Compression height of pistons? 1275 vs 1500?

[1riktigsvensk]

Quick question; anybody know the compression height of the stock pistons? The OEM 1275cc pistons? 1500 7.5CR pistons? 1500 8.8CR pistons? Apparently the 1500 pistons are the same bore as the 1380 pistons....73.5mm. Plus, the pin is the same size. Now, I need to know which piston to use. The low compression or high compression piston for the 1500's. Any info would be greatly appreciated.

 

[Billm]

I just micked some stock 1275 pistons, compression height (center of wrist pin to crown) measured 1.496" but these are older so I would guess that it was 1.500 originally.
Bill

 

[1riktigsvensk]

Anybody have a 1500 piston for comparison?

 

[jim79]

Not sure if this is any help to you but I took the 7.5 pistons out and installed the 9.1 pistons(uk if I remember correctly). Seems like everything was the same but the pistons made the difference. If you need me to I can try to dig out the 7.5 pistons but I can't remeber if I have any extra 9.1 laying around.?

 

[1riktigsvensk]

If you have one laying around I would be ever so grateful for any info you could post.

EDIT: I just thought of something.....the 9:1 CR pistons would be taller to create the added compression. So if the 7.5:1 pistons have a compression height of 1.5" we should be in business.


thanks

 

[jim79]

Okay sorry it took so long to reply. I measured from the center of the wrist pin to the top of the piston and I got 1.290"I had to eyeball the center but it is not even close to the 1.5 you were hoping for.
If you got anymore questions let me know.

 

[jim79]

Oops forgot to say it was the 7.5 compression piston that I measured.

 

[1riktigsvensk]

Hmmmm....then my guess is that it is the 9.1 pistons that I need to use. Anybody have one laying around?

 

[Dadandson]

I am not sure about the 1500 pistons but the compression ratios on the 1275 pistons are changed by the top piston cavity, or lack of one. The wrist pin to top of piston I believe are constant. I may be wrong and if so it will be corrected.

 

[Hap Waldrop]

I am not sure about the 1500 pistons but the compression ratios on the 1275 pistons are changed by the top piston cavity, or lack of one. The wrist pin to top of piston I believe are constant. I may be wrong and if so it will be corrected.

LOL, not picking on you, but yes you can buy 1275 piston that 9.7 to 1 over the 8.8 to 1 replacment pistons, same dish volume, the 9.7 to 1 piston just have a bit more compression height.

AE 21251 8.8 to 1 piston
AE 21253 9.7 to 1 piston

As far as using 1380 piston in a 1500 block, I guessing the original poster has now fiquire this out, it won't work, but you can use the 9.0 to 1 piston and then cut the head to get the compression up, the 1500 has very healthy head deack, racer have cut these head .120"+, so a street can get to 10.0 to 1 (which is as high as I recommend for street use) pretty easy. You can use std. TR6 piston in a 1500, this works out to be .040" in the 1500, and of course you could go oversized with the TR6 piston, like .040 (which would be .080 over in a 1500) to get a over bore, these pistons require alot of decking but is do-able, I've built SCCA limited prep 1500 race engines dong this, it requires shorter tubular custom length pushrods and using the 1300 head gasket, but it works, it's old Rick Cline recipe. Not like alot of folks are thinking about it, buit time will change that, there are some formula to build some pretty serious 1500 street engines, that can give the A-series engine a run for their money, I've built 3 of the SCCA LP 1500 race engines, and one of them blew my Huffaker 948 away down the straight. Even though I primarly am a A and B series MG builder, I kinda hope one day a street guys wanting to build a pretty serious 1500 gives me a call, I think it could be a very fast A-series killer (ok, don't you A-series lover all smack me at the same time ) All jokes aside I see a new breed of 1500 Midget owners, and this new breed is looking to making the 1500 more powerful, and they are wanting some other answer than "just put a 1275 in it", and trust me, a whoop-butt 1500 it is very do-able.

 

[SilentUnicorn]

I like the way you think Hap




Mark
1500 owner