light on this? Thanks, Scott
I had one crack that was milled .125" (1/8").
No noticeable power gain.
Could've been milled previously...dunno.
I believe what you need to do is get some PlayDoh or modeling clay and place it on top of your pistons and valves, then set the head down on the block without the gasket. I'd put in a couple of head bolts so that when you crank the engine over by hand the head doesn't move any. Then, carefully remove your head and measure the thickness of the clay. That will tell you how much room you have to work with. Decide how much of that you want to shave off, I think you could get pretty darn close with no problems since you will then be putting the gasket back in between.
If it's an original head you'll have plenty of room above the pistons, it's the thickness of the cast iron over the deck plane that limits how much you can mill off a head. Look at the holes to the water channel between the mid cylinders, there you can see how much meat you've got to shave from..
I think there are canadian heads with a thicker casting, I would try to find such a head if I wanted to shave off more than about 0.125".
Raising compression by milling a std head isn't very effective, a high head increases from 3.8:1 to 4.2:1 in compression by milling 0.125" according to the Tulsa club's measurements:
Last week I milled a low head for a '15 that was badly pitted around the water jackets, particularly on the rear, and losing compression out the back. It cleaned up at about 0.035 in. An 1/8th inch is a lot to take off. When you set one up on a rotary broach, you can only take off about 0.010 per pass, anyway.
Measure how high the valves and pistons protrude from the block and take a depth measurement on your head. If it hasn't been milled at all, or very little, there is quite a bit of clearance.
I have photos of how bad it was at 0.010, 0.020, and cleaned up at 0.035, but I'm no good at resizing for the purpose of posting here.
Thanks guys, You have giving me the info I was looking for. Scott
If you want, send me your email through a p.m., or whatever, and I'll send those photos to you directly so you have some sort of visual reference.