Zak Shaker

Hello,
I'm doing an engine rebuilt.
what are the difference between HF and HF4 pistons ?
I broke my STI my99 which came with HF4 pistons
I got a new block, which has HF pistons. both pistons set are ok, so I don't know which one I should use in this new block.



HF seems to be high CR than HF4,
anyone could confirm this ?

Zak Shaker

anywone ?

I beleive HF are VER 3 standard pistons while HF4 are VER 7
anyone could confirm ?

Kwik

iTrader: (10)

If they are technically the same bore but higher CR then the difference would be in length or crown of the newer pistons. Although there are apparently very slight different bores in each block, so i'd suggest putting them back as you found if original pistons are fine (though i'd change the rings).



Sorry i don't know the difference but hope this helps.

merlin24

iTrader: (1)

The piston on the left looks like a STi Version 5~6 and the one on the right looks like a STi Version 3~4.
You can measure the Compression heights of each piston to see if they are different or using one gudgeon pin,link the 2 pistons together and sit them on their skirts - you will see any difference in Compression height.

Mick

Zak Shaker

thanks for your help

merlin24

iTrader: (1)