It's from using mis-matched parts. You have to have a plan, and use whats within your budget. Way too many people just toss stuff together... Spend a little time, do some research, look into what you have. What year are the 351 heads you have? What year bottom end are you using? Is it stock? What cam? What intake? What carb? What transmission? It's a total waste of money to just assemble an engine with parts you have laying around. Big heads are no good without a cam to match, then they have an affect on intake and exhaust...one thing leads to another. A good start, and they can be found cheap, is an 95-96 Explorer motor.
If you're going to use the 351W heads, you also need to replace the rocker studs with screw in types. Me personally, I'd opt for a fresh roller 5.0 short block, your choice of hydro roller to fit, top it with the W heads with the exhaust lump removed (I'd also raise the roof of the ports and widen the sides to match the header gasket while you're at it) Then top it with a high rise dual plane intake and a 600-650 cfm vacuum sec carb. I would also first see exactly what heads were on your 302 before passing judgement on it. There are many casting numbers for SBF heads, lots are simple duplicates of older numbers, so it's not impossible to have different heads that are basically the same.
The 351 heads are '69. C90E-G. I don't know what year bottom end this is, I do know that besides the pistons it is stock. I was thinking an E303 grind cam I have lying around is what I'd put in. I'm thinking of taking the port-injected intake from an Explorer motor and controlling it with a MegaSquirt (I already have the MegaSquirt) and I'm using a C4 transmission.
It means your 351W heads have pressed in rocker studs, unless someone's already converted them to screw in types. Any cam bigger in profile (and springs) than about a Comp Cams 268H will tend to work the studs right out of the heads. That includes the E303 and any other roller cam.
The Thermactor bump is easily removed on your own., takes all of 30 minutes with a diegrinder and a carbide bit (or a drill) to remove all eight. AS for the stud conversion, any competent machine shop can do that operation. You'll need to but the studs and guide plates, you'll also need hardened pushrods to replace the stock rods too.
If you've got a good carbide porting bit, it won't. But it may lead you into going further with the bit and opening up the port, then the bowls under the valves.........................................
NEW PLAN: We were not able to get a set of GT40's, but I found a '92 5.0 HO Thunderbird at Pull-A-Part and took the whole top end except cam from it. They're E7TE heads, which are just about the same combustion chamber size as the 351W heads however they have smaller valves. The 351W heads have a couple burnt valves and one bent valve, so it seems like they're going to be a lot more work and expense than I had originally hoped. Do these E7 heads flow better than GT40's? I also grabbed all the pushrods and roller lifters, are the pushrods hardened?
E7TE's flow less than GT40's Still a good head, and easy to clean up. Don't pay more than150 for the pair.
I only payed like $100 for the pair, so okie doke. Also: Does anyone know how well C9TE-C heads flow? They were the heads that were on our '65 Mustang and it seems like the 351W heads are going to be way too expensive to fix, though they would be the best considering I absolutely cannot find any GT40 heads for under $300.
do the 351 heads have hardened seats? It will need them if you plan on driving the car a lot otherwise the valves will sink into the head and run funny. personally I would go with the late model heads if your not looking to make a lot of power and keep it on the cheap, hardened seats can get pricey also the guides may need to be replaced before the seats are done. honestly I'm a big advocate of aftermarket heads, I work at a machine shop and see people end up spending $750-900 just to rebuild a set of stock heads that need seats and guides surface possibly may need some or all valves then if you need screw in studs, guideplates, positive stem seals it can get even higher. Most aftermaket heads already come with all of this, but unless your building a high HP 347 ci or bigger engine you dont need tons of cfm or big valves it will just hinder performance. If you want to take your heads to a machine shop make sure you just have them take them apart and inspect them for what the need before you agree to any other work and ask how much everything will cost. dont just go in there and say you want a valve job and surface because they will tell you it will be $350 then they will call you and say you need seats, guides, valves ect and it will be a lot more.
The 351 heads do have hardened seats (my uncle built up a 600 RWHP monster 347 for his Fox body Mustang without hardened seats and didn't run leaded fuel and ended up killing the heads after a few thousand miles) but yeah they do need 4 valves and it sounds like the work on them is going to be way more expensive than I thought. So I'll either run the E7's or the '65's old C9TE heads.
Run the E-7's, make sure to check PTV clearance when installing cam. You can shave off the thermo bumps on them as well if you have the time and tools. 300.00 for gt-40 heads is pretty common around these parts to, unless you get lucky and find a set at the pull-a-part.
