This is sort of a complex situation, but here goes:
I'm checking out a '79 F250 4x4. Someone spent a fortune on the top end and externals, but ignored the bottom, so the the rear main seal went out in a bad way. (PCV maybe) A new seal was installed, and the truck then was only driven short distances a few times. I live about 20 mi from where it was sitting, and it blew the seal again and started knocking about half the way home. It maybe ran 10 minutes total with the knock and this is the first time it's knocked.
1st question: I'm assuming this is main bearings considering the seal problem and the knock and the fact that it holds pressure till hot.
Would this asssumption be correct?
2nd question (I've read some posts, but not the exact scenario) Can I replace the mains and rear seal in this motor without loosening the AT transmission?


I won't ask longevity questions or crank questions, I pretty much know what to expect according to what I find when I see it. This is a darn decent old Ford, and I hate to put it out to pasture. Sorry so long. Any help appreciated.

 

My suggestion is to unbolt the motor mounts and tranny. Pull the motor out, change bearings/seals/etc. and drop it back in. Put a jack under the tranny and some wire to keep the torque convertor in place. If you try to do it with the motor in the truck it's going to be a major pain in the @$*. I know from experience that the oil pan will not clear to come out through the bottom with stock front suspension and motor location. Also, if you pull the motor out you could probably wash it down, maybe put some paint on it.

Best of luck though.

 

I replaced the main bearings and rear main seal in my 351m/C6 combo. I didn't have to do anything with the A/T. Just loosen up the main caps and roll in new bearings and rear main. I put on a HVHP pump while the pan was off. My bronco is lifted 4" so the pan cleared everything without any problems. The whole job took me about 2 days because I replaced the timing chain and water pump at the same time, plus I work slow.
It sounds to me like you have more problems than just loose mains. Main bearings don't knock, rods do. If it had very low oil pressure then it might cause a rod bearing to knock but I would guess the damage may be done. Does it still knock when it is started cold now? Or only when it gets hot and the oil thins out. The rod bearings may have to be replaced or one rod could have stretched bolts or cap. If you wanted to go low budget you could probably get the motor usable again for another 30k but it might be better to pull and fix it right.

 

Thanks guys.

It sounds like the smart money is on pulling the motor and doing it right. Since the top is new, I could have a keeper in this one.
OffOrOFW, It only knocks noticeably when it gets hot and then only if you rev it.
samickguy15, Thanks for the oil pan advice - It has no lift.
I can get the truck for $600 bucks and other than minor body damage in 2 places, it's jam up. AC needs a charge and the body is not rusted through anywhere.
Here's the next question:
The steering gets a little spooky in a curve. Not bad, just a little. What's it take on the F250 to tighten this up? How much $$ to get it perfect?

 

I would suggest an anti-sway bar. That will keep the body roll down to a minimum. I would suggest checking out some of the pieces that Jegs and Summit Racing carry. Summit carrys one for Ford trucks and Broncos for $150. It's made by Addco. That should help out a lot. Best of luck.

 

who put the rear seal in.did they leave a main loose.at any rate your in for a crank.

 

312,

from what I understand, the shop simply put new seals in. Front and rear seals. They only had it for one afternoon. It was POURING oil in a bad way. Now it drips bad when hot. I am pulling the motor tomorrow afternoon and hope to get a look at the crank this week. I'll take it to a shop and have it miced. I hope I don't need a new crank, but what will be, will be. I cranked it again today and let it warm up. It runs perfect till it gets hot, then you begin to hear the mild knock and the engine starts to drag, as if it's being pulled down by a load. It's been run very, very little since the knock started. This will be my first time on a Ford motor pull, and first time in the bottom of an engine. Any advice is appreciated.

 

If you are going to pull it apart, I would put ARP main bolts in and ARP rod bolts in it after having the rods resized. Turn the crank, New rings, rehone, And oil pump.

 

OK guys,
I have the 351M upside down on a stand with the crank out. Here's what I found:
1) the time chain was loose enough to hit the block and the cam gear had 2 broken teeth.
2) the crank/mains and rod bearings are very worn. The back main is grooved a good bit.
3) the wrist pins all feel good, side to side, but no up and down play that I can feel by hand. The cam looks fine, visually.
4) other than this, I see no failure that, in my limited experience, could cause the very obvious knock it had after it got hot. (no knock when cold - check first post)
I have a crank kit (no cheapo), new time set, HV pump, water pump to pick up in the morning.
Any suggestions on where the knock came from?? Time chain slap and worn bearing combo maybe? It would knock after warm and start to bog the engine. Very noticable.

 

I think you answered your own question, (crank/mains & rod bearings worn).

 

You did not say what the block looks like. Is there a cylinder that is worn on one side, like the wrist pin has come out and hit the cylinder when hot?

 

abroom Did you plastigauge the clearances? Its a good cheap way to measure bearing to crank clearances. Plastigauge is a plastic wax like strip that you put between the bearing and crank. You tighten it down and squish the strip. Measure the width of the strip and it tells you how loose you clearances are. There are different sizes of strips. Get one of each. Napa has plastigauge. I can explain it more later if you haven't used it before. I bet you will find a rod that is looser than the rest. The fact that it only knocks when the oil gets warm and the OP goes down means it could be a combination of a loose bearing and low op caused by loose bearings. A Rod cap/bolts could be stretched from over reving. This could be fixed cheap if you want to just get it running.

How do you know the top end is in good shape? You know the rings/cylinder haven't been touched if the crank bearings are shot. The worn timing gears tell me the cam is stock. You say they spent money on it, but it doesn't sound like they went any deeper than a manifold. Do you know if the heads have been off? How many miles are on the truck? For $600 I'd say you got a smoken deal even with a bad motor. You could part the truck for $2500 hear in CO if it has a dana 60 up front. I bet they knew about the problem and thats why you got it cheap.

You could probably get this thing patched together for a few years of running if you want. If you want to do a re-ring and hone then I would completely rebuild it and do it right. It just depends what you want to do. You could get it running for cheap and rebuild it right in a few years. Or you could go for the full rebuild now. I wouldn't do anything inbetween. I've done both. Don't waste money turning a 351 crank! Yours if prolly ok anyway

A loose piston will typically knock while it is cold, before it expands in the cylinder. A piston knock will get quiter as it warms up so I think the cylinder/pistons are probably OK. Don't worry about the oil leaks you had. Main seals won't last if the main bearings are way shot. It is common for someone who doesn't know about Ms to replace seals only to find it leaks after a short while after. Seals just can't hold a crank in place.

I wish I could have got to you earlier before you pulled the motor so you could check compression and OP so we could have a better idea of what kind of shape the rest of the engine is in. Don't do anything drastic because of what anyone here said. Hope this all made sense, it's late. Good luck

 

Thanks guys.

I have put all the advice to work.
OffOrWFO, Somebody put brand new heads on it, new radiator, starter, belts, hoses. Don't know why they did not go further. The mains were shot, and time set was shot (bad). I've dropped in a crank, bearings, new time set.
It should be up and running by Thanksgiving weekend. It has been good stress relief, and is going back together smooth.