switching to 350 heads on a 403
February 25th, 2017, 10:37 PM
#1
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switching to 350 heads on a 403
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.
February 26th, 2017, 10:03 AM
#3
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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.
February 26th, 2017, 08:46 PM
#4
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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.
March 1st, 2017, 06:12 AM
#5
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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.
March 3rd, 2017, 08:31 AM
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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.
Last edited by olds 307 and 403; March 3rd, 2017 at 08:34 AM.
March 3rd, 2017, 10:01 AM
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March 3rd, 2017, 08:09 PM
#10
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August 9th, 2017, 12:02 PM
#15
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403 rebuild
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.
Thanks, Frank
August 20th, 2017, 12:06 PM
#20
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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.
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