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1275 S Pistons (9 replies)

colinandvenie
6 years ago
colinandvenie 6 years ago
colinandvenie
6 years ago
colinandvenie 6 years ago

Hey guys,
When I stripped my engine after many years of sitting in a garage, I am sure I pulled out flat top pistons. They were unuseable, as the rings had welded themselves into the pistons. So I used 1300 pistons, with the dish, to get me mobile.
All the new pistons that I have looked at are dished too-maybe not quite as much. Would I be losing too much compression with my existing ones??
Would the compression be too high with flat top ones???
I am going to strip the engine in a few days, to do the cam and an oil leak, so was going to look at a rebore-plus 10thou- to give me 40thou over, with new pistons
Thanks
Colin

22g68
6 years ago
22g68 6 years ago

Pistons

Colin.
Strange that you would have flat tops in a 1275...unless it was used for competition. Using the 1300 pistons would drop the compression way down...possibly around 9:1....be rather 'doggish'

Standard 1275 S piston will give you 9.75:1..if the block/head are also standard.

You need to establish what the cc is on the cylinder head...standard is 21.4 cc ish!
Then, you will need to check what the deck height (top of piston to top of block.( This may be around 1cc capacity depending on the height).005' is close to 1 CC.
Need to assemble the engine..(just need one piston instaleed..using a magnetic attached gauge to find top dead center..then CC the volume of the deck

Decent compression is approx 9.75:1 or even a tad higher. Of course it depends on what you want to use the car for.
The old hepolite dished pistons were what everyone used for the 1071/1275..think..if my memory is correct...is around 4.5 cc dish.
The 970 had flat tops as standard..nrequired to get decent compression.

Gasket is approx. 2 CC. Taking all of this into consideration...would provide an approx. compression ratio of 12:1...a tad high for the street. However,if your deck height is standard...then the C R will be lower also, and the bowl in the pistons will change everything...the 1300 pistons will drop this down as I mentioned above.
I use a burette and parrafin mixture to get an accurate CC reading of the volumes.'do not use water solutions as this will not give an accurate reading. Hope this helps.

colinandvenie
6 years ago
colinandvenie 6 years ago

I think it was used for racing. The head has been ported and polished, like you would'nt believe, the firewall had a weber hole cut in it and the cam had no markings on it whatsoever.
Also the deck had been shaved(I think) because there was no engine tag.
Looks like I may have to measure it all up and find out the volumnes
Cheers for the info
Colin

22g68
6 years ago
22g68 6 years ago

Pistons

Hi Colin
I suspected it may have been a competition unit. Now you have to start from scratch. If the deck has been shaved down...aim for .008-.005' deck height.
This is safe and will give good combustion. 9.75:1 is ideal for the street..gives nice torque is very tractable..with the proper cam
Good luck.

colinandvenie
6 years ago
colinandvenie 6 years ago

I measured dish volumn of my pistons today, as far as I can tell, the dish is 11.2 ml
I don't know how to translate that to cc ??
Colin

DaveShreeve
6 years ago
DaveShreeve 6 years ago

They're different ways of expressing the same thing, 1000 of each to the litre.

colinandvenie
6 years ago
colinandvenie 6 years ago

So, that would be 11.2cc ??
Colin

DaveShreeve
6 years ago
DaveShreeve 6 years ago

Exactly....

colinandvenie
6 years ago
colinandvenie 6 years ago

Also, the measurement from the top of piston to the deck is a whopping 60thou !!
No wonder I am not getting the performance!! I definitely need to sort this engine out !!!
Cheers
Colin

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