Gentlemen,

As mentioned in another thread, I'll be opening up the crank case of my 1979 351M from the bottom soon to replace the piston rod bearingsand crankshaft main bearings. This is because of a terrible knock that developed and has been ID'd as a bearing problem. When I open it up, I expect to find at least one bearing fubar - most porbably a rod bearing. But I'll replace all the bearings while I'm in there for good measure.

Question: How critical is it that I remove the piston rod in question and have it re-worked? A friends suggested that the banging that was doing the knocking will have made the piston rod end non-round, and a new bearing will just be worn again in a month.

If I DO have to haveit reworked, can I at least get it out from the bottom to avoid having to take off the head? ('cause if I have to take off a head, I'm going to be tempted to say I might as well take them both off and do a valve job, and while the heads are off and the piston rods uncapped, I might as well pull out all the pistons and do a cylinder hone and a ring job, and then my one-day bearing job will turn into a month long rebuild......)

Thanks.

 

it gets worse. like your bud told you, you might get away with just changing the bearing for a little while, but what about the crank where the bad bearing was beating on it? you may find a situation that you need a crank kit and the rods resized. i would for sure have the crank checked out to see if it is good to go again. you might be able to have the big end of the rod checked if you haul the whole engine to the machine shop and pull the rod out as far as you can for the guy to take measurements. if the bearing is still intact and in place, you might be lucky and able to do bearings. for the amount of work involved to R@R the engine, tear it down, clean it and freshen it up! oh, hold on, you plan on doing this on your back? good luck! i would seriously consider finding a swap block to overhaul, unless you want to do it who knows how many times?, or for who knows how long?

 

If it's knocking, it's too late to put in a set of bearings.

 

that's the way i figure it, until the disassemble and inspect proves otherwise. my winter beater spun the first main 3 or 4 yrs ago, had a local shop(because it was about to snow and i live in an apartment) patch it together with bearings. the guy said he didn't give it much of a chance, the crank is buggered up. i told him to do the best he can and the thing still runs good! it likes straight 40wt and lets you know when it needs a change with a little knock knock knock. i know i could be walking anytime, but i also know that i might as well try to keep driving it if it lets go. it keeps trying to commit suicide and i keep patching it together at absolute bare minimum cost, if any.

 

I'd love to wish for such luck. I hate it, but down deep I know what has to be done. It's not possible! Weeks of body/chassis restoration, to have the engine self-destruct the first 100km it's back on the road? This stinks.

OK, ranting and raving tantrum finished. Back to the garage...

 

Don,
Do you need a repair or a restoration? There are a lot of choices for you to make. It is possible to swap the bearings and pop in a new oil pump and oil pump drive shaft and be on your way. How bad is the crank? If it looks OK, swap the bearings. If its damaged, you could polish it or you could replace it. Replacing it means pulling the tranny. Generally, the piston pins can survive a knocking rod. I've gotten away with it and I've been burned, but I've never seen a piston pin fail. Does the motor burn oil? If not, do your quick fix. The real question is: Do you need a repair or a restoration?
KingFisher

 

Hesitating on taking action on a terrible engine knock that I've been putting off doing something about for a couple of weeks. (Truck's been sittingin the garage since detected, not driving it.) And looking for experienced opinions on how to attack it....here's the topo:

My '79 F250 with a 351M sat in a field for two years. I put a couple quarts oil in it, started it, and drove it the 300 miles home to my shop. A few months of chassis work later, it was certified. So, I changed the oil and filter, poured in a can of STP oil treatment in with the oil, new plugs & wires, and put it to work. Over the course of the next three days it hauled garbage, dirt and rocks, and I started noticing this knock in the engine. Very faint at first, it increased slowly and gradually, until by the third day it sounded like someone pounding a hammer on the block.

I've beenaround the engine with a broom-handle stethoscope. It's strongest in the rear-passenger corner of the engine. Less loud whenlistened to from the rocker cover, but I can't really distinguishany significant difference between listening on the head or on the crank case from below.

