The fun has begun...

Finished getting the engine completely torn down today (see my previous thread here https://www.thirdgen.org/forums/tech...ml#post6553083

My only real concern is the shallow rust depressions in cylinders 4, 6, & 8 as seen in the pics. While I can't snag a fingernail in these marks, I can feel (barely) that they do have some discernable depth. Using a telescoping gauge, biggest/deepest measurement I can get is ~4.003, with the surrounding areas right around 4.000.

This is my first SBC teardown, and I'm apprehensive about tapering, or otherwise screwing up the bores if I tried to hone the spots out myself, so my questions are;

- Do y'all believe this is something a machine shop could/would hone out?
- If so, what should I expect to pay for having the block cleaned, cylinders honed, and maybe cam bearings installed?

Thanks in advance for your input! - Maker

 

Doesn't really look like rust or that bad to be honest. If you oil the walls and rotate the engine by hand they may "fade". A ball hone may be worth a shot before taking it to a machine shop. A ball hone will just dress the surface and let you reuse your internals.

 

I've put around 10k miles on the vehicle, and I'm thinking those spots had to have been in the cylinders since before I bought it. I did a compression test before tear-down and there was only 5% variation among the cylinders, with an average around 132 psi. Maybe I am over-thinking it, and no more than a honing might be all that's needed. I found some copper showing in a few spots on the crank bearings, so the rod and crank bearings will be replaced. Do you s'pose I could keep using the existing rings? they move freely in the grooves and all look good.

 

The marks you identified notwithstanding, you may want to check for that taper and out of roundness first. If it's not in spec, then an overbore will clean it all up. Yes. More $$$.
Do you have access to more precise tools? A dial bore gauge may be an inexpensive investment.
Check some of the available resources for taper and out of round. You may find something like: " On cylinder taper and roundness, with typical honing and measuring procedures in auto-machine shops, .0002" is considered good and .0005" about the max acceptable. (though some will say .001" is fine).
If they do measure OK, a light hone and new rings might be all you need.

 

If it's running fine and not eating oil, than it's probably fine. However, if you're going to rebuild you'll need new rings. New hone surface = new rings needed.

 

Any conclusions?

 

I took the block to a local machine shop on Wednesday to be assessed and possibly honed. They're supposed to get back with me mid-week. I'm working on the peripherals in the meantime.

 

If you're targeting a compression ratio, given what you have to work with:,i.e.: piston shape, chamber volume, you may want to consider a block decking Was the block delivered as an assembly? If so, the piston below deck can be noted and then there's a better guess going forward. As stated earlier, the factory piston at TDC is .025" or more down the hole. This isn't a problem onto itself but it looks to be that head gasket choices are limited when going for a 040" piston to head clearance. The benefits of a tighter quench being well documented.

 

Hey Skinny z, I was supposed to get the block back today or tomorrow, but based on what you've told me, I called the shop when they opened this morning and asked them to deck the block .011, leaving the clearance at .014. Checking on the Mahle gasket you recommended, I found many complaints from folks stating that they received .040 gaskets instead of the advertised .026". So, I'm eyeing the GM #14096405 w/.027compressed ht. That combination would put me ~ .041 quench and 9.5:1 CR. Thank You so much for the input. Now I'm sorting whether the crank will get along alright with -.010 bearings. That's what was in the block when I took it apart, but dang it seems like it'd be too tight for the 2.0886-2.0891 measurements I'm pulling off the rod journals. Looks like the mains will be okay with new -.010 bearings giving ~ .0018-.0024 oil clearance.

 

@Maker-Wright I see that Summit lists it as a .040" compressed gasket. I had a look through the Mahle catalog to confirm that it is actually .026". I'd be willing to bet that those negative reviews aren't exactly accurate. (Like how do they know how much the gasket will compress really. Answer is: they don't).
But that's a moot pint now seeing as you have the GM gasket. Which I also used on the 1st Vortec 350 I assembled. You got a decent quench going (the piston top profile notwithstanding) and the compression is looking better than it would have alternatively. So that's good.

As for the bearing clearances, I gave up on that game entirely. I found that I didn't have the needed size selection on hand to make a change if I needed to. So I've had two very good shops assemble my shortblocks. Taking them apart for an inspection isn't an issue but messing with two or possibly more bearing sets for rods and the same for the mains didn't seem economical. Somewhere in my paper records I have all the clearances. If you find yourself in a quandary, I can dig them out but it looks to me that you've got a solid handle on it.

