Technical Can I Get Away With Running My Engine As-Is?

Long story short, I have a Ford 390 block that I dropped off at the machine shop so they could check it out. The block is already bored .040 over. Guy at the machine shop says the cylinders are .0003 off from being perfectly straight, and that the engine would run fine but might smoke a little. My buddy who's helping me put the car together says not to worry about it since it'll be a weekend car and that it wouldn't even be enough smoke to see.

I know what I should do, but that would mean having to buy new pistons and having to go another .010 over. I'm on an extremely limited budget right now and would like to get the car out of my buddy's shop so he can make room for people who would pay a lot more than I'm paying him! haha

So, do you guys think I'd be okay running the engine as-is, or am I asking for trouble? Thanks.

 

If that's all that's wrong with it ; Build It!! Those 'specs' are closer than most shops are actually capable of! (Now don't you 'high-performance' shops get your panties in a bunch: we all know those numbers are fine for many miles of ordinary use, & that the machine shop was just 'covering his butt'!)

 

do you mean 0.003" ? Usually the wear near the top of the bore, the rings wear the bore as a result of combustion pressure pushing them outwards.

If it has that much taper, and you give it a clean up hone (or glaze-breaking) it will probably work fine, and not start smoking until it gets tens of thousands of miles on it. of course without looking at it, and knowing exactly what you're doing, this is only a guess, based on experience.

So yeah...you can put it back together, and drive it. In the old days, we'd call it a "ring job".

 

Ream the ridges, hone well, clean the pistons, and go with inexpensive cast rings. Clean all the parts, oil well at assembly time, use a good geade of motor oil with some lube for flat tappet cams, and it will last for years. For oil, use an oil meant for a diesel, or any good quality 10 W 30 with a bottle of STP, or the ZZPD additive.

 

Ford specs the wear limit at .005", you're at a bit over half that.... run it.

 

like squirell said glaze break and re ring , I did this to a pontiac in college and it lasted 10 years till my ex wife decided to drive without water in it . only when I raced with the headers open and at the traps when I shut down would it blue fog any.

 

I don't think you should have any problems. I say if the machinist is telling you that you don't need to give him more business, you're in the clear.

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Indeed I did mean 0.003! I always mess that up. haha

I'm getting much more positive feedback than I was expecting, so I feel good about running it. Thanks guys!

 

When I have one that is a little loose I always go with file to fit rings.

 

That engine will be fine. Porknbeaner is correct about the ring choice. I might also suggest if the pistons are a little loose to ask the machinist to knurl the piston skirts. It will tighten up the piston as you can wet sand the piston to get the exact fit you need. That will also hold a little oil on the skirt and stop the piston from rocking thereby making the rings last a lot longer. If your machinist has been around awhile he will know all about this process. Good luck.

 

Knurling the pistons is very traditional.


Knurl them.

 

If you're only going to do a light hone, then you don't need to knurl the pistons. The skirt clearance should still be acceptable. If you hone the taper out of the bores, then you'd want to knurl the pistons.

 

If it's not getting an overbore, I'd hone out the taper & knurl the pistons. It's all apart & in the machine shop already, so I'd do at least that.

It'll run fine with .003 taper, but it'll run a lot longer without blowby if you straighten the taper.

 

If you have straight hair it's .003 thick.
That ain't squat.
Like my Dad used to say; "If it was running when you took it apart it ain't gonna be any worse if you just put it back together."

 

Pistons always run with a little clearance. .003 taper is nothing to be concerned about IMHO. The real question is the gross clearance. If your machinist is not concerned with that there is no reason for you to be concerned either.

 

Maybe it's not so bad.

But the engine's torn down, and at the machine shop. If not now, then when?

I can't see them charging much for a quick hone and skirt knurl.

 

I haven't seen anyone knurl pistons for decades. It's a lost art.

 

If you're on a budget the hone and go sounds like the best bet.

 

I get a lot of "deer in the headlights" looks when I ask for something that isn't done much anymore. If they'll do it reasonably go for it. If not hone & go like mentioned before.

 

I don't know why you even tore it down. Normal service spec says you don't need to rebore until taper exceeds .007 and I have seen engines with .010 reringed and ran good.

Hone it with a bottle brush hone, install new rings and go. If you take decent care of it, it should go at least another 50,000 miles probably more.

 

I looked in the mirror this morning. Some fuzzy dinosaur was looking back at me.

Probably nobody knurls pistons now because the basic cost of machine work is so high, that a new set of (imported) pistons is too cheap (by comparison) not to use. When I was young it was done whenever you wanted to do a cheap hop-up. Hone, re-ring, and knurl, and you were back to the races.

 

Like squirrel said... last set I knurled was back in the 70's I doubt you can find anyone who does it anymore.

 

I did the "ring job" Jim speaks of on my 430 in my Stude, lost track of how many drag strip passes, and terrorizing the streets I have done with it, (knock on wood) Haven't had any oil consumption problems.

 

You can still get pistons knurled if you know where to look. One of the reasons that it is not don't much any more is that pistons are relatively inexpensive in the whole scheme of things and it takes a certain amount of the liability off the shop. Knurled pistons is not commonly accepted as a long term repair.

One thing that some of us used to do (and a few of us still do) when an engine is right on the edge of being out of spec is replace the cast pistons with forged pistons and hone the cylinders to the proper clearance for the pistons. Forged pistons are set up more loosely then cast. But buying new pistons does away with budget build.

Someone mentioned using a bottle brush to hone the cylinders. This is a good idea, a bottle brush breaks the glaze on the cylinder without removing as much material as regular stone cylinder hone.

 

Haven't seen it either, but for good reason. I remember having it done on a 283 back in the 60's, and it lasts for a very short time. The fresh hone, and small surface area of the knurling combine to wipe the knurling off in short time. Took my old 283 apart to swap heads about 3,000 miles after the rebuild, and when I popped a piston to check the knurling, it was just the indentations left in the skirt. None of the raised knurling was left above surface height. Waste of time and money.

 

I've worked in several machine shops and knurled a lot of pistons. Our local NAPA closed It's shop and had an old Hastings unit left which I picked up. It came with a manual. It said only knurl the non-thrust side which makes sense. All the shops I worked for had been doing both sides.

 

Sunbeam,
It was a common practice to knurl both sides of a piston. Obviously the thrust side got wiped but the engine was tighter when it left the shop and that fulfilled 10 minutes or ten miles warrantee.

 

Beaner
Do you think knurling both sides is why it got a bad rap?

 

Knurling was a cheap hop-up trick. That's all. It doesn't replace new pistons.

But if you want to get a few more runs from your expensive high compression pistons, new rings and a knurl will bring back the compression and reduce the rocking/knocking/slapping business.