All,

I'd like your opinion on the maximum allowable bore wear that you would consider just a hone instead of an overbore.

Some of my cylinders exhibit a slight taper at the top. Bottom/middle measurements look good, but we are seeing as much as 0.005" wear at the top.

This would be rebuilt as a stock L46 motor. Would like to reuse the pistons. Skirts show minimal wear.

Thanks for your input.

 

No expert here Pete. Although it may be technically over the limit, I would not hesitate to put it back together paying particular attention to ring gap not too tight. I'll probably get flamed for saying that so this disclaimer: 11:1 is pretty high compression and rounder is better. lol

 

nothing like that is optimum but i think with forged pistons you could get by . i think file fit rings are a good idea. as long as your not driving it every day or plan to run 100,000 miles with it. may be more valuable being an unbored engine. some body will take strong exception no doubt but like i said not optimal.

 

I discussed again wtih my machinist this morning. He is comfortable with it because only the rings will be riding where the 0.005" wear is, so he is not concerned with any piston slap because the pistons are in great condition and the maximum wear at the middle and bottom of the bore is 0.002".

The motor has approximately 130,000 miles on it already. I run about 2,000 a year. I think it will be many, many years before I have to rebuild the motor because of excessive wear.

 

I would lightly torque plate hone it to get a good ring seat and as round as possible. Take a few light passes and see where your at. If it is very out of round you might not have a choice but bore it. If you use file fit rings check them down in the bore where it is not tapered. Not optimal but I see where your coming from.

 

Bore it. The cylinders are worn oval. If you replace the rings they'll be round until they wear to fit the oval cylinders. I personally don't think you'll find a 130,000 mile engine that doesn't need an overbore. If you put the old rings back in I'd mark the ring gaps and replace them as close to where they were. There are plenty of engines running around with more clearance than what you have that aren't making noise, but if you've got it apart fix it. With that many miles the heads probably need done too.
If you put it back together and have poor vacuum, compression and or blow by you'll just be repeating work and spending additional cash for a gasket set.
The only people bringing heads in with those miles who don't get seals and valves ground are used car lots.

 

If it is out of round which I believe it will be, a new set of pistons is $400. Is it worth the chance? Pretty nice car!
http://www.summitracing.com/parts/sl...make/chevrolet

 

Using my GM shop manual for my '74 says the L82 (forged piston) spec is 0.0036" - 0.0042" for production but can run out to 0.0061" in service. And D. Vizard says pretty much the same for forged pistons if u measure with a feeler gauge between piston and cyl wall - up to 0.006" okay but dont run with 0.008" piston to wall clearance even with forged pistons.

Yea with a limited use car thats not going to be raced just ball hone the cyl and install new rings - should be good for 50K miles.

 

Hi Cardo,

I saw that note in Vizards book as well. Well within spec if checked by Vizards method.

The 1969 CSM states 0.005" maximum wear for service.

 

If you're seeing .005 wear, then you need to add this to whatever clearance you had to start with. If you had .004-.005 clearance to start, then you're now at .009-.010 where it's worn. That's way out of spec. If the cylinders are worn .005 at the top but in spec in the middle, they're way out of straight (tapered) and out of round. Time for a bore and hone IMO.

 

I missed your response earlier Mako. This is unfortunately where the budget comes into play. This started off as a gasket replacement job since the car sat for a long period and developed massive oil leaks everywhere after running it a couple times. It progressed to rebuilding the heads because of valve keepers falling out, which progressed to new valves because they were so thin. The bottom end was going to be left alone until I noticed two broken top rings. So what started off as a cheap project to replace gaskets turned into a completely torn down motor. I'm already in $900 at the machine shop and $500 in other parts. An overbore and new pistons would tack on to my overblown budget.

Scott, since we don't know what the original clearance was, you have to assume that wear includes the initial clearance. In other words, my bore at the top is 4.005". My bore at the bottom is 4.002".

