My recently purchased 77 f-150 has a 400 the previous owner had rebuilt about 4,000 miles ago. It seemed down on power compared to a previous truck I had. She didn't know much other than it has an RV cam. From the quick measurements I took it appears to be a comp 255. After tweaking the idle screws and checking the timing it still only had about 15" of idle vacuum. Not good with such a small cam. Also the power brakes seemed to not have enough assist.

Compression test results were 135 - 150 on all eight. Not too bad but not very good for a fresh motor. I suspected maybe the cam lobes were worn, so I pulled the motor as the engine bay needed some cleaning and painting anyway. Pulling the intake off revealed a nearly perfect looking set of lifters and cam lobes.

The truck sat in the garage for a few months as I was busy with other things. Finally this weekend I got around to doing a leak down test. The results were not good. 30 -40 percent on all cylinders. And none of the leak coming from the valve seats, all going past the rings.

So now I turn to you for advice. What to do? I need to tear it down further, but it does appear to have been bored, the pistons look new.

Did the machine shop not leave a rough enough wall finish? Did the rings not fully seat? I guess I need to tear it down and see if I can get another hone at this size. If it needs bored again I'll probably get some Tim Meyer pistons to bump compression.

 

Hello hivoltj,

I agree with you that a vacuum of 15'' is sh$t (with a 255 cam) as is the broad range of cranking pressures.

I would measure the deck height and combustion chamber cc's etc, etc, to determine what the expected cranking pressures should be before stripping the block.

With the block, pistons, cam lobes etc looking good, I would thoroughly check the piston ring end gaps.

 

You will need a bore gauge reading in .0001" and mics as well. These will tell you whats up between the bore and the piston. If the cylinders are straight, round and the pistons have the right skirt clearance then maybe they didn't finish hone the block with the right grit stones for the rings they used.

 

As per previous post, check compression ratio. Stock pistons are what, low 8s? And cam timing. If the rebuilder used a stock timing set, the cam is retarded 4 degrees iirc. Low compression, a cam with longer duration and retarded cam timing might contribute to your low cylinder pressure and low power.

As long as you have it out, I'd flip it over and check bearing clearances too, just for giggles.....

Good luck with it.

 

don't know if a few of you guys saw where I said it had 35% average leak down (past the pistons) I talked to a machine shop in town and he was confident we could hone it and throw on some new rings, but of course needed to mic the bores first. He said the leak down was bad enough they may have put the compression rings upside down.

I'll tear into it when I get back from vacation and update.

 

You'll never know until pulling some pistons. I know I swapped a used Chevy 396 to my brother for a fresh rebuilt 350 LT-1 out of a '70 Camao, I wanted the LT-1 and the 396 was just taking up space. He needed a BB for a new project as well.
Reason I got a rebuilt LT-1 was because when he first got it rebuilt it smoked a lot and was a bit hard on oil so he figured a botched job, he put back a JY 350 and sold the car.

I get the 396 loaded and to him and we load the 350 and I haul it home. I set it on the stand, start pulling it down to the block and pull the first piston / rod out, brand new bearings! new forged pistons, "pink" rods, steel crank, and every ring gap lined up in a straight line on the piston. All 8 were found to be lined up.

I can see a "mechanic" with his stuff laying out, ready to reassemble, letting some shop helper / groupie / ? help resinstall pistons and never having been told to rotate and stagger gaps.

Any rebuild is only as good as the person doing the job and a lot of $$$ is for their knowledge and attention to detail.

Rotated the rings staggering the gaps, checked bearing clearances as reassembled, and that motor never smoked as long as I had it, ran great in that little SB C-10 stepside.

 

Tore it down last night, realized a few blocks from my house that I forgot the damn camera, but I was already pressed for time so I just used my ****ty camera phone.










Didn't find anything really alarming on teardown. The factory numbered rods were not in the right cylinders, but I'm not sure how that would matter, as long as the correct rod cap is with the right rod (which they are) Either someone didn't pay attention when putting it together, or there is some kind of weird balance thing going on. I better weigh each piston/rod assy. and make sure they are close to the same before going back together. The cylinder crosshatch looks great to my eye, just as it should for the miles, doesn't look glazed at all. Someone had suggested maybe the (compression) rings were on upside down, they weren't. I pulled off the top ring of the first piston I pulled, and there is no dot on either side, it is a square face ring, not tapered on one side, so it doesn't matter which way you install it. The second ring has a dot and facing up, so I'm assuming it's tapered, but didn't pull it off yet. I wonder if they got the top and second rings mixed up, because (it's been years since I've torn into a 400) it seems I remember the tapered ring always going on the top and straight ring second. The faces of the compression rings looked too shiny in the middle to me, which makes me wonder still if they never seated. I'll get over there tonight and get some better pics with my actual camera, need to clean up my mess anyway...

 

Hmmmmm......makes you wonder if they were running full syn oil?

 

If you don't find an end gap problem or rings in the wrong position or any other smoking gun, then maybe:

 

Well, it has stock-replacement (Badger I think) .030" over pistons, so the static CR is probably about 8.2 to 1

That still would not explain the leakdown numbers though, only explanation for that is poor ring seal.

Currently I'm trying to decide if I want it to be faster/better, or just fixed. Somebody took a grinder to some of the crank counter weights, don't know what that's all about. I'll snap some pics this weekend

 

Finished tearing it down (almost) today. Tried to get a good pic of one of the top rings, I do believe they didn't seat 100%. The top and bottom edges of the ring face are darker and dull, but the middle is shiny with spots on it, I'm thinking it should be a dull cast iron color the whole way through.

Still don't know what the cam is for sure, there are no markings etched into it anywhere, so I'm guessing it's not a comp unit. There are some numbers cast into the core, but none of them really made sense.

Bearings all look great, only slight scuffing on the coating, obviously I'll be replacing them anyway.

The pics of the crank show the grinder marks I'm guessing from a balance job, but I've never seen it done that way and I don't think it's right. Also the oil holes on the crank appear to have little to no chamfer on them, not sure at this point if I'll end up being able to use the crank or not...

 

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Question? I have a buddy trying to get me to use some 2v Aussie heads he has collecting dust. Need a valve job but they should be good. If I'm changing pistons to up the compression anyway, should I use his Aussie heads? Compression would be about the same with the proper Tim Meyer pistons, but I've always heard a small chamber and dish piston makes more power than a large chamber & flattop at the same ratio.

I've though about re-using these stock dished piston (with the aussie heads), I assume with the piston being so far down in the hole, it's not a good situation for quench, power and detonation.

 

One of the tricks to raise comp was to put a set of aussie heads on the standard block. The cc heads are supposedly better for reducing pinging, but from some of the comments on this forum some have struggled with that. I didnt when I had my top end rebuild on my 400. Good fuel, decent cam and correct timing very important.
Aussie heads with a set of suitable pistons very good idea especially if you get the heads cheap.

 

Dug into the heads on my rebuilt motor a little last night. Looks like they were machined and had hardened seats installed, so I'll be keeping them as long as I don't find any problems. Hopefully a cleaning, spring change and new seals and they'll be good to go.

Fun fact :

400 shares the same bore spacing with small-blocks, so I figured the exhaust port spacing might be the same? It is. I held up an old 302 manifold I had laying around and it matches the port spacing. This gives me a great idea, more later.