One thing - the noise doesn't start immediately upon starting the engine cold. After about a minute it starts faintly, then builds over the next 5 minutes or so of warming up. When I first noticed it, you could only hear it when revved up, but now it's there even at idle, although definitely worse at rpm's.

As a desperate try before opening the engine, I tried changing the oil and filter again, this time with a Motomaster filter instead of an afermarket non-Ford brand. MAde absolutely no difference.

My theory is a thrown crank or rod bearing. But that noise would have appeared suddenly and certainly wouldn't require warm up to appear. So I wonder if all the work to open it up and replace those will be worth it. Freinds and neighbours have suggested everything else from a chipped piston, to a blocked oil pump intake, to a lifter.

Any ideas?

Thanks.

 

I have never owned a 351/400 that hasn't done that. Most likely a main bearing. the 351/400's are pathetic when it comes to holding the lower end together.

 

Does it make the noise when the engine is under load (like, during acceleration as well as during a cold start)? It sounds like it, given what you've mentioned.

Run the vehicle at idle, and make sure the noise is there. Meanwhile, use a pair of insulated pliers (or fuse pullers, which wirk just as well) to remove and then replace the spark plug wires at the rotor cap (if you can't reach that, try it at the plugs) one at a time. IF the noise goes away during a removed plug wire, then the problem is in that cylinder.

To make a long story short, if you can reproduce what I did by using the same procedure, then the problem is traceable to the piston journal bearing, which could be wearing out - the noise is the sound of your piston rod slapping against the crankshaft when the fuel mixture goes off in that cylinder.

I have a 460 in my '79 F-250 that traced to the journal bearing, and it only makes the noise while under load, and only after it warms up (when the bearing warms up and gets loose...) I'm just going to go ahead and change out the motor, since it would be cheaper for me (in Utah, where Ford parts are as plentiful as green jello recipes) to do that, than to go through the time and parts to jerk the engine out and (while it's out anyway) rebuild what I've got. (Trust me, simply dropping the pan and just changing bearings on a 460 is not a viable option in my opinion, given the monster undercarriage that gets in the way... you might luck out with a 351, though.)

HTH, and as a disclaimer I just work on trucks a lot, but am not a professional mechanic by any means.

/P

 

Do you have an oil pressure guage or an idiot light? I'd say your mains are shot and you have little or no oil pressure once the engine warms up.

 

I vote for rod bearing, I have seen many act just like you describe, they only knock right away when they have severe slop, as the oil warms up and thins, the knock begins to be noticeable. I rarely have luck with polishing the journal and slappinng some bearings in, your only sure fix is to pull it out and have it properly machined, or wing another motor in.

 

Possibly a nasty piston slap?

Maybe a/the cylinder bore had a good amount of rust from sitting in the field

and after running a few hours the rings/piston skirt got good and shaved down by a shot rusty bore....

just a guess as to why it was ok on initial start and run but went to hell afterwards

happened to me on some old motorcycles I restored that sat for years...initial running was good then they were blowing smoke and all hell out the tailpipe a day or 2 later

 

The engine is defeintely bad.

I suspect the previous owner put in heavy oil to deter the knocking. One case I knew of, a dealer put 80w90 gear oil in the motor! to make it run quiet. When its oil change was due, 10w30 oil was put in. Discovered a terrible knock.

Or it just quit on you one day. Either way, it went south. Maybe that's why it was on the pasture?

 

Sometimes you can see if it's a rod by pulling spark plug wires one at a time to see if it lessens the knocking. The cylinder with the bad bearing will quiet up somewhat when it's not under load. Not that it's really going to help anything down there, might just help to satisfy your curiosity.

 

I loooove the spark plug wire pulling trick. Will definitely try that! Thanks.

And here I have more opinions in favour of bearings. OK, I'll open her up. On this truck it looks like it can be done frombelow through the pan, so it's already up on blocks with the oil draining. Others have mentioned the low success rate for slapping in new bearings without machining, but I gotta try. At least the bearings are cheapso if it doesn't work, well, I've not wasted toomuch and then I'll go for the full rebuild.

Thanks all. Will let this forum konw how it goes for future information.