 

Finished bore sizes are usually nominal. Pistons have the clearance built into them, so a finished bore of 4.000 would already have the clearance included. Anything more is extra clearance.
Something else to consider (now that you mention broken rings) is that the end gap on a ring changes by ~.003" for every .001 of bore increase but like has been mentioned, your bores are not round now, either.
You don't need to bore the block. You might be able to find +.005 pistons, but even .010 pistons are only a hone job away.
The broken rings are trying to tell you something. Remember this saying:
There's never enough time or money to do it right, but always time and money to do it over.

 

Maximum piston "wear" is showing 0.002" from nominal.

Your last sentence rings true, but money will always be a limiting factor in all projects, otherwise our cars will set on blocks for years. I have installed a new valve kit, and ordered new rings, camshaft, lifters, rockers, and pushrods. The machine shop has verified all critical measurements, cleaned the pistons, block and heads. A helicoil was used to fix an intake bolt hole in the cylinder head. The crankshaft journals have been polished. The crank, pistons, and cylinder bore have all been checked for wear. The block deck height has been verified, cylinders honed, new cam bearings and freeze plugs installed.

In my 4 years with this motor alone, I have had detonation issues. This is an understandable explanation for broken piston rings.

I have found there to be a very large grey area that encompasses "doing it right" and what is "acceptable." Vizard is comfortable with 0.006" wear. Others say never reuse pistons.

If I can get 30,000-50,000 miles out of this rebuild, It'll be another 15-25 years before I have to tear it down again.

 

So you have .002" piston wear and from .002 - .005 bore wear...
No matter. You'll do what you'll do. Just trying to offer some clarification on the numbers. Good luck with your rebuild.

 

Thanks Scott. I did make the decision to trust my machinist. He's well experienced.

 

if i understand the engine was fine but broke a ring, i don't see why repairing an otherwise good engine won't work as well as it did before by putting rings in it. as i said before it isn't optimal , but i am starting to have less issues with it than i had. this isnt a daily driver or race car its an occasional use car . if it burns more oil oh well , then you know. it is a shame new pistons aren't in the budget and maybe waiting a couple months would make it a reality.

 

A good question that's been asked is, "why did the piston rings break?" You can make a valid argument that it was due to fatigue as the ring constantly flexed through that tapered bore. You can also make a valid argument for detonation.

I think to get it 100% right, the cylinders should be overbored. With all these costs, it would have been best then to wait even a couple more months and just purchased a crate motor. Then we can say, "wait even longer and get the 383!"

This is a disease people. We all suffer together.

 

There isn't a crate engine made today that I wouldn't immediately disassemble and inspect very closely.
The only thing missing here from having basically a zero time engine would be a set of pistons. Everything else was done and paid for. FWIW there can be 30-35hp in bore finish alone, and that's comparing one fresh bore and hone to another.
Excessive piston rock (due to excessive bore/piston wear) can lead to rings that don't stay square in the bore and never really seal. That = loss of power.
Like mentioned, tapered bores cause excessive ring flex. I'm not sure how I would gap those rings, either. If you're using pre-gapped rings I guess it doesn't matter.
Out of round bores can lead to rings that don't seal, and if the oil rings and second ring aren't doing their job, you can have excessive oil consumption which leads to oil in the chambers and that will cause detonation and loss of power.
Bores that aren't straight or round will also promote more rapid wear of the rings which means you won't get the life out of your rebuild that you might expect, and it's never right from the get go...and never going to be. You think you're saving money, but you're really throwing about $.50 of your $1.00 out the window.

 

years ago some people would knurl the piston to get a little tighter fit i don't know if anyone does that anymore bit you could ask your machinist.

 

Well Scott, that is a ridiculous statement. I understand you disagree with my decision. It's very easy to make these decisions with other people's money.

I'll trust my very experienced machinist and have moved forward with just a hone. I appreciate everyone's opinions.