Small Block Ford Need basic engine help

[Bubba1235]

Situation:

Owner brought his boat (1996 OMC Cobra SX) over for me to help with trouble shooting.

Engine Ford Windsor 351 rated at 265 HP

Flame Thrower Distributor and Coil

New carb, intake and exhaust manifolds.


Sypmtom: Missing at any RPM above idle but at idle it runs smooth as silk. No power, will not get above 3000 RPM. Engine has roughly 5 hours on it since a complete rebuild. Owner says has never ran right since rebuild. Rebuilder is out of business and gone.


Did all the usual troubleshooting and found no real issue. Decided to do a compression test, was shocked at results as ALL 8 cylinders are running 50 to 55 PSI compression. Did a wet test (oil in cyl.) and no real difference. (Maybe 5 psi improvement.) Did leak down, could not hear anything.

Thought possibly the were over tightened so backed them off until loose and re-did compression test, no difference. All 8 cyl still at 50 - 55 PSI.


I am at a total loss as to what the heck it could be and am thinking I may have to pull the engine and do a tear down to see what is happening but I REALLY do not want to.

Anyone have any thoughts on why all 8 lungs on a newly build 351 would be running such low compression.

 

[a70eliminator]

Re: Small Block Ford Need basic engine help

wrong pistons, 351 has a 4.00 bore and 3.50 stroke right?
302 has 4.00 bore 3.00 stroke right?

I'm not 100% sure but i think the 302 351 pistons are the same bore only the wrist pin location is higher with the bigger stroke, get them mixed up you either have piston crashing into head or pistons way too low in bore and nill compression, good luck with that one.

 

[Bifflefan]

Re: Small Block Ford Need basic engine help

with compression that low on a new engine, id say you need to pull it and open her up.
When it runs is there any knocking? like the pistons are standard and the bore is 10 over?
You really should hear air coming from some where when you do a leak down.
have you tried doing a comp test with the other plugs in? just to see it the head gaskets are in there?
Can you find the guy and drag him to small claims court?
My best advice after building many fords ( im a ford guy) is to tear it down and look at everything and start again with a reputable rebuilder.

 

[Bifflefan]

Re: Small Block Ford Need basic engine help

I
My gut is telling me there is something seriously wrong in the piston/rings/cyl. but I would have thought that if that were the case doing a wet test would have shown different results.

I dont think a wet test in this case would make any difference. if it the wrong pistons (to small or to short) there is just too much gap to fill with the oil.
On the up side you should be able to reuse most of the gaskets ( not the head gaskets) as its just been sitting. So the cost ma y not be as bad as you think. once you get it apart let me know in a PM if you need some parts. we got tons of stock parts laying around that are used but still good.

 

[dubs283]

Re: Small Block Ford Need basic engine help

if there were a problem with the piston rings/cylinder bore you'd think for sure you would hear something on the leakdown test through the dipstick tube

i am leaning towards a piston/ring/cyl issue myself. maybe just enough grip there to not show up on leakdown test?

could be the rebuilder saw the end was near and did something shady to turn a quick buck?? shameful, but it happens

 

[a70eliminator]

Re: Small Block Ford Need basic engine help

A 351 piston piston installed in a 302 block will potrude above the head exactly 0.136" a 302 piston has a 0.136 difference in compression height, you certainly can measure that with the proper tools through the sparkplug hole.

 

[Bifflefan]

Re: Small Block Ford Need basic engine help

I really do think its a case of standard pistons in a 10 over bore.
This could have been a legitamit mistake, but i doubt it.
The only real way to tell is pull a head and stick a feler gauge between the piston and wall. this will tell alot.

 

[Bifflefan]

Re: Small Block Ford Need basic engine help

bifflefan

Quick question. Once I get the heads off, what should the measurement from the top of the piston to the top of the block deck be?

it should be flat with the top of the block. there may be a small dish in the piston but the outside is flat with the top.

As for the leak down, it may be that you will only hear very minimul ( that dosent look right) air passing as the rings are seated and the valves are good. A leak will tell a broken ring, but i can recall ever hearing of this problem on a new motor.

you can get a rough measurement of the bore with a digital caliper. 4" is standard. Then eather check the gap to the pistons with a feeler gauge or run one to the top and try to get a measurement on it then.

