# Piston options for 9A w/JH head



## drew2fast (Jul 27, 2004)

I'm replacing my 84 jh lower end with a 9A. Does anyone know what my piston options are with a jh cyl head?
It has been difficult finding the interchangeability of these engines with the search.
So far I know that they are basically the same except the ABA has a 16mm taller deck and 16mm longer con rods, same bore and stroke, 82.5X92.8. The 3A, ABA (93-95), came with an 8v head, oil squirters and forged internals. The 9A is the same except it came with a 16V head.
It seems that I should be able to run 3A pistons in the 9A block with the 8v JH head. Let me know if this info is wrong. Does anyone know the CC volume of the 3A 8V and the JH 8V?


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## B4S (Apr 16, 2003)

*Re: Piston options for 9A w/JH head (drew2fast)*

Huh? What are your goals? If you put the JH head on the 9A bottom end, you'll get super high comp (13:1 or so IIRC), no custom pistons required. If you're interested in building an 8V with the squirters, get a PG block from a corrado.


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## drew2fast (Jul 27, 2004)

*Re: Piston options for 9A w/JH head (B4S)*

My original JH bottom end lost the crank thrust in a bad way, found a 9A short block locally and would like to increase the displacement. I would like net 9.5-10.5:1 compression with either an OE cast piston or forged after market unit.


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## Fox-N-It (Jul 23, 2003)

This question has come up quite a bit recently. Run through the fist 4 pages or so of the 8v forum and your sure to find something.


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## Ben B (Bengineer) (Dec 20, 2006)

*Re: Piston options for 9A w/JH head (drew2fast)*

I recently did a block swap in my 86 Scirocco. I used the ABA block with a JH counterflow head. I cc'd parts on both engines (ABA and JH)... 
JH head combustion chambers were 29.5 cc 
Crossflow head was 31.5 cc 
Both heads were without milling and cleaned down to metal.
So, when using the ABA block(1996) and JH counterflow head (1984) the compression was *10.3:1* (same as the Audi 3A). (the 3A and JH had the same piston shape, but the cc of the dish is different due to the 82.5mm bore on the 3A and 81.0 mm bore for the JH pistons. I have not checked the piston dish vlume of the 3A, because I have never had one.
I also have a 9A block and checked into fitting the JH head...after checking the compression ratio is way too high for a daily driver street engine on pump gas....13.4:1 cr.
I think the 3A pistons would work (for piston dish volume and overall height (not stick-up above block )...as long as the piston pin is the same diameter and type.


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## Ben B (Bengineer) (Dec 20, 2006)

*Re: Piston options for 9A w/JH head (drew2fast)*

From archived post..........
Re: (Campbell) » « » 8:44 PM 1-25-2007 

Guys, I just got done testing several heads for volumes (with a clean, un-milled head, plexiglass/oil water syringe, etc..
The JH 1781 cc enine used a counterflow cylinder head with a head chamber volume of 29.5 cc to be exact. I checked everything, so here goes (I had to go back and look at what I wrote down).
Displacement Volume = 445.25 cc (stroke x bore area x 4) (1781/4)
Cylinder Head Volume = 29.5 cc (cylinder head chamber volume)
Piston top Volume (just in piston) = 18.9 cc 
Head Gasket Volume (compressed) = 8.1 cc (hole is 82.3mm) 
Air gap (above piston at TDC) (-quench pad) = 2.4 cc

Formula: CR = DV + CHV + PV + HGV + AG
-------------------------------
CHV + PV + HGV + AG
CR = 8.56:1


Ben B (Bengineer)
Member
Re: (Ben B (Bengineer)) » « » 11:35 PM 1-25-2007 

2.0 L ABA (1984 cc) info:
Displacement Volume = 496 cc (stroke x bore area x 4) (1984/4)
Cylinder Head Volume = 31.3 cc (crossflow head chamber volume)
Piston top Volume (just in piston) = 14.3 cc 
Head Gasket Volume (compressed) = 8.3 cc (hole is 83.5mm) 
Air gap (above piston at TDC) (0.009 inch, 0.25 mm) = 1.2 cc
Compression Ratio Formula:
DV + CHV + PV + HGV + AG
-------------------------------
CHV + PV + HGV + AG
CR = 10.0:1

This ABA block and the counterflow JH head equates to 10.3:1
using the 2.0L 16V metal head gasket.
See my other post: ABA block with JH head - compression ratio and ping control......

*NOTE:* I did find out that the 3A and 9A pistons have 20 mm wrist pins, due to the standard (JH, 9A, 3A) 144 mm rods with 20mm pin bores. They both also have notches for the piston oil squirters at the bottom corner. So, I think they will work!
(You can't use the 3A pistons with the ABA block and rods (unless you could bore out the piston wrist pin holes to 21 mm and have to cut the pins much shorter (ABA pins shorter due to narrow section of piston). 
The 3A pistons look almost exactly like the JH (except 82.5 mm vs 81.0 mm), these pistons are much heavier than the ABA also.
The ABA block is actually 16.5 mm taller, while the rods are 159 mm (15 mm longer than the JH, 9A, or 3A at 144 mm) 
Hope this helps.



_Modified by Ben B (Bengineer) at 6:29 PM 4-8-2007_


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## drew2fast (Jul 27, 2004)

*Re: Piston options for 9A w/JH head (Ben B (Bengineer))*

Thanks for the info.
The 3A, 9A and ABA have the same basic configuration, same swept volume etc. The CC volume is close enough between the ABA and JH head, so that will not to be a factor. The only problem with running ABA pistons is that they use a 21mm wrist pin vs the 20mm for the A & Jh engines. Also the skirt on the ABA piston would need to be shortened on one side for added crankshaft weight clearance.
The 3A engine piston would be a nice drop in solution, but it seems they are only available from one source (Autotech) and they only come in a 83.5mm size, +1mm. Not to mention they're forged JE's which are a little pricey and way over kill for this application.
I would like to find a set of OE 3A cast pistons in either std or .5mm sizes for a reasonable price.


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## Ben B (Bengineer) (Dec 20, 2006)

*Re: Piston options for 9A w/JH head (drew2fast)*

I guess that would be the ticket...find a nice set of cast 3A pistons at the stock size (82.5 mm) or slightly over (0.5mm) if you need a rebore...but a re-bore and hone would require a lot more money and time for the engine removal and tear-down.


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## drew2fast (Jul 27, 2004)

*Re: Piston options for 9A w/JH head (Ben B (Bengineer))*

Boring is not an issue since the engine is completely apart and I have access to complete automotive machine shop. The bores are in great shape with no ridge and at their highest wear point they only measure about .001" over. I would risk just a few strokes on the Sunnen hone and std pistons. Although I would prefer a set of .5mm over pistons. 

_Quote, originally posted by *Ben B (Bengineer)* »_I guess that would be the ticket...find a nice set of cast 3A pistons at the stock size (82.5 mm) or slightly over (0.5mm) if you need a rebore...but a re-bore and hone would require a lot more money and time for the engine removal and tear-down.


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