Piston clearance for long-life cruiser

Hello,

I sent my '73 010 SBC block to a dodgy machine shop and afterwards to a proper one for plateau honing and sleeving of #6 cylinder.

I'm at the point of measuring piston ring gap (filed by the dodgy shop) and pistons. The one that was sleeved by the second, good, shop, has a 0.003" clearance consistently but the other seven are all over the place. They measure between 0.0045-0.0055" in various directions. The rings they filed are also not very consistent and some are too tight.

What do I do? This is a street engine I want to last. How bad is this 0.0050" piston clearance?

The pistons are Wiseco 5506A3 (4032 alloy) and the rings are JE moly rings. The block is a '73 010 SBC block.Regards,

Henrik Morsing

An expensive lesson, and I think we've all been caught at some point - I had almost an identical issue on a SBC as well, that ruined a, then rare, 4 bolt block!

It's not what you want to read, but I would suggest the best, overall, option is to order a new block and have the TRUSTED machine shop take itout to the 4.030 bore required, and check the decks for squareness and correct deck height. It's quite expensive, but that way you will know it's RIGHT!, and will LAST! - heck, when you take into account the full build cost, it may not be quite that expensive.

Alternatively, see if you can trade the +0.030 pistons for a set of +0.040, which isn't the optimum for wall thickness, but a lightly stressed engine 'should" be fine. Third option is to have all the other cylinders sleeved, as well.

I would NOT give running the crap bore you have more than a nano-second's contemplation.

If your trusted machine shop is agreeable, you could have them write a report on the "work" the dodgy shop did, why it was unacceptable, and take a small claims claim against them to get your money back. It's only a few quid, and you might win.

Good afternoon Gord,

Thanks for replying.

- "It's not what you want to read, but I would suggest the best, overall, option is to order a new block"

This is just not going to happen, ever. Even just trading the pistons for 0.040" over would set me back 2 600, so that will only happen if the court claim goes my way.

I have heard of companies that can do piston skirt coatings that will add 0.001" but I can only find them in the US. Anyone aware of any UK companies that do this?

The second shop is sadly not willing to get involved but an itemised invoice might be enough to show what was corrected. I was recommended ACE Automotic Forensics by the FER to write up a damage report, so that is my next plan and then take the first shop to court if necessary. It will set this build back a year though.

Regards,

Henrik Morsing

Uh, I don't know where you get 2600 (pounds, Euros?) as a trade in deal, as even directly importing from SUMMIT, or similar, should be closer to half that after taxes and duties - but check first, because I'm not in the UK, so don't know if things like VAT would apply.

Then put the 4.030 bore pistons on E-Bay with a quick summary of why you're selling them as "new/unused", and you should get a good percentage of your money back. Oh, if you DO consider this, talk to your 'good" engine shop first, as the sleeve may complicate things.

I forgot to ask which engine size, or rather crankshaft stroke, you have, because there are different piston pin heights for the 302. 327, 350 and the 3 3/4" stroker crank, if using the stock length 5.7" rods, whether OEM or aftermarket. I assume the common 3.48" stroke 350?

It's something to aware of, even without some of the aftermarket options for different length connecting rods.

There's no "cheap" option, you're just going to have to decide where you place the line for the "good enough" Vs cost option.

Ok, money aside, I wanted to know if I could continue the build which, as I expected, is a no.

So my second question, the cheapest and easiest fix, was about piston coatings. Even if I have to post the pistons to the US and back, it's still a more appealing solution, but would be nice if I could find someone here that could do it.

Regards,

Henrik Morsing

I think you may be thinking of the ablative piston skirt coatings some piston manufactures use, that are designed to 'self clearance' the piston to wall clearances? I can't think of anything else that could posible apply, except maybe anodising, but that's usually adding negligible material.

With the coatings, they're specifically used on pistons designed for the coatings, and because of their nature cannot be used as you are thinking. That's ignoring the point that your cylinder walls are out of shape, and they're the problem not your pistons. Oh, and they're applied to the skirts, so will have NO affect on the upper part of the pistons, where the ring lands are. I guess, if it comes down to it, plasma spraying the pistons, and having them remachined, would be a VERY expensive, and daft, option, and still does nothing about the bores.

