Hey guys,
Im looking to rebuild an extra motor I have from my 87 mr2... the valve covers have been replaced, but its a 7 rib, big port head, with TVIS... I would really like too use some of the later 4age high compression pistons used in the SP engines. My bit of research has shown that the later pistons use a 20mm pin, while my engine uses an 18mm pin. So my question is, is there any way I could source a factory high compression piston with an 18mm pin? I've read that different markets (JDM, US, EURO) had different variants of the 4age with different pin sizes. Thanks for the help guys!

Have you measured your wrist pin?
7 rib should be 20mm.
I can get Wiseco pistons with an 18mm wrist pin.
I think there was a higher comp 3 rib piston in like Thailand or something but good luck finding it.

I think the mk1a mr2 (blue top) high comp pistons are 10.3:1 with an 18mm pin - the red top 20mm pin are much more common.

Although oddly I had some low comp 9.4:1 20mm pin pistons come out of my corrola ae92 lump.

Alex170984 wrote:I think the mk1a mr2 (blue top) high comp pistons are 10.3:1 with an 18mm pin - the red top 20mm pin are much more common.

Although oddly I had some low comp 9.4:1 20mm pin pistons come out of my corrola ae92 lump.

The red top blue top thing does not work on our motors.
There have been three variations in the US and people always mix them up.
There was the three rib large port which had the 40mm crank and 18mm wrist pins with 9.4:1 compression.
There was the 7 rib largeport with 42mm crank and 20mm wrist pins. It had the same 9.4:1.
Then there was the smallport. Needless to say it had a 7 rib bottom end as did all following 4As along with 42mm crank and 20mm wrist pins. It had 10.3:1 compression.

holy crap.... waaay too much mis-information in this thread for only 4 posts.

40mm/18mm N/A internals: 9.4 to one throughout the world, except Singapore, and areas in Europe without a cat - these areas got 10 to 1 pistons.

On the left is 9.4, on the right is 10 to 1.




42mm/20mm N/A internals: 9.4 to 1 from 1988, and 1989 (SOME 1987) piston crown looks identical to left piston in photo above. 10.3 to 1 compression was introduced in 1990 the piston looks VERY similar to the piston on the right in the view above. I do not have as up close a photo of the high comp, but I do have a photo... note the coating on the piston crown

So what I need to do is pull apart my motor, see if I have the 20mm wrist pin, and if so I should be able to order factory high comp pistons through Toyota for a later 4age, correct? I believe I found a set through toyodiy for some sort of euro or jap corrola... as long as I end up with the correct oversize and wrist pin, I should be good to go?thanks everyone for the (mis)information!

Also, in your photo it seems that the 9.4 pistons combustion area sits just below the ring around the edge of the piston, where the high comp one is just above the machined ring... is that what im seeing?

benniemr2 wrote:Also, in your photo it seems that the 9.4 pistons combustion area sits just below the ring around the edge of the piston, where the high comp one is just above the machined ring... is that what im seeing?

yes....

There is a factory high compression piston with 18mm wrist

If want help sourcing it lmk but only if your serious

13103-16030 are the 10:1 oversize cast pistons with 18mm wrist pin (Singapore & some Euro market). This was just posted in OST's most recent engine rebuild & head porting topic. There is no Toyota 4A-GE 10.3:1 piston with 18mm wrist pin. I guess 10:1 would be "high enough"