Compression good but engine needs to be bored over?

[thegreatjustino]

Some of you have been following my 351 nightmare for the past couple of years. After finally pulling it back out and taking it to the machine shop to have it inspected, I'm curious about something.

Prior to pulling it out, I compression tested it, and all cylinders were right around 130/135 psi.

Upon disassembly, the machine shop showed me there is a ridge at the tops of several of the bores and there is no longer any cross hatching on the cylinder walls. The machine shop feels it needs to be bored. I'm curious why this would be the case if the compression numbers came out so high. There is obviously some wear to the engine, but enough to bore it over?

 

[blubuckaroo]

While a compression test is a good trouble shooting tool, it doesn't tell much about the expected life of a motor. A cylinder leak-down test is more appropriate for that. You can squirt motor oil into a bad cylinder and get a good compression test.
The taper in the bore will cause the rings to flex, wearing out the ring lands and ruining the ring tension. If there's more than .006" difference in bore diameter the cylinder needs a rebore. Your motor would probably have a short life.

 

[Timmy390]

My 351W from a 96 E359 van had minimum wear (running when I bought it and not a smoker). No ridge at the top of the cylinders and you could still see some cross hatching but not much. It measured out at between .008 and .009 of wear. I discussed just a hone and re-ring with the machine shop and they suggested a bore and the kicker is, it was only $40 for a bore vs. a hone. you have the added expense of pistons too I guess.

Based off .008-.009 of wear still showing a small bit of cross hatching and you saying your had none left, I bet the wear was .010 or greater.

Tim

 

[rguest3]

If the block is currently at a standard bore and has not been touched but does show signs of wear, then hone or an over bore of +.030 with new pistons and rings is the next common step. Perfect time to raise compression a little and build the 351w as you like.

 

[73azbronco]

Was it honed or bored when you first installed it or is it factory original? If rebored already and now you have a ridge, Might want to consider how rich you are running and you might be washing the cylinder walls.

 

[cldonley]

You can hone the cross hatch back in and run new rings, probably work fine for a long time. If you have a significant ridge it signifies you need to bore it out. If you hone a loose cylinder and go back with stock rings, wear will speed up. One check you can do is put a new ring in the cylinder and check the gap with a feeler gauge. Many manuals will give you a min/max for that gap. As has already been said, if you bore it .030" inches and install new rings and pistons you'll be assured of great service for a really long time, plus your engine will hold up well if you increase compression/cam/etc. Good luck!

 

[thegreatjustino]

Engine was already .40 over. Will need to go .60 over if it's bored. I know there are mixed feelings on the board about going that far over.

 

[WheelHorse]

You may want to seek out a new block.

 

[Hozr]

Not sure if your original question was answered but it sounds like the rings weren't gapped properly or were too big to begin with. Over sized rings would cause excessive wear without compromising compression. You could sleeve the cylinders as well if you don't want to go 0.060. I know the 302 blocks don't do well over 0.040 but I'm not sure about the bigger blocks.

^ Hey that's my 1000th post!

 

[Timmy390]

Engine was already .40 over. Will need to go .60 over if it's bored. I know there are mixed feelings on the board about going that far over.

JMO, I would not go .060 over unless the block is sonic checked and it's verified it can take the bore without being too thin.

As suggested, you might want to find a different block. If the walls are too thin, it will overheat after the rebuild and you will not be able to keep it cool thus you will be out the cost of the rebuild and back at square one looking for a block.

Got my running 96 roller 351W for $225 complete. Bought the van for $800, parting it out had yielded $275 in parts sold (so far) and it's going to crush for $300. If a few more parts sell, I might break even.

Tim

 

[blubuckaroo]

Why are pistons only made in .030, .040, & .060 oversize? What happened to .010, .020, & .050?

 

[Timmy390]

You can still get them it's just .030 is more popular. Sites show the most popular ones first in the search. Expand the search to "all" and you will fine them.

http://www.summitracing.com/parts/slp-463p20/overview/make/ford

Full kits too
http://www.summitracing.com/parts/fem-mkp6116a-213/overview/

Tim

 

[76Broncofromhell]

Some of you have been following my 351 nightmare for the past couple of years. After finally pulling it back out and taking it to the machine shop to have it inspected, I'm curious about something.

Prior to pulling it out, I compression tested it, and all cylinders were right around 130/135 psi.

