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Old 12-19-2010, 10:23 AM   #1 (permalink)
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Lightbulb Valve cutouts on Piston Heads

I am partially rebuilding my 98 EJ25D , because if a siezed idler pulley broke a pulley ( DOHC ones were made of Metalic carbon? Looks like plastic but Brittle.) Which in turn caused Valve piston interference . But the other issue is the Cometic Head gasket i think is to thin according to the oem 1 which is s .051 ", the 1 have is .040" which i believe is find for a EJ251 Sohc engines , my piston deck clearance is slightly different then EJ251 Sohc , plus a difference in the pistons heads and length, slight difference. So instead of sending them back i decided to cut some half moon valve shapes into the top of the pistons , not very deep just enough so i can use the thinner head gasket. I won't have it done for a while as i need a adapter for my dremel . I would love it if i could get enough clearance as to make it a non -interference motor. This will give a slight boost in Compression because of the thinner headgasket. I also have put on a STI 10 MM oil pump.d Will update.
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Old 12-19-2010, 10:30 AM   #2 (permalink)
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This is work best left to a machine shop. I can't imagine doing this well enough with the engine together to run well. I've heard of old timers welding washers to the tops of pistons to increase compression, but seems a bit half-ashed to me. If you do it, make sure you leave no burrs.
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Old 12-19-2010, 10:42 AM   #3 (permalink)
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This is work best left to a machine shop. I can't imagine doing this well enough with the engine together to run well. I've heard of old timers welding washers to the tops of pistons to increase compression, but seems a bit half-ashed to me. If you do it, make sure you leave no burrs.
Yes there will be no sharp edges or burrs to cause hot spots. Its been done before by other DIY guys not on this forum , its someting i think is doable and i don't want to go thru the hassel of trying to get custom cometic headgaskets, since the guy i bought them from would have to get the Cometic company to make them , it would take weeks . I'm not doing this to get more compression , but it is a benefit.
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Old 12-19-2010, 10:59 AM   #4 (permalink)
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Did you pull the pistons or are you doing it in the engine? Can you take before/after pics with the 2 pistons next to eachother? I would be curious to know just how much would need to be removed to become non-interference?
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Old 12-19-2010, 11:43 AM   #5 (permalink)
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Did you pull the pistons or are you doing it in the engine? Can you take before/after pics with the 2 pistons next to eachother? I would be curious to know just how much would need to be removed to become non-interference?
I would like to make it non- interference , but i would have to spend time with figuring out the exhaust and intake lift and the space in the stock piston , gasket thickness and how thick are these pistons , how much can i actually remove without compromising the integritity of the piston. I don't know if i can spend that much time on this, it is a good idea. I will take some pictures , and yes i will being dong this with the pistonns in the block on a engine stand, I will take care to make sure coolant ,oil passages and piston area is sealed up and properly cleaned out before its put back together.
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Old 12-19-2010, 11:48 AM   #6 (permalink)
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You wont have valve to piston issues at using a .040" HG.

The issue with the EJ25D is that the piston protrudes above the deck height and Subaru uses the thick HG as a method to prevent the pistons from hitting the heads. The valves will still clear the pistons even with a .024" SOHC NA HG. As IIRC all the EJ25 NAs all have around a .100" valve to piston clearance to start with.

Thin head gaskets in DOHC ej25 (don't do it) - NASIOC

I can't say however if a .040" HG is thick enough to prevent the piston from making contact with the head. Best way to find out is to measure the piston deck height. Then if the installed(compressed) height of a .040" HG is more then the piston deck height it should be good to go. You can find out the installed height from the gasket manufacture.

If you are working off factory number for calculations you also need to know if the block has been milled, you will need to know this for the numbers to work. Best way is to manually check.

Milling the block adds piston deck height.
Milling the heads reduces piston to valve clearance, but does not change the piston to head clearance.
HG thickness plays a role in both valve to piston and piston to head clearance.
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Old 12-19-2010, 12:22 PM   #7 (permalink)
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You wont have valve to piston issues at using a .040" HG.

