When my '78 is apart for resto I'm planning on pulling the 351M crank out and goin' to a 400. Is remachining the block completely necessary or would new bearings be enough? If theres enough compression in all cylinders and oil pressure is good now, couldn't I get away with just the new crank/pistons/bearings? What are good compression figures for the 351M/400? Oil pressure? Is cylinder honing standard at this point? Theres currently 124K on the block. This isn't a high dollar build(I hope), and not going to be a show queen; I just want a truck that I can be proud of and still have some torque figures to go with her. Any help/opinions are greatly appreciated...

 

well, if your are not changing your camshaft it only depends on your cylinder walls, how are they? see if you can clean them with a 30 or 40 overbore and buy pistons acording, but the rest can work stock... oh you will need piston rings also but its all cheap...

 

You won't know until you mic the cylinders. Your machinist can do this for you if you don't have the instruments or know how. If the cylinders are good enough, you can simply use cast iron rings for the rebuild and leave the cylinder walls alone. Cast iron rings are the only type that will seat on glazed cylinder walls. For a rebuild involving rebore and hone, use a higher quality ring.

As far as adding torque, make sure you use the correct rod/piston combination for the 400, don't simply change the crank and leave it at that. If you do the pistons will not be at the right height.

To build a torquey 400, either use Aussie heads or flat top pistons. Use a mild cam, four barrel intake and dual exhaust. You can make a torque monster out of a properly built 400. I did.

 

can the cylinder walls be honed without reboring and then use good rings? Or still use iron rings? Why would it be advisable to remachine the block if I were to re-cam it?

P.S: don't be afraid to use tech talk when it comes to machining, I was a machinist for four years, I'm just new to the motor rebuild game and need guidance on the practices of it.

 

You can indeed hone the block and use moly rings. With cylinders that are worn, but not so worn that they won't rering, I have found cast iron rings to be the best choice. You don't grind away any of the remaining cylinder wall with the hone and you're not going to get the engine life of a fresh engine anyway, so the shortened life of these rings is not an issue. Also by not honing, these rings break in with virtually no wear to the cylinder wall and rings, leaving more life in both.

BTW, my recommendation for cast iron rings on a glazed cylinder wall is against what many folks will tell you. It is, however, a scientifically proven overhaul method. It was determined by Ryder labs in the seventies to be the most effective overhaul method. They put lots of money in the research. I once had in my posession a cast iron ring that came out of a truck that had been run only 500 miles after cast iron ring on glazed wall installation. The engine was taken apart for other reasons. The ring was very obviously properly seated.

I've used cast iron rings on glazed walls numerous times with great success.

If you have mics and a snap gauge, a set of inside mic's or better yet a dial bore gauge, you can mic the cylinders. Find an overhaul manual to give you limits. You are looking for taper and out of round in the cylinders as well as basic ID.

Also since you can use mic's, mic the crankshaft and look up limits in an overhaul manual. It very well might be that the crankshaft does not need turning.

If the engine has had regular oil changes it very well could be that once you mic everything you might find that rings, bearings and a valve job will fix you up. Tearing apart an engine is exploratory surgery and like a box of chocolates, you never know what.... well you know.

I would highly recommend a book How to Rebuild Your Ford V8 351C, 351m and 400. My copy is probably 20 years old but I know it is still in print. It will give you wear limits.

If everything mics out okay, you could go back with rings, bearings and a valve job and there would be no reason not to cam it a little. These are torque motors though and I would not get carried away at all with a cam. A long duration cam will kill the bottom end torque. A cam of about 212@.050 intake duration would be the absolute upper limit and I wouldn't go that far.

Good luck,

 

Wow, thanks Doc, thats probably the best answer I'll get. However I won't need crackshaft measurements since it will be replaced with a returned 400 crank and new pistons. I sure hope I don't need major overhaul machining, I'm afraid I wouldn't be able to afford it. Thanks all.

 

Just make sure to get the High Cr pistons...

 

Yes, I wish the flat tops would have been available when I built mine. You can get them from TMI.

 

He MBDiagMan, whats in your built? On what vehicule is it? Have you dynoed it? I would like to ran a Symulation on yours..

 

It's in a 78 short bed F150 4X4 that my wife and I bought new.

It has standard pistons and standard heads. I pocket ported the heads and match ported them to an Edelbrock with a 600 vac secondary. It has a cam that is something like 204@.050 .488 intake and 212@.050 .510 exhaust. It has stock manifolds with dual 2 1/4 inch exhaust through turbo mufflers. It also has an MSD ignition.

It makes good torque and runs great. The only thing is that the big tires on the truck soak up every bit of that torque. Small tube headers would do wonders for it according to desktop dyno, but I only lost about 2% of power due to the stock CR. I did shave the heads as much as possible and used shim headgaskets to get as much compression as possible.

I have never dyno'd except on desktop dyno.

Were I building it fresh today, the only thing I would change would be the CR and use the small tube headers which it may get sometime in the foreseeable future.

BTW, if you are planning on replacing pistons, you should go ahead and bore it. Unless something is fouled, there is no reason to change pistons unless it needs boring.

 

Yeah, you can actually get a lot more torque from some headers than from a higher Cr....besides if you milled already your heads all you could, i think the safe limit is 0.030", you should be in the range of 8.7:1-9:1, very good for you cam choice, concerning the tires maybe a lower gear could help you...i was just curious...

 

Yeah, I would have pursued lower gears some time ago except I don't use this truck right now. I replaced it about 4 years ago with a 2000 F150 4X4. Last year I started stripping it. The truck is very straight and rust free except for lots of surface rust. The cosmetic restoration is a long term projectl

When the truck goes back on the road, I won't have the need for the big tires. It will be a parade truck, so I'll put stock wheels and tires back on it and I expect it to feel like I doubled the torque.

 

My pistons will need to be changed anyways converting 351M to 400...

 

Check around and see what a bore job would cost. I got one for $125. Not bad considering a crank and pistons will cost more.

 

If you have a choice of machine shops, try to find someone with a Rottler boring bar. Many second tier shops have an old fashioned bar that sets on the deck of the block. This does NOT give an accurately bored hole. The bar has a mind of it's own and does not make cylinders that are perpindicular to the crankshaft centerline.

With a Rottler, it is a large machine in which the block is mounted such that the hole is indexed perpindicular to the centerline of the crank.

Some shops will tell you that if they deck the block to make it parallel to the crank centerline THEN use their bar on that deck it will make a perpindicular hole but DON'T believe it.

Some shops will tell you that their Sunnen hone will make the holes perpindicular while honing. Don't believe that either.

If you can't find a good shop with a Rottler you might have to compromise, but do your best to find a good shop with top equipment.