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Here is some info to get started. All of these measurements are approximate and the pistons are original Subaru parts.

 

1987 EA71 (JDM) OHV 1600 top of piston pin to top of piston (flat top - no valve reliefs)  note, stamped "B"

0.851" or 21.61mm

 

1984 EA81 (Hydro) OHV 1800 top of piston pin to top of piston (flat top - no valve reliefs)  note, stamped "B"

0.830" or 21.08mm

 

1988 EA82 (Spfi) SOHC 1800  top of piston pin to top of piston (flat top - with two valve reliefs)  note, stamped L or R and UP

0.862" or 21.90mm

 

I'll try and post a link to some pictures soon. I'll also record clearance to the deck at TDC with one of each type installed. This block has .005 removed from each case half deck surface for clean up, and both heads have .020" removed each. The intake bolts do still line up without any problems.

Edited by Crazyeights

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I just received a new set of 0.5 OS pistons, EA82 flat tops with valve pockets. I'm using them on an EA81 rebuild. This it what I measured so far:

 

1979-1984 EUDM EA81 pistons - FLAT TOP - 33.0 mm CH

1979-1984 EUDM EA71 pistons - FLAT TOP - 33.0 mm CH

1987-1992 EUDM EA82 MPI pistons - FLAT TOP with VALVE POCKETS - 32.0 mm CH

 

I ordered the EA82 pistons (EUDM MPI are the same as USDM SPI) at Subaru Genuine Parts - Jamie was really helpful BTW.

 

The EUDM EA71 and EA81 pistons have the same part number, at least according to my 1979 parts catalogue. The EA71 has 9.0 compression ratio, the EA81 has 8.7 compression ratio, so the volume of the bath tub in the EA81 heads is bigger. Can post pics tomorrow of all three pistons if you like.

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Here is some info to get started. All of these measurements are approximate and the pistons are original Subaru parts.

 

1987 EA71 (JDM) OHV 1600 top of piston pin to top of piston (flat top - no valve reliefs)  note, stamped "B"

0.851" or 21.61mm

 

1984 EA81 (Hydro) OHV 1800 top of piston pin to top of piston (flat top - no valve reliefs)  note, stamped "B"

0.830" or 21.08mm

 

1988 EA82 (Spfi) SOHC 1800  top of piston pin to top of piston (flat top - with two valve reliefs)  note, stamped L or R and UP

0.862" or 21.90mm

 

 

Here's some more data, actual measurments in a EA 82 block. The EA 81 piston at TDC was .040 down in the cylinder. Using the above measurments the EA 71 piston would be .019 down in the cylinder. The EA 82 piston is flush with the deck, so it's taller than both of them. The EA 82T piston sits above the deck by like .017. Both the EA 82 pistons would reduce quench, less prone to detonation. Left to right, EA 82 SPFI, EA 82T, EJ 25, EA 81. The center piston is out of a Top Fuel Dragster. Guy handed it to me after a quarter mile pass between rounds rebuild. The early EA pistons weigh a lot more than EA 82 pistons.

18436572.jpg

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Wow, that's one whacked out picture. Right after posting I had to kill off some intruders inside my computer. I'll post a better one soon.

 

Edit: EA 82 SPFI, EA 82T, EJ 2.5, EA 81.

 

18436572.jpg

 

 

Twitch, what you would need to do to see which piston has the higher CR is take a large syringe graduated in cc, piston in the hole and fill up that space from top ring to the deck. Record the CC it took.

 

Do the same with the EA 82 piston, fill in the valve reliefs and top ring to the deck. Record the CC it took.

 

Or do some fancy calculations...however, the EA 82 piston still has less quench, piston to head.

 

HTH

 

Doug

Edited by Quidam

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The EUDM EA71 and EA81 pistons have the same part number, at least according to my 1979 parts catalogue. The EA71 has 9.0 compression ratio, the EA81 has 8.7 compression ratio, so the volume of the bath tub in the EA81 heads is bigger. Can post pics tomorrow of all three pistons if you like.

 

Another possibility is a difference in head gasket thickness. 9.0 to 8.7 is like the difference between a Genuine Subaru head gasket and a Fel Pro headgasket. The Fel Pro is about .010 thicker.

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OK I didn't know that. Unfortunately I forgot to measure the thickness of the (original) EA71 and EA81 head gaskets when I disassembled the engines.

 

I have also just ordered a EUDM 85-91 EA71 piston, it has an 'AA' partnumber so I suspect it will be of a similar design as the EA82 SPI but with flat top crown. That would be one great piston.