The issue with the EJ25D is that the piston protrudes above the deck height and Subaru uses the thick HG as a method to prevent the pistons from hitting the heads. The valves will still clear the pistons even with a .024" SOHC NA HG. As IIRC all the EJ25 NAs all have around a .100" valve to piston clearance to start with.

Thin head gaskets in DOHC ej25 (don't do it) - NASIOC

I can't say however if a .040" HG is thick enough to prevent the piston from making contact with the head. Best way to find out is to measure the piston deck height. Then if the installed(compressed) height of a .040" HG is more then the piston deck height it should be good to go. You can find out the installed height from the gasket manufacture.

If you are working off factory number for calculations you also need to know if the block has been milled, you will need to know this for the numbers to work. Best way is to manually check.

Milling the block adds piston deck height.
Milling the heads reduces piston to valve clearance, but does not change the piston to head clearance.
HG thickness plays a role in both valve to piston and piston to head clearance.
Thanks !!Flstffxe. i tried to get this answered before and the only people that did was C&G Sand rails , they said i need .040" clearance ?? Which is what i thought that gasket was giving me , but like you said once it is compressed maybe they felt it is to risky. I have no milling done to the heads, I also read on another forum someone had a problem with the STI Gasket .024" HG on a DOHC , with it tapping. I guess i will do the valve cuts anyways i just can't bear having any more valves bent because i did't stick to the oem gasket which is .051" thick. It will be a learning experience . Then when this is done i have MY 01 to do but i may get i little more Creative.
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Old 12-19-2010, 01:09 PM   #8 (permalink)
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Okay looking at the HEADS I see know how it is possible for the pistons to hit the heads , i kept thinking the heads had a full piston cut out in the heads but no so i still need to make sure the compressed HG .040 is enough.
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Old 12-19-2010, 02:15 PM   #9 (permalink)
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If it were mine I would suck it up and get the OEM or get the correct thickness after market gasket.
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Old 12-19-2010, 02:50 PM   #10 (permalink)
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If it were mine I would suck it up and get the OEM or get the correct thickness after market gasket.
I do have a Headgasket made by Eristic, sitting here but that would be to easy.It looks like a decent quality but i don't want to chance it , i sent a email to Sport Cometic about the compressed Thickness if the H.D.If its not enough i will lay the headgasket on the head and trace the circumference and grind out .002"
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Old 12-21-2010, 06:08 PM   #11 (permalink)
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.040" should've been plenty thick for the EJ25D. I have ran mine that way with no tapping. When you look at the pistons, you'll see the bathtub dish they have, so valve reliefs aren't going to do much of anything. The outer ridge of the piston is what's gonna hit. Yes, .024" is way too thin but even as thin as .027 should be just enough.
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Old 12-26-2010, 01:25 PM   #12 (permalink)
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[QUOTE=JStorm;948151].040" should've been plenty thick for the EJ25D. I have ran mine that way with no tapping. When you look at the pistons, you'll see the bathtub dish they have, so valve reliefs aren't going to do much of anything. The outer ridge of the piston is what's gonna hit. Yes, .024" is way too thin but even as thin as .027 should be just enough.[/QUOTE Okay Thanks , Were they Cometic H.G.s??
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Old 12-26-2010, 05:35 PM   #13 (permalink)
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.040" should've been plenty thick for the EJ25D. I have ran mine that way with no tapping. When you look at the pistons, you'll see the bathtub dish they have, so valve reliefs aren't going to do much of anything. The outer ridge of the piston is what's gonna hit. Yes, .024" is way too thin but even as thin as .027 should be just enough.
Okay Thanks , Were they Cometic H.G.s??
Yes, they were Cometic. That's all I buy anymore. :)
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Old 12-27-2010, 01:10 PM   #14 (permalink)
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Yes, they were Cometic. That's all I buy anymore. :)
Perfect !!, i am waiting for my Viton valve stem seals and i will start putting everything back together. Thanks again. J
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Old 12-27-2010, 06:34 PM   #15 (permalink)
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No problem. There's two different colors for int. and exh. valve seals. Damned if I can remember which is which right now.... food for thought!
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