I have a set of 7a heads from a 350 Olds that I would like to have machined to fit my 403. What I need are the bolt hole size and locator pin size that need to be drilled. My engine is back together and I would like to drive it while I gather the needed parts for the upgrade. I want to take these in to the shop but i don't have a 403 head to send with them for the measurements. Any help would be appreciated. Thanks.

 

They have to be drilled for ½" bolts. As far as the size of the locator, that I don't know.

 

As said 1/2" bolts so slightly oversize so bolts don't bind. The dowels, one on each head, needs enlarged to 5/16", I believe. If they are going to the machine shop, 2.07"/1.625" valves with the bowls opened up and a clean up mill are a good idea. Also any worn guides replaced, new seals and upgraded springs.

 

Thanks for your replies. I never thought to measure this stuff while the 4a heads were off, but like I said, I would like to drive the car this summer while getting the parts ready for the modification.

 

17/32 is the drill bit but any competent machinist can figure it out. Things I have discovered on my stock 403 build. The pistons are .030 in the hole. Consider about a 70cc combustion chamber and a standard fel pro gasket at .040 crushed thickness and you're only looking at 8.5:1. I just got .011 MLS head gaskets from Smitty. That will give you 9.0:1.

 

MLS, multi layer steel? Who made the gaskets you got from Smitty?

 

No idea. There's no identifier on there or anything. There is a chance he went right to the gasket maker and had them custom made. I can't find that thin of gasket anywhere.

 

I believe Smitty did exactly that, mentioned it on here. I just bought the 330-455 .028" gaskets from Rocket Racing, very nice for the price. I wish they had composite 403 gaskets in the .028" thickness.

 

http://www.cometic.com/i-24769372-ol...tegory:1280258

 

I should have said for less than $100 each.

 

Well....lol.

 

Smitty's are $110 plus shipping. AND they're reusable.

 

110 each or is that a pair, wishful thinking.

 

That should be a pair, Contact Smitty, use Paypal.

 

What dish amount do those pistons have or are the flat top?

Thanks, Frank

 

Factory 403 pistons have 21CC dish.

 

Thanks for that info.
Frank

 

Yeah...they're not small... 7.9:1 Compression back in '79

 

Good to know since my '70 is supercharged, I would want it to be about 8:0 to 8:2, it is now 8:12.

 

For what it's worth, .011 gasket and .030 in the hole will set you up for a .041 squish distance, which very close to the optimal 0.040 squish.