I am in the process of tearing down my 351W from my 84' F250. I discovered, among other things, that the bearings are 0.010" undersized and the pistons....yikes....are 0.40" oversized which is what bothers me.

I'm assuming this engine has been rebuilt once already (at least). I can't find any piston sets that are more than 0.040" over standard. Does this imply that 1) larger pistons are not available/very expensive or 2) I'm at the service limit of the block? At some point, the strength of the block comes into question.

The good news is that the pistons look to be in good shape with little wear on the cylinder walls. There are a few shiny spots and I can't see any crosshatch marks but there is no ridge at the top of piston travel. Compression was ~180psi before I took the engine apart, and it didn't burn any oil past the rings. Should I just leave the pistons/rings alone and just do the bearings and heads (valve seals are shot). I would like to do something to the bottom end though....since I'm in there, just if it is to clean things up.

I need to figure this out before I start spending money on it, there is so much available in the way of upgrades compared to my other vehicles that it makes my head spin. I do want to get more power out of but I want to do some reading first.


Thanks,

Bryan

 

Most people don't like to go past 4.030, due to potential overheating problems. Your best bet would be to actually check the bores/pistons or have them checked for wear, you may be able to re-ring it and go. All isn't lost as long as the block checks out, maybe the block was redone more recent than you might think and in excellent shape. If it requires to be bored further most would suggest finding another block. There are .060 pistons but you will more likely than not have an overheating issue.

Whats wrong with the motor that you felt it needed to be rebuilt? 180 psi is good, I would think you could leave the shortblock alone and be good. What kinda oil pressure did it have?

Your not really gonna gain any hp by adding parts to the bottom end, well except a camshaft designed for your needs.

 

There were numerous problems, the engine needs a new rear main seal, it was overheating and losing oil pressure. It would drop down to 20-30psi on the highway when it was hot.

I pulled the oil pan and found the crank bearings were in need of replacement. Also, #7 had a bad valve and the valve seals were gone. That cylinder (my earlier post was wrong) was actually 130psi, the other 7 were 180psi. So at the least I am going to have the heads done and new bearings and seals. 15 years of baked on oil didn't help either, I like to work on clean engines. I had the engine out anyway for the rear main so said "what the heck". Besides, it seemed like a good time to bolt on some extra HP :-)

It sounds as if the big question what will happen of the block. I guess I'm not going to worry myself with any performance upgrades right now. A stroker kit is probably out of the question now, at least on this block.

Does anyone know why overheating becomes and issue when the block is bored out? It doesn't seem like you are removing enough material to matter.

Bryan

 

Well as a shot in the dark if you were experienceing overheating issue part of may be to blame on the .040 overbore.

What happens is the walls when you bore them become thinner, which means more heat is transfered and more is required of the coolant, too thin and its gonna overheat. Due to casting shift the cyilider wall thickness is gonna vary, .030 is safe so you don't end up with a hole in the cylinder wall when its overbored. Clearances of an engine are tight, the slightest variation and your either gonna improve performance or hurt the engine.

Low oil psi when warm does point to worn main bearings, seems you knew that.

It seems the low psi was due to the valves so that is good, if you can determine that the overheating was due to another reason then, I would say use it, but if your not sure if the overbore was the bulk of the problem, you might consider getting another block, it would suck to throw money at something thats gonna be trashed, the good part is sounds like you can reuse it without having to machine the block, so if you get everything together and it overheats you can swap your heads and crank over to another block, just cost you time. Who knows? Good luck