New pistons or new block (overhaul advice)

[wmax351]

Hi everyone, I've been going through the engine on my 72 '02, which is a 1975my block with an E12 head, and looks like I have a ton of piston/bore clearance, and is overbored +1mm already. Looking for recs on pistons. 

 

 

Was rebuilt in 1998-9 with 10:1 pistons, has a 300 degree regrind, and hasn't had that many miles that I could tell, but did mostly sit for 10 years or so. It smoked on overrun and a ton on initial acceleration after an overrun. Figured it was valve seals (which are old and several looked bad). Head is at a good shop getting worked on now. 

 

On to the block: there's a ton of cylinder/wall clearance. Like 8-10 thou. I'm going to clean off the pistons and measure them at 70f rather than my freezing garage to confirm. But I can slip a 8 thou feeler gauge between the skirt and the cylinder wall at 45f. 

 

I'm debating if I should get it bored/honed to 90.25mm and get the fairly standard (albeit maybe custom still) pistons, or if it should get it touched up the absolute minimum, and get custom pistons. I'd favor low expansion pistons, be they forged or cast. I street drive this primarily. 

 

It got 10:1 Arias pistons at the time, for a 1mm over-bore based on the records I have,  and Carillo/Arias was able to pull up the old spec sheet for me. Per their spec sheet, pistons made for a 3.544/90.018 bore, which was likely just rounding from a 90.015 bore. Pistons were supposed to have a 6 thou clearance, and measure 3.538 but currently measure 3.534-5. The shop sheet from the engine build noted a 0.040 over bore, which would give it an extra half thou or so. The bore measures (without a torque plate) at 3.545-3.547, and there is no ridge. The shorter dimensions are longitudinal, so I guess that's related to the torque plate or maybe decreased wear since they're not getting thrust load. The areas at the bottom where the piston rings don't touch are 5.4555-5.5465 in both directions. There's a tiny bit of discoloration at the site where the rings sat. 

[wmax351]

One option would be the wiseco set that is 9.5:1 (or 10:1 reported in some places) that comes in 90.25mm. WKE326M9025

[tzei]

Pistons or block are both good ways to go about. With new block you’ll need pistons thou. If it’s old cast you should have meet to go over size than that.

[John76]

55 minutes ago, wmax351 said:

there's a ton of cylinder/wall clearance. Like 8-10 thou.

How are you measuring this?

If using a feeler gauge, do you measure 4-5 thou on each side of the piston (180 degrees apart) to the cylinder wall??

You would need 2 feeler gauges to do it this way.

Hope some engine builders can chime in with an accurate way to measure piston clearance without a bore gauge and calipers.

 

[wmax351]

Measured both with a bore gauge + micrometer as well as feeler gauge. Bore gauge has a clearance of 0.010-0.011. Feeler gauge x2 was similar. With a single feeler gauge it was 0.008 or so. 

27 minutes ago, John76 said:

How are you measuring this?

If using a feeler gauge, do you measure 4-5 thou on each side of the piston (180 degrees apart) to the cylinder wall??

You would need 2 feeler gauges to do it this way.

Hope some engine builders can chime in with an accurate way to measure piston clearance without a bore gauge and calipers.

Edited February 8 by wmax351

[wmax351]

Is there any issue with running 90.25 bore? It's not getting flogged, but it does produce more power and I will be driving it regularly with some decent distance. 

[kaptanoglu]

I have used JE pistons from Top End Performance

Top End Performance - JE Custom and Off the Shelf Pistons for all Import and Domestic applications at Discount Price. Shipped Worldwide.

WWW.RACETEP.COM

---

JE Custom and Off the Shelf Pistons for all Import and Domestic applications at Discount Price. Shipped Worldwide. Pistons, Pins, Locs, Rings, Everything JE.

 

They sure are purdy.

[stephers]

How much taper is in the bore?        Have you measured the piston skirts to see how they are

 

Thanks, Rick

[wmax351]

No taper in the bore. And the skirts were about the same. 

 

I got some 90.5 JE pistons From top end performance in the queue, and found a shop to take it to 90.5

[OldRoller]

Pistons don't look bad at all for the time they've had. Your smoke issues are likely valve seals/guides, you've got that in hand already. You didn't note whether or not you had any piston slap on cold startup.

The old school of thought was forged pistons always expand more than cast pistons. It depends greatly on alloys used and procedures of manufacture. Some do, others not so much. Of greater important is cylinder bore specs. 

It would be interesting to pop a couple pistons in the oven, heat to 180-200F and check the specs measured in two axis before and after.

Been out of racing for many years, but we used to set up engines a little loose, a little "freer". In your case, a couple thousandths would not worry me in the least... but then it's your engine. Follow your instincts. 

[wmax351]

Yeah, the spec from manufacturer for the pistons was 006 clearance, but they did say I'd probably be best replacing them with a clearance of 10 thou. The pistons are 3 thou down from original, and the bore is 3 thou above what the pistons were made for. 

 

Didn't specifically notice piston slap on cold startup, but it is somewhat noisy. It much have been there in the calcaphony. 

 

I'll check the specs at 200f and see what they look like. I'm curious. 

Edited February 8 by wmax351

[wmax351]

There was also quite a good flow of air out of the breather and though I didn't check leakdowns, I believe a lot was going through the crankcase. 

[66Vette]

Seems pretty odd to me things are so far off.  I looked at your piston spec sheet and the alloy was not specified.  I would guess if they are calling out .006" clearance, the pistons are made from 2618 aluminum alloy as opposed to 4032 alloy.  2618 is typically used in applications where the pistons will see more abuse.  2618 is more ductile but has more thermal expansion so piston clearances go up.  At least that is the old school pitch on piston material.

 

If you have a build sheet from the machine shop, it seems like (maybe) they were somewhat reputable.  Hard to believe they were that far off in dimensions, and I doubt you would see that kind of wear.  Is it possible someone was trying to make this a turbo build and added even more clearance for boost.

 

Either way, rebore and new pistons would be the way I would go.  You've got the whole thing torn down, a shame not to make it right.  Modern pistons and rings are also better.

 

 

[wmax351]

Yeah, I was pretty puzzled. The shop that bored it in the 90s is some long gone random shop in Albuquerque. So could see them having gone too far maybe? Could have been a shop usually working on old American stuff and added some extra clearance "to be on the safe side." 

 

I'll put the pistons on the classifieds cheap. They'll work great for someone who is going to bore to 90mm, and they can just stop at 89.92mm or so. 

 

What is so weird is the non-wear areas are also too big. It really looks like it was all bored an extra thou or two over. 

 

It was built to be high performance, so thus might be what they were going for. But definitely never turbo. Nothing in the POs notes had anything for forced induction. 

 

I did get a call back from max-sil and they still have pistons for the 2002 for $130 each. So that's another good option for folks in the future. 

[wmax351]

Hmm. With the piston to wall clearance spec, is it bore diameter - piston diameter, or is it 1/2 of that?