EDIT: Went through the build sheet.
.0028" on the mains.
.0025" - .0026" on the rods.

 

I'm getting bleary-eyed from searches for piston/wall clearance values. Lots of info/opinions about aftermarket hypereutectic and forged pistons, but I can't seem to find definitive info on stock cast pistons. Can anyone please tell me what the piston to cylinder wall clearance range is for a 1988 iron-head L98 engine with OEM cast pistons. Thanks

 

My 87 GM shop manual lists a production clearance of .0007-.0017. It also suggests a service clearance of .0027 Max. It specifically calls out the L98 as well.

 

Thanks!

 

Got the block back from the shop today. Shop said the cylinders had been "lightly honed" as I'd requested. Started measuring the bores and I'm going bonkers. I figured I'd want to make the first/top measurements directly below where the "ridge" used to be, right where the top compression ring dwells at the top of the stroke, which is about 1/4"-3/8" down from the deck. Measuring at that location, I'm getting readings with the dial gauge of up to ten thou over my avg piston diameter. So, I go on the YouTube, and the guys in the videos tend to generalize and say "you wanna start with your first measurement about half inch to an inch below the top of each cylinder...". If I measure 5/8" or lower down my bores, the world is a happy place again, taper and out-of-round aren't bad, and all measurements land within .003 of my piston diameter. So, is it acceptable that I'm getting + .007 over-spec readings 1/4"-3/8" down from the deck, but go back into spec just 1/4" farther down the bores at 5/8"-3/4"?

 

What tools are you using to measure?

 

.0005" Dial bore gauge.

 

Same tool as used here although I've a three point gauge rather than two point (as in a snap gauge) that I've used. I can't produce repeatable results with the latter although I can get close. Are your results repeatable from cylinder to cylinder? The reason I ask is I see it unlikely that there's that kind of spread in such a short distance. How does the "light hone" look in the area?
Slide in a squared up piston ring to the 3/8" mark and measure the gap. Then repeat at the "happy place" 5/8' down. 7 thou should be feeler gauge measurable and it may serve as a reference for the bore gauge.
One thought is that if there was ridge and it was reamed out, it might have been overly aggressive (assuming a ridge reamer was used at some point).

 

My gauge has the outrigger wheels. I've zeroed the bore gauge at 4.0000" with a calibrated micrometer. The measurements 3/8" down vary from +.0067 to +.0095 across the thrust axis and +.0030- +.0080 parallel to the crank. The rest of the measurements, 5/8 down and from mid to lower bore are between +.0010"- +.0028".

With so many measurements being so close to 4", I can't imagine there was ever much of a ridge.

I've measured many times and come up within a few 10 thousandths of the original #.

I'm wondering if they may have hit it with a ball hone so all the highs and lows are cross-hatched.

I'm starting to wonder if the compression/ignition force tends to hammer the top ring causing a shallow groove at the top of the bore, and that's why many say to start measuring just below the top ring witness mark, or a bit lower in the bore. Because it's above the lower compression ring, maybe it's not seen as a problem... but, yeah, .007" variation in a 1/4-3/8 span bothers me.

 

I was in a somewhat similar situation although it was barrel and taper that were the culprits. End result was to go to 40 over from the previous (high mileage/high abuse) .030". I did however get a very forged piston and thin ring pack in the deal (Not that that helps your situation any). But you may be at the same crossroads.
You might want to enquire on an engine site I frequent (Speed-Talk) and ask directly about the area at the top of the bore and how it can be beat up as yours appears to be. Far more knowledgeable people there (than me) not to mention professionals, that may have come across exactly what it is you're describing.

 

Thank You sir. I am contemplating stabbing it out .020", getting flat tops, and moving to higher octane.

 

Going back to the 1st pictures posted and seeing that very distinct boundary between the top of the cylinder and where the top ring resides, and if you're confident in your measurements, the 20 over approach may by the way to go.
If nothing else, you've done your due diligence.

 

I'm surprised they didn't give you the final numbers.
​​​​​​I just had rods recon'd and crank polished they gave me a sheet with so many numbers it a good 5m of staring at it to figure out what was what.