Should I buy these pistons?

[Oakland Jeff]

The seller bought these Ross forged pistons and 144 mm rods years ago from Ireland Engineering.
According to the label on the box the bore size is 3.5430” or 89.992 mm

piston diameter is 3.5390 or 89.891 mm

If I did my math right this means that the pistons are for a 4th overbore hole, which would be about 91 mm. 
conflicting info on this site says that up to 92mm is fine… or that you shouldn’t go over 90mm. 
 

The rods and pistons will be shipped to me along with an M10 block and a crank that look good but have not been checked by a machinist. He says he “believes” that the block is standard bore.  According to the seller Ireland Engineering said that the pistons will work with his E12 head, which I’m also buying. My car is a ‘75. 

As long as the block is good is there any reason this would be a bad idea?

[2002iii]

I don't know what kind of math you did,but the bore size is just under 90mm so you should be fine. 

 

The box says for e21, usually e21 and e12 are not compatible.

 

I would call Ross and send them the above pictures and see what they say.

 

Those look like very high compression pistons, all of this stuff is going to have to be checked out by a machinist for fit and clearance.

 

BTW I'm also in Oakland, send me a message if you want to chat or hang out or need help. I have a 1974 2002 in pastel blue, I do most of the work on it myself.

[Oakland Jeff]

You may have just saved me a bunch of money! The tech at Ross said no go with a E12 head, so but that I should contact Ireland engineering. I’ve sent them an email, hopefully they will respond.

 

where in Oakland are you? I am near Lakeshore. Picture taken last Saturday at Sonoma Raceway. Car handles fantastic, motor is weak!

Thanks, Jeff

[mac]

Don't buy the E12 head and buy an e21 head instead. 

 

Mac. 

[TobyB]

From memory, the E12 head's right around 60cc.  Subtracting 16 from that takes you down to 44 cc, which on a 500 cc

cylinder gives a pretty high compression ratio, even allowing for squish, quench, valve recession and crown recession...

 

The E21 chamber is a bit bigger- but not that much.  There are numbers in an old thread... somewhere.

 

t

would run those.  bkuz racekar

[Oakland Jeff]

Found the thread:

 

if my math is correct these pistons with an E12 head will yield 11:1 CR

[kbmb02]

On 3/28/2024 at 12:39 PM, Oakland Jeff said:

As long as the block is good is there any reason this would be a bad idea?

 

 

Are you building a street or race engine? If street, investigate the forging material of the pistons (if 2618 alloy, they’ll likely be noisy due to required clearance spec… and, other possible things you may not like). -KB

[Cabbage Fumes]

6 hours ago, Oakland Jeff said:

if my math is correct these pistons with an E12 head will yield 11:1 CR

The reality, that has already been determined, is these pistons and an e12 head are incompatible. Why is the e12 head still being discussed at all? Buy these if you intend to also buy an e21 head. Otherwise move on.

Edited March 30 by Furry Camel