GT40 p heads 351w HO

So I came to the show this year with my 84 f-150 with the 351HO. I was parked up next the the cross on the church ( 2 tone tan/brown) with black steel ram 20’s. Anyways, my question is this. I know it’s well documented that the 351 can be equipped with the GT40 heads, BUT: I have found zero info on someone swapping them onto the HO with the Marine grind cam. It has a bigger lift and my only fear is piston to valve clearance. Now I might be worrying about nothing, but I already have the heads, and the ole truck sat for years I got it running and driving and after my drive from NW Arkansas to the show and back, it started puking out the rear main, the oil pan and trans pan gaskets. So I’m just going to take it apart, and I felt while I was in there, put the GT40 heads on and the long tube headers and get it running a little better. Any and all advice would be welcomed at this point.

I don't know anything about GT40 heads, but I assume you are worried about piston to valve clearance because the GT40 heads have a larger valve diameter?

If that's the case and you are committed to keeping the pistons you have, I would play-doh the heads you have now to get actual p/v clearances, then compare that to the valve size of the GT40 heads.

The heading says GT40P.  If that is the heads you have, I don't think the headers you have will work on them. Something about the spark plugs hitting the headers. You might check it out. Not sure about the valves, clay the head like he said to check.

HOG F-150 wrote

So I came to the show this year with my 84 f-150 with the 351HO. I was parked up next the the cross on the church ( 2 tone tan/brown) with black steel ram 20’s. Anyways, my question is this. I know it’s well documented that the 351 can be equipped with the GT40 heads, BUT: I have found zero info on someone swapping them onto the HO with the Marine grind cam. It has a bigger lift and my only fear is piston to valve clearance. Now I might be worrying about nothing, but I already have the heads, and the ole truck sat for years I got it running and driving and after my drive from NW Arkansas to the show and back, it started puking out the rear main, the oil pan and trans pan gaskets. So I’m just going to take it apart, and I felt while I was in there, put the GT40 heads on and the long tube headers and get it running a little better. Any and all advice would be welcomed at this point.

The GT40 heads should be fine with pistons with valve reliefs.  The only time you run into issues with piston to valve interference is with large valves such as the 2.02" valves.  The GT40 heads use 1.84" intake and 1.54" exhaust valves and the GT40P has a 1.85" intake and 1.46" exhaust valves.  You should be fine, the only issue you will run into I believe will be the non adjustable valve train and having to find a proper length pushrod to obtain proper valve train geometry.

Im running AFR Renegade 165 cylinder heads with 1.95" intake and 1.60" exhaust valves with a crane truckmaxx roller cam that has 0.520"/0.542" lift at the valves.  Pistons are flat top hypers with valve reliefs and I didnt even bother checking for clearance cause these 58cc heads are good with OE style valve relief pistons up to 0.550" lift up to 240* duration.  It doesnt hurt if you want to check it but I believe you will be fine with your piston to valve clearance if you have dished pistons or pistons with valve reliefs cut in.  If you have true flat tops then you will most likely have a problem.

On the headers be careful, the GT40 and GT40P have differences, the GT40P relocated the spark plugs to be straight in like a sbc vs angled like the GT40 heads.  This here makes the requirement of special GT40P headers to clear the relocated spark plugs as regular sbf headers will not clear the relocated spark plugs.

Yeah, peanut plugs are a requirement in most cases because plug sockets just don't have room where the head pipe hooks around.

Also note that stock P heads are limited on exhaust lift because of the valve rotators.
So.... I'm not sure which "Marine cam" you have but if lift is at or above .500" it's likely you're going to need new valves, collets and springs to keep from crushing the valve seals and smacking the guides

I run the Edelbrock 5023 heads on a stock 351W HO bottom end, 1.9" intake 1.6" exhaust valves with a .461" .474" lift cam and there is no piston to valve clearance issues with those pistons and the larger valves. Should be fine. Need to worry more about the OEM heads being able to handle a higher lift camshaft without modifications to them.

let's not forget to check the head bolts. "gt40 p" heads are made for the 5.0 and are drilled for the7/16" head bolts. gt 40 heads were installed on the 5.8 and are 1/2" head bolts. i have two 5.0s in my garage right now with the gt40p heads and they are working great. one is my bronco 2 with a .060 over 5.0 and gt40p heads and exhaust manifolds. primarily due to the narrower frame and that the passenger exhaust manifold maneuvers around that very well. the other is an f150 with 5.0 and gt40p heads but we used a set of headers that happen to work quite well. just got lucky, I guess. no special plugs needed. but let's not forget that even in factory form the gt40p heads do use special manifolds and they also use steel spark plug boot heat shields. ford is going to go out of their way to save money, not waste it. it was/is necessary. using the soft heat shields is highly reccomended

Thanks for the reply guys. Well I did a test fit of the long tubes, they actually seem to fit well, and with some 90 degree boots and wire protectors I should be fine. Now the new issue. I scored a comp cams 262h brand new with lifters off marketplace for $80 bucks. At that price I couldn’t resist. Now the only issue will be what valve springs to use for that came on the GT40P heads.

HOG F-150 wrote

Thanks for the reply guys. Well I did a test fit of the long tubes, they actually seem to fit well, and with some 90 degree boots and wire protectors I should be fine. Now the new issue. I scored a comp cams 262h brand new with lifters off marketplace for $80 bucks. At that price I couldn’t resist. Now the only issue will be what valve springs to use for that came on the GT40P heads.

When it comes to a cam, it is best to run the springs that the manufacturer recommends.  They have an installed height and a pressure spec, you don't have to use their springs if you maintain the same specs.  I have AFR springs on my AFR heads and they are the recommended upgrade spring package they offer that is a close match to what Crane recommended.  The AFR springs are only like 5 to 10 psi higher pressure than what Crane recommended.  Before I made my purchase, I called up Crane and asked them and they said it would be perfectly fine to run those springs with the slight increase in pressure.

Need springs and keepers for the GT40-P heads in a Mountaineer/Exploder not an '80's 351.

Definitely get rid of the 'P' springs and exhaust rotators.
But you're going to need collets with a lower retainer notch to use the existing valves.
Then you need a check spring and gauge to determine how much to shim the springs for proper installed height.