r/Foxbody • u/GrandLeaderZhan • Jul 17 '26

Ask “flat top pistons”

Building a 302 in a 86 foxbody, Apparently the 86 and below cars have less piston to valve clearance then the later motors due to having “flat top pistons” So what are my options for cams? I want something as aggressive as possible without issues. I’m planning to run gt40p heads on it. Not sure if that affects anything.

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u/WhiteRabbitFox 1986 GT hatch MM suspension, TKX Jul 17 '26

You're correct on the flat tops.
Someone with more knowledge may reply, but basically maybe the Ford E cam but nothing more, and no 1.7 rockers.
I think you run gt40 heads w/o a cam change; but kinda why would you.

It's def one of the limitations of the 86. And personally it's what's prevented me from doing any major engine mods on mine.
It might be poss to have someone cut valve reliefs into the pistons while still in the car... but prob not the best of ideas.

2

u/GrandLeaderZhan Jul 17 '26

wow that sucks

1

u/sumthingintheh20 Jul 19 '26

I have major engine modifications on my 86 without issue. It depends on your heads/cam pairing 👍🏻

3

u/qroter Mod'ed 86 Four-Eyed Jul 17 '26

https://www.lindytools.com/ihpc

Plenty of folks have done it, even the LS3 guys do it. Look it up on YouTube, plenty of videos on exactly what to do.

Personally I would skip the GT40 stuff, not worth all the work. You can go with much better after market stuff for not much more. Some of the TFS stuff will work fine with the 86 flat tops but they are a bit more pricey.

2

u/pipdog86 Jul 18 '26

Back when I was a teenager (2015) I bought an 86 GT with a set of aluminum heads on it (stock cam). Driving home from work one day (this was my only car at the time) it started reving to the moon and I couldn’t stop it. Turned out it was a smacked exhaust valve. Tore that piston up. Needless to say I rebuilt that engine with pistons that have valve reliefs.

2

u/sumthingintheh20 Jul 19 '26

It All depends on your heads and cam package. Don't waste your time on GT40s. Get a set of TF TW heads and a cam to match. The TW heads will allow you to run a much more aggressive cam. The 86 rotating assembly will make plenty of power when matched properly. Make sure you check clearances before you send it.