As I said, there's NO "cheap" option - cheapest would be if you found a good s/hand original (worn) bore block, or maybe a 20 thou' over (pistons were available in that size, but usually they were taken straight out to 30) block that will clean up at 30.

Just to clarify, the 010, for the "high" tin block, is behind the timing cover, and not part of the engine cover, and that's why you want to use the block?

If it's to keep it "numbers matching?", you're going to have to have the other 7 cylinders sleeved as well, by your trusted machinist - might even be the cheapest option, too.

You don't actually mention what sort of build you're doing - if you're going to be leaning on the engine hard - forced induction, high rpm, etc, and you have a two bold block (they came in both), you should also be factoring the machining for 4 bold mains caps, etc.

Hi Gord,

The title says it's a cruiser. It's NA, mid-RPM range cam and low RPM-range inlet manifold and heads. It will be running through a TH200-4R and a 3:42 Auburn limited slip differential. The car is a 1970 Chevelle. I expect this engine to last 200 000+ miles.

The piston skirt is where the piston clearance is measured and where it matters. The top of the piston is handled by the rings and clearance it less important there.

I want to keep the block because I hate waste, both physical as in throwing useful things out, and money. I don't mind spending money, but I dislike wasting it.

I did in no-way anticipate a machine shop destroying my block. As much as possible, this will have to be paid for by them, but not fixed by them as I don't trust them.

Regards,

Henrik Morsing

That's where the piston skirt is measured, true, to establish the piston to bore clearances, but I REALLY think you should actually learn more on the subject, as it appears you are mistaken in the importance of the entire piston's clearances and design.

Yes, I agree - I also hate spending money, especially when other people "f" things up and STILL get payments because it wasn't obvious on pick-up - and I am also relatively happy spending a bit more when it's for a good reason.

It's not easy to assess costs, being in different countries and all that, but sleeving all eight bores, or finding a "stock" bore old engine would seem to be the best value options. I would strongly advise against any snake-oil "solutions".

The ideal would be a new block, because that way ALL the clearances and heights will set correctly, but $$$

You might give some thought to the heads and, especialy, intake manifolds - can't recall what the fuelling was on that engine, if it's still a carb' or the early EFI?

Hi Gord,

I totally agree, but how? The Internet is the blind leading the blind, people just talk nonsense all the time. There are some chargeable courses on here, and again, I don't mind paying for useful information, but it's hard to know if the courses are useful to me or not.

I don't know anywhere near enough to figure out if you would gain from the courses.

If it's regarding the pistons, there are plenty of very good on-line guides, including from the manufacturers.

In this instance, the upper piston, with the rings in their grooves, needs to be a close fit in the bore to keep the rings square to the bore for sealing, and reduce flutter and other issues. This is actually the area where the "piston clearance" needs to be adjusted for the fuels and/or boost, because the combustion heat causes the alloy to expand and the clearances required are reflected in the skirt, measured, nominal clearances. The rings should be free to move within the piston grooves and it's the piston that stabilises the rings, not the other way round as you seem to think.

I'm not sure that's what I thought, I was just saying keeping the piston stable and straight was down to the skirt. I know what the rings do and how they fit.

Even manufacturing guides don't really answer my question. I emailed Wiseco before Christmas, but I am guessing they have been closed and/or busy.

I am now booking ACE (https://www.ace-uk.website/) to come and inspect this. Not sure they can tell me what's right, but they can at least measure it all.

'afternoon,

I have spoken to the second, competent machine shop and he said it wouldn't be a problem. There might be a bit of piston slap when cold, but it wouldn't affect the life of the engine.

I will continue.

Regards,

Henrik Morsing

Wiseco has now come back with this:

"Dear Sir,.004 is standard clearance with no power adders. 4032 alloy only additional clearance if adding NOS.

It really needs to be at .004. Will it hurt to run probably not maybe noisy on start up until up to operating temperatures.

Thank you.

Wiseco Pistons"