Ej257 head gasket thickness/ compression ratio!!
 

Hi need some advise I'm awaiting my block and heads to come back from machine shop,it will have had a 2"skim on heads and 2" of block,the pistons will be mahle 2618,and I believe will be first oversize.

Building the engine myself and need to order some head gaskets as not using the OEM ones in kit,was looking at cosworth,RCM, cometec any recommendations please tell free to advise,but the main thing is what thickness gaskets,

This is where I need help working out,or do i not really need to as the way i see it 4 thou of surfaces shouldnt make alot of difference in the big scheme of things,unless the piston dish is different from oem and with a slight oversize piston make that much difference? Standard crank and Manley h tuff forged rods..standard heads..

If your having 2mm off your block & heads there scrap, its usually 0.0508mm to clean them up, If its std spec 2618 Mahle pistons, & not the high comp type, you can fit std thickness gaskets 0.8mm & a very small rise in comp ratio, less than 0.2, or go & fit 1.1mm gaskets & keep if near OE.

Sorry typo error 2" thou of each head and on block = 4" thou total skim
So to be clear I will be fine with standard 0.8mm head gaskets,on standard compression forged mahle 2618 99.75 oversize pistons, on standard length forged Manley h tuff conrods,standard heads.. what's the best brand of gasket cosworth,ECM cometec or are they much of a muchness.

wouldn't entertain Cometic on a lawn mower...............

best and most cost effective option in our experience is the RCM

The only way to do it correctly is cc the pistons at tdc, cc the heads & work out the gasket required, as for gaskets its already been answered, & I would also not use a Cometic gasket on an EJ engine.

Yes they should be standard compression,4" removed during machine work, but the pistons will be .25 so 99.75,standard stroke would the 1.1mm gaskets keep near the oe spec 8.2.1, I can not find the cc of the cylinder heads online,otherwise I've gotta buy some equipment and try to measure everything,but I don't know what the compressed size is of a 1.1 gasket,I have the old OEM 0.78mm ones to refer to only.

What cylinder heads are you using, if 2.5 you have 25D WRX or 25B STI they are the common ones, using a 1.1mm gasket is the usual route after skimming, but if your having it re mapped you might as well go for a compression hike,& use a 0.78mm, although small, it will help spool & response.

Car is a 2.5 sti 2011 sallon dual avcs, Been told it will be 2 though off heads and 2 though off block,standard stroke 79mm. Mahle 2618 standard compression,but 99.75 oversize. Manley forged rods,As long as fitting 0.78 won't cause a problem with clearance and drivability,also if better response off boost is a plus, as long as when mapped i get nice boost pressures during the mapping and doesnt cause det etc

There is only one to check everything & that's measure it all, on a dummy build, seeing as you are doing the build yourself, you do not have the required equipment to do it, so its a guessing game.

Ok if i am to measure and use the online calculator i need to input head gasket thickness,on these calculators does it need to be the compressed or uncompressed thickness,i believe it is compressed but i cannot find out the compressed thickness of a 1.1mm hls head gasket.i have the oem 0.78 and can measure them.but need this to find out the corrext compression ratio.

Hmm, I've always assumed that the head gasket thickness given (i.e. the 1.1, 0.78 etc) is the gasket thickness after the head has been torqued down?

I've only rebuilt standard engines, though, so maybe I've just gotten away with it :)

I too would assume 0.78 is the compressed thickness. There's no point listing the uncompressed size.
You don't need to measure anything to get a good idea of what you need. The following works for a standard engine so may need adjusting for mods.
The swept volume is the engine size/No of cylinders. For a 4 cylinder, 2 litre thats 500cc per pot.
The combustion chamber volume above the piston at TDC is the sept vol/compression ratio. 10:1 CR means 50cc combustion chamber volume.
You are reducing that volume by skimming the heads and block. The reduction is 3.142 x bore x amount removed.
You now have the new combustion chamber vol above the piston at TDC. The new CR is swept vol/this vol.

Difficult to describe but easy to do. Hope this helps.

Hi firstly thanks for reply,
i have oem gaskets on the 2.5 sti removed/ used and the thickness is 0.69mm compressed,new 0.78 mm so i don't agree with this,and i was after 1.1 mm gasket compressed size.so if anyone has stripped there engine and they know it had 1.1 mm gaskets on could they clear this up for me please.

Stripped some 1.1mm RCM gaskets from an engine yesterday & they measure 0.94 to 0.96mm if that helps, other makes may vary, the 0.79mm OEM ones you mention, I get 0.65mm on some here.

Wow, I had no idea the gaskets were measured before being compressed.

Every day is a school day :)

there is no std new-age 2.5 engine that ever had 1.1mm gaskets on........they didn't use 0.78mm either as std but that is commonly used to take account of skimming, etc.......

I never said 2.5 sti std had 1.1mm gaskets! Please refer back up to the original question! I Asked what the compressed thickness was which has been answered by motors gt.
And the standard oem gaskets are 0.79mm from new uncompressed and you would need a thicker gasket like the 1.1mm to take account of skimming not thinner!!!!.. i am a technicain by trade, but trying to get correct info on here is very difficult sometimes,theres a few people on here that are very knowledgable don being one of them and thanks motor gt you have been of great assistance

I for one count Stockcar as one of those very knowledgeable people. I don’t understand why you’ve decided to go on a rant at him for what was technically correct and useful information. But then, what would he know about Subaru’s ;) :lol1:

:thumb: Was going to write something similar myself but decided not too glad you did.

I've not gone on a rant at anyone!!, and nothing has been directed at him,,i was thanking the people that had answered my question, Read it back or is it just me !:confused:.. but this part number 11044-AA770 is stated as a 0.80 mm gasket if measured is 0.79mm and fiited to 05 to 2010.