 

[parrisw]

Re: Small Block Ford Need basic engine help

What about cam timing? Maybe the timing chain is installed incorrectly, check that when you take it apart before taking the chain off!! Then you know for sure.

 

[mkast]

Re: Small Block Ford Need basic engine help

A cylinder leak down will show where the leak is.
Rebuilt engine, are the rings seated yet?

 

[KRH1326]

Re: Small Block Ford Need basic engine help

Hey Bubba,

I just skimmed through the post. I am not sure that I din't miss anything.
If the piston is way to low from the deck at TDC, I would try to get numbers off the crank and off the connecting rods. You might have to resort to measuring them.

I have had and rebuild alot of 351W's from my good ole' days of banging around the woods in my good ole' Ford pick ups. Off the top of my head, and if my memory serves right, alot of 302 and 351w parts are interchangable EXCEPT: Crank (the stroke is different), pistons (most of my 351 pistons were always deeper dished than your photo, and they always hade cuts to relief for both intake and exhaust valves.) and the actual rod length if I remember right the 351 rod should be shorter to compensate for longer throw of the crank. ( The intake manifold wouldn't fit in this dilema).

I could actually see the builder, accidentally or no, using a 302 crank ( will result in too short of a piston throw ( both up and down). A short throw (with nice new rings and all) could be giving you a steady and consistant low compression test.

Correction:

Sorry Bubba, I should have read more carefully. I think I need glasses too. If it were only .2" and not the 2" that I thought I read, I would be looking at those pistons, too. Did the guy even need a ring compressor to get that in?

What a deeeek, that builder was.

 

[Wakesilver]

Re: Small Block Ford Need basic engine help

Is it going to live, doctor?

 

[Bifflefan]

Re: Small Block Ford Need basic engine help

From the looks of the pic, there is a LOT of gap around the top and sides of the piston. Have you had a chance to measure the bore and piston sizes yet?
As for haveing a lip, its probably not the same block that you gave him. He tool one he had and pressure washed it, painted it and gave (sold) it back to him.
If the one you have is still good, looks like it should be, then id find a local hot rodder and ask him who does a good job. then go and talk to the guy to see if will work with you on getting it bored and new pistons. It will probably run in the 400.00 or less range for the work. then put it back together your self and save a few bucks.
If this is the first engine take pics of it as you go so you dont forget where stuff going and mark the wires with tape. also remember the knoch in the top of the piston geos to the front of the engine.
Ill answer any questions i can for ya.

 

[a70eliminator]

Re: Small Block Ford Need basic engine help

Hey Bubba,

I just skimmed through the post. I am not sure that I din't miss anything.
If the piston is way to low from the deck at TDC, I would try to get numbers off the crank and off the connecting rods. You might have to resort to measuring them.

I have had and rebuild alot of 351W's from my good ole' days of banging around the woods in my good ole' Ford pick ups. Off the top of my head, and if my memory serves right, alot of 302 and 351w parts are interchangable EXCEPT: Crank (the stroke is different), pistons (most of my 351 pistons were always deeper dished than your photo, and they always hade cuts to relief for both intake and exhaust valves.) and the actual rod length if I remember right the 351 rod should be shorter to compensate for longer throw of the crank. ( The intake manifold wouldn't fit in this dilema).




I could actually see the builder, accidentally or no, using a 302 crank ( will result in too short of a piston throw ( both up and down). A short throw (with nice new rings and all) could be giving you a steady and consistant low compression test.

Correction:

Sorry Bubba, I should have read more carefully. I think I need glasses too. If it were only .2" and not the 2" that I thought I read, I would be looking at those pistons, too. Did the guy even need a ring compressor to get that in?

What a deeeek, that builder was.

It can't work, a 351W crank has 3" mains the 302 mains are much smaller, the two cranks will NOT even fit the block if swapped.

But.... there is an easy way out, using whats there.

 

[fast50]

Re: Small Block Ford Need basic engine help

302 mains is 2.248 and i the 351 windsors were about 3 inch on the mains .a 302 rod is 5.2 inch a 351 is 5.7 inch bigger because of different deck height