Upon disassembly, the machine shop showed me there is a ridge at the tops of several of the bores and there is no longer any cross hatching on the cylinder walls. The machine shop feels it needs to be bored. I'm curious why this would be the case if the compression numbers came out so high. There is obviously some wear to the engine, but enough to bore it over?

If there is a ridge in the cylinder it means you have significant wear and usually pretty gnarly cylinder taper. That is going to require a .030+ over bore.

Compression is not really the thing you care about, since that has more to do with ring tension and gapping, what your current setup will allow is huge amounts of oil usage and blowby due to the oblonging of the cylinder in the middle part of the stroke.

As said earlier, .040 is about as high as I'd go on a 351w, so I'd recommend looking for a new block. In all truth I think people are nuts to rebuild something these days, just buy this shortblock for $1250 and be done with it:

http://www.summitracing.com/parts/hpe-sp09

 

[Timmy390]

In all truth I think people are nuts to rebuild something these days, just buy this shortblock for $1250 and be done with it:

http://www.summitracing.com/parts/hpe-sp09

Some truth to that statement for sure.

I can't speak for everyone that does a rebuild, but for me I enjoy the challenge and the creating process of building something that's "mine" not to mention the satisfaction of a job well done. I grew up in a time without fast easy internet shopping that gives people 100's of choices and allows them to "click" and have a short block at their door in a week or so.

And yes, it was uphill both ways to and from school in 5 foot of snow....

My block and crank are back from the machine shop in a week. I can't wait.

Tim

 

[Hozr]

Brake jobs have gone the same route. It's cheaper (in most cases) to just buy new rotors than to resurface. In CA I couldn't find a shop under $40/rotor due to environmental fees and taxes. Amazon prime fixed that when I can get new rotors in the mail next day for less and not waste the time with the machine shop.

 

[76Broncofromhell]

Some truth to that statement for sure.

I can't speak for everyone that does a rebuild, but for me I enjoy the challenge and the creating process of building something that's "mine" not to mention the satisfaction of a job well done. I grew up in a time without fast easy internet shopping that gives people 100's of choices and allows them to "click" and have a short block at their door in a week or so.

And yes, it was uphill both ways to and from school in 5 foot of snow....

My block and crank are back from the machine shop in a week. I can't wait.

Tim

I've rebuilt my own shit many a times, but cost/benefit says that you are going to be in it more money doing it yourself. The same applies to cylinder heads, and brakes.

Alternators and power steering boxes are still much cheaper to rebuild yourself however.

 

[Timmy390]

cost/benefit says that you are going to be in it more money doing it yourself. The same applies to cylinder heads, and brakes..

As it stands now, I'll have $800 in parts and machine shop labor in my short block.

Tim

 

[TwoDalesDad]

Over bores

Today you can pickup a runner for $600.00 To build an engine your looking at $2500. The crate engines are great for street use , but they will be lacking on the trails. You really don't want to even go 40. Over because the engine will always run warm - hot..you can also go to pep boys or Walmart even...but your stuck with what you bought....Find a decent machine shop...they will gladly take your cash...

 

[74strokerbronco]

Just some FYI , I had a .030 over 69 351w block that was just sonic tested at my machinist shop and was very surprised to find at .030 over it was down to .095 wall between some of the cylinders. it didn't have any overheating issues but the motor had broken a 2nd ring in one cylinder, and would need .060 bore to clean it . I feel it's to thin to go .060 even tho the thin areas are not in the thrust zone of the bores. to save this block I'll probably sleeve the one bad hole.

Not sure what yr of block Justino is dealing with here but chances are if it's a older block to thin for .060, Altho I have heard of some of the older blocks being sonic tested and able to go .060 it's not common.

 

[Broncobowsher]

Why are pistons only made in .030, .040, & .060 oversize? What happened to .010, .020, & .050?

When I redid my last small block I could still see the crosshatch and didn't have a ridge at the top. But there were a few scratches in the bore I wasn't happy with. I would have loved to do a .010 overbore. But pistons don't exist. You can have that much wear and the engine still run good. So the next piston I could get was .020 and that was what I got. .030 is a super common size as that will clean up all but the worst wear on a standard block. .040 does exist but isn't that common. For when .030 isn't enough and .060 is too borderline. .060 does exist to rebuild an engine for the second time. But that can get sketchy with the thin cast cylinder walls. The cast iron is getting really thin at that point and distorts under load. Maybe fine for grandma who just idles through traffic, but falls apart when put under load. Which leaves the .050 which there just isn't any real demand for. If you are going that big it isn't for a hard working engine anyway so just get the common .060 and be done with it.