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Opel Rallier since 1977
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Discussion Starter #1
Hey all,

I am taking a long shot that someone might have some good info on this question. I have a set of old Arias forged pistons that are probably 15-20 years old, that I am planning on putting in a block for street use. I have had them over a dozen years.

My machinist and I are debating the clearance to run... my only experience was with the old TRW and Hepolite's and I would run them at .0035 to .004" with no issues rallying. He is thinking more like .006", which I don't like for just running around on the street.

These have no real markings.... the are flat tops, 2 eyebrows, and a raised .052" high flat dome. Under the crown they say 'Arias' and 'Alcoa' and '40' in cast letters. The material coloration is bright aluminum, rather than any sort of grey like the old TRW's were, so IDK the actual material. There are no steel strips in the skirts to control expansion. So any ideas on the piston-to-bore clearance would be appreciated.

Thanks!
 

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1,567 Posts
.004 if piston has oil groves in the skirt and .006 if it has the smooth skirt with floating pins.
HTH
 

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Opel Rallier since 1977
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1,587 Posts
Discussion Starter #3
Tnx Guy. Is that from some rule of thumb, or some Arias knowledge, or where? These have grooves across the skirts, not the usual fine ones for production type pistons, but individual ones, maybe .040" wide and spaced about 1/4" apart. And floating pins.

What is left of the Arias site is not much use anymore.... CP-Carillo bought them 3 years ago. I shoulda taken pix before taking to the shop.
 

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I have used 3 sets of Arias piston, of that early style.
The new design pistons now available are an improvement and I like the thinner ring designs and that has skirt and piston top coatings.
 

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Opel Rallier since 1977
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1,587 Posts
Discussion Starter #5
Ah OK, that is good. Yes these are definitely older pistons.... I have had them for more than a dezon years and got them off of eBay from what may have been an estate. So do you think these are the .004" or the .006" variety? Thanks very much.
 

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Hey Manta Rallier, You can try to contact the SCTA-BNI.org (organizers of Speed Week at Bonneville). The whole Arias family were members of the Gear Grinders Club and Nick 3 (I think) is still a member. I'd ask him the next time I see him, but I'm guessing that August might be some time off your schedule! You can also try an email to [email protected] Howard is the current Gear Grinder secretary so he could forward your questions to Nick. HTH -- Doug (former Gear Grinder Secretary) :yup:

The GG are a member club of the SCTA & BNI. Southern California Timing Association/Bonneville Nationals Inc
 

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Opeler
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49 Posts
I used a lot of Arias piston and Carrillo rod combos in the latter half of the 70s, mostly 95mm bores in 1.9 blocks for rally or road racing motors.

The rods were great, never a failure, and never even measurable deformation requiring re-sizing.

Most of the pistons worked fine, but I did have one set that lasted only 5 minutes before expanding to the point of destroying themselves on the cylinder walls. Arias replaced the pistons, evidently they used the wrong piston blanks that had a bad expansion profile, something from their drag racing side.

So, can't help you with the clearance required, my 70 yr old memory isn't that good, but I think they mostly were designed for smaller clearances.
 

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Opel Rallier since 1977
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1,587 Posts
Discussion Starter #9 (Edited)
I found what looked from the pix to be the identical Arias piston slugs, but cast a little larger and machined for a Porsche 928 engine. The Arias spec sheet showed .003" clearance.... BUT that 928 engine is an aluminum block V8. So for putting that piston in the lower expansion rate cast iron block, I decided it better to be in the .005-.006" range. I don't like too much clearance for the street, but I also know how these bores can distort when the head and bellhousing are attached, and I'm not using torque plate in the block when boring/honing, so better safe than sorry.

See post #70 in tis link for the piston sheet if you are curious: https://rennlist.com/forums/928-forum/229007-928-motorsports-can-repair-scratched-cylinder-walls-5.html

Thanks again for all the input!

BTW, BruceB, have you ever posted over on RallyAnarchy?
 

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Living in the past
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2,280 Posts
Piston skirt clearance

I found what looked from the pix to be the identical Arias piston slugs, but cast a little larger and machined for a Porsche 928 engine. The Arias spec sheet showed .003" clearance.... BUT that 928 engine is an aluminum block V8. So for putting that piston in the lower expansion rate cast iron block, I decided it better to be in the .005-.006" range. I don't like too much clearance for the street, but I also know how these bores can distort when the head and bellhousing are attached, and I'm not using torque plate in the block when boring/honing, so better safe than sorry.

See post #70 in tis link for the piston sheet if you are curious: https://rennlist.com/forums/928-forum/229007-928-motorsports-can-repair-scratched-cylinder-walls-5.html

Thanks again for all the input!

BTW, BruceB, have you ever posted over on RallyAnarchy?
The rule of thumb for a cast iron block street motor in usually .001 per inch diameter of the piston, which would be approximately .004. If you are going to run these in street motor I would run no more than .005, and if you are going to run them with floated pins I would run the teflon "buttons" in the pin holes to prevent the possibility of "pin walk" and keeping the piston from rocking in the bore. I ran Arias piston in the race motors and never had a problem even at 7500 rpm's. I would look into total seal rings with the gapless ring in the second groove for much better sealing on compression.
 

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Opel Rallier since 1977
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1,587 Posts
Discussion Starter #11
Tnx nut10. Yeah, I don't want to go larger than necessary. I ran the old TRW forged pistons at .004" but these are a bit different, and have no steel strips in the skirts for expansion control like the old TRW's had. And my old school local machinist is of the same opinion to be around .006"; I gave him a .005" to .006" range to work to.

As for the Total Seals: I tried them once in my rally engine and they just wore the bores down fast with no improvement in cranking compression whatsoever. I bet they do indeed help leakdown rates quite a bit, but unless you are running WOT all the time and looking for every last HP, like in your drag racing efforts, the amount of time at WOT is very small for regular street operation, and the improvements in leakdown rate are pretty meaningless as the cylinder pressures are way down anyway at part throttle. So they just add drag and bore wear to no good purpose, IMHO.

I'll be sticking with some good moly top rings in a standard Hastings ring set. Interestingly, I found some true moly 'insert' rings (not moly plasma coated ones); I bet you can remember those early moly types!
 
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