Cam specs - PTV clearance
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Laughing Gas
Joined: Dec 2003
Posts: 900
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Cam specs - PTV clearance
Hey everyone, doing some brainstorming here for a future project. I have a spare 351W from the mid 70's as well as an early 90's 351W truck motor. Thinking of building a fun little street toy with my 78 F100 shortbed using AFR 185 heads with 2.02 valves.. Considering either of the stock factory shortblocks mentioned above, when do ya'll generally start running into PTV clearance problems in terms of cam lift and evc and ivo points? Regardless, I'm going to check it when that time comes and I know a lot depends on head gasket thickness, deck height, cam advance / retard etc. Just looking for a "guideline" in terms of cam specs when it starts becoming an issue with the factory shortblocks. I've been considering the 35224HR Bootlegger cam. (276/300 224/236 @ .050 .590/.590 108 lsa) I don't see a spec card on their website. Thanks.
Cargo Master
Joined: Oct 2020
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The cam lift is never a factor when it comes to piston to valve clearance. The duration and lobe centerlines are what do make a difference. Probably with a very short duration similar to a stock cam the 2.02 valve might not be an issue even with no valve relief in the piston its tough to say.
The problem with using the factory style pistons with a valve larger than 1.94 isn't just due to the valve size but also due to its position. In order to get a valve set larger than 1.94/1.60 the centerlines must be spread. In some heads they may also move both of the guides toward the exhaust valve side of the chamber just like what's done on a Chevy with 60-40 valve locations. This makes the valve relief in the piston not only on the small size but also in the wrong location. I've had to fly cut pistons before for a guy building a budget 393 who happened to come across a good deal on some heads. It worked fine.
The problem with using the factory style pistons with a valve larger than 1.94 isn't just due to the valve size but also due to its position. In order to get a valve set larger than 1.94/1.60 the centerlines must be spread. In some heads they may also move both of the guides toward the exhaust valve side of the chamber just like what's done on a Chevy with 60-40 valve locations. This makes the valve relief in the piston not only on the small size but also in the wrong location. I've had to fly cut pistons before for a guy building a budget 393 who happened to come across a good deal on some heads. It worked fine.
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Laughing Gas
Joined: Dec 2003
Posts: 900
Likes: 3
I may rebuild one of these and use different pistons, trying to avoid it since all the machine shops around here closed their doors years ago. I knew the Trick Flow heads required different pistons due to how the valves are positioned. Was hoping 2.02 AFRs would clear. I'm thinking the 90's truck had a dished piston??? Thanks for all the insight everyone, I appreciate it.
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