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International Harvester engines were like that, Rebuild them with a brand new Fel-Pro Gasket set and they run like poop.

(heh, I typed "poop")

All because the original head gaskets were just a single thickness of copper.

The Fel -Pro replacements were about three times thicker!

didn't look like much, but it was enough to gap the valley pan clearance, (read that as OIL LEAKS from Heck!) and drop the compression ratios down to about 6:1...

Not bad if all you wanted to do was look at it, but starting and running it? Don't get in a hurry to go anywhere.

 

Shaving the heads (or decking the block) Made all the difference in the world!

 

Maybe for subarus it would too?

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Looking for new/nearly new EA71 pistons for a high comp EA81 build.

And finding these little buggers is proving to difficult. Not more than I anticipated, but still tough.

I can get ring sets all day long, but the pistons themselves are tougher finds.

I'd go to a wrecking yard and get some, but with as much strain as they'll be under, I don't want to toss something in there that may have issues.

Because pulling this motor back apart may not be an option for some time.

And it'll be dealing with me trying to tune it and figure out what octane it'll take it to prevent ping...

Anyway, anyone got any to sell? I've got cash and am willing to deal.

 

Twitch

 

PS: OEM Part number is 380050100 if anyone was wondering

 

 

Yes,

 

http://www.subaruparts.com/cart/?pn=380050100+

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Something to keep in mind. A hi po build like you're doing benifits from tight quench, piston to head clearance. You can probably go as close as .025 with this build, but not any tighter. That's IF you have enough valve to piston clearance. The tighter the quench makes it less likely to "ping".

 

The FelPro gasket is probably just a regular head gasket coated with teflon, hence the greater thickness. I'm not saying there is anything wrong with them except the drop in compression AND adds .010 to piston to head clearance.

 

It's good to have a choice where adjustments need to be made. I'd actually like to see someone make a .030 thick gasket for the EA 82...instant compression jump, tightens the quench to .030, and with our low lift cams, plenty of room for it, piston to valve.

 

Just some thoughts, hth.

 

Doug

Edit: Keep in mind with your pushrod engine, when you deck the block and/or heads, it alters the valvetrain geometry. I have used adjustable pushrods before in an Oldsmobile, and custom pushrods are readily available for other makes, probably not for your engine though.

 

Look at the rocker arm pad where it pushes on the valve. You'll notice the wear pattern is centered on the pad. I've used prussian blue and even a sharpie to check this during assembly.

 

With shaved deck and heads that pattern shifts to a different spot. In short it sideloads the pressure on the valve guides and increases wear on the guides and is not a smooth action compared to being centered.

 

Oil...I would use a break in oil with your reground cam, more zinc and phos. Highest point of friction in your engine is at the cam lifter interface. Insurance against wiped out cam lobes.

 

Just thinking...hth

Edited by Quidam
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In that case you could also put spacers under the valve rockers (which I did many years ago when I shaved over 1 mm off my EA71 heads). Makes it a little more difficult to seal the valve covers though!

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That is a eraly ea82 piston the later ones are short skirt WITH DRILLED OIL RING FEED HOLES why is that piston so hard to find ?

 

I've got a Loyale EA I'm tearing down and I'll see what's in that. '88 and '89 I have in hand has slots, not drilled. I'd like to see one, anybody got a pic of one?

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In that case you could also put spacers under the valve rockers (which I did many years ago when I shaved over 1 mm off my EA71 heads). Makes it a little more difficult to seal the valve covers though!

I looked at the PAECO web site yesterday and they do offer new pushrods, custom length, for $24.00 each. Ouch.

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Just throwing this out there...as I know a bit about pushrods. A checker adjustable push rod can be had from many sources. With it you measure how long your pushrod needs to be. I had one but got a car with a non adjustable valve train and it had a slight tick on one cam lobe only. Well, I put that checker in and never looked back.

 

I take it you're dealing with aluminum push rods which I've never laid eyes on.

 

Anyway for instance.

 

Empi has a VW bug kit for $65.00. It's only the tip of the ice burg of what's out there from multiple sources for lots of vehicles.

 

Chromoly push rods are lighter and stronger than regular
steel push rods. The 3/8" tubing offers maximum strength to maintain no flex
operation at higher RPM’s under the stress of high lift cams and high-rev. valve
springs. Custom Cut-to-Length style.
 

 

4034
-
Cut to Length, 11.600"
Long, 3/8" Chromoly Push Rods, Set of 8

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