OK I'm finally getting close to starting some work on my 351M in a 1980 F250 4X4. I'm fairly new at this and am looking for some feedback about my constantly changing plan. It started with the need to replace the clutch, I figured to remove the engine and fix leaky rear main, pan, front cover gaskets at the same time. While I'm in there I might as well replace oil pump and timing set. There is some evidence of leaky valve seals, might as well replace them too. While I'm that deep into it might as well replace/upgrade cam,lifters, and springs. I really didn't want to get into a full rebuild right now, maybe in a few years so I need to scale back a little.

I did some testing and the engine is not in too bad shape. 120K miles, compression between 145-155, vaccuum steady at 18". It is down on power in low rpm and backfires thru carb when cold. I
don't think it burns oil, plugs are mostly tan colored. Timing is set at 10 degrees, mechanical advance doesn't move between idle and 1200 then kicks in only 5 degrees and stays there up to 2000 so I have some work to do there too.

Now I revised my plan (for the last time I hope) to just replace the gaskets/seals, timing set, and oil pump. For now I want a reliable (already is) non leaky motor with a little more low end power since I have 33" tires with 3.54 gears for daily driving and a little hauling/towing. later I would do a complete rebuild with a 400 crank and pistons. Any comments/suggestions would be appreciated.

 

Don't mess around trying to save a few bucks and mixing worn and new components. You are spending a bunch of time just pulling and re-installing. Spend the add'l 500 to 1000 and do the valves rings bearings etc. and have a goo power plant you can depend on for 100K + miles.

 

I second that Ecuri

 

I'm sure glad I didn't have that attitude when I freshened up my 429 to pull a 6 x 12 trailer from one coast to the other. I did almost exactly the same process, and that was 50,000 miles ago.

He is mainly talking about replacing leaky gaskets and seals. The cam/lifters/timing chain is a complete replacement of a system. There is no mixing of parts when you replace all.

The short block assembly seems good, not burning oil, no noises. Probably good for at least another 100k if he treats it right.

Replacing the valve springs and seals can be done with the heads on the motor. It's just a freshening up.

The backfire and low RPM stumbling he describes is probably carb or ignition issues. Unlikely that anything he's described so far will have any impact on that.

 

I successfully finished my project, it went smooth thanks to all the info available in these forums. I ended up replacing front/rear seals (almost half of the rear seal was missing), oil pan gasket, timing set (old one had 1 broken tooth), oil pump, distributor, carb (base was cracked on old one), valve cover gaskets, and all vaccuum hoses. Also replaced the clutch with a Centerforce unit which is much easier to push in than the old stock one. I also found a clogged pushrod, when spinning the oil pump with a drill there was no oil flowing to the #5 intake rocker. When I removed it and blew some crud out I then noticed that it was bent and replaced it. I did not replace the cam/lifters/springs, after snapping the exhaust manifold bolts and seeing how rusty the intake bolts were I didn't want to risk breaking more bolts.

The engine fired right up but I got a little nervous when it kept burning oil for the first few minutes, it eventually stopped. There are no leaks and the engine seems to have much better throttle response than before. When i had the cap off to replace the rear main seal it showed a lot of wear (copper showing) but no grooves. After adjusting carb vaccuum is steady at 21". Right now oil pressure looks good with 10W30 but maybe I need to go with 10W40 because of the bearing wear. I'm just happy that there are no leaks, no major problems, no busted knuckles, and the engine looks great since I painted it.