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Andre and friends
I am rebuilding a 2JZ-GTE motor and I wonder if you could advise me on the clearances. Of course I am aware of the factory specs and tolerances but the engine features now forged HKS pistons which expand more and the power output mark is around 850 bhp. Given the engine will be operated at a higher than factory output level, I gather from engine builders that the clearances must be increased and I will very much appreciate when you share your vast knowledge here.
Thank you very much.
There's an "old rule of thumb" and that's 0.001" of clearance for ever inch diameter, I believe a few members on here use this without any issues. you'll just need to watch your oil pressure as the extra clearance will allow more oil flow which will drop the pressure slightly
Thank you for the reply.
do I understand correctly, this rule of thumb includes both, piston to wall and bearing, clearances? And does it, somewhat, include diffent piston materials and power levels?
Looking forward to all of your inputs, thanks.
no sorry, this is for bearing clearance. piston/piston ring clearance is target power dependant
0.001" per inch of journal diameter is indeed a good target for clearances. Generally I would aim for the loose side of factory tolerance and this should be adequate for a high power application. If you go too loose then you're going to potentially see low oil pressure, particularly at idle. I tend to base my clearances on what I see during a tear down of the engine too - ie how are the bearings wearing, and is there any sign of metal to metal contact? That of course comes after the fact though and with no experience on a particular engine you need a starting point first.
Piston to bore clearance is a bit harder to advise on, particularly as I haven't used the HKS forged pistons. A big consideration here is the material the pistons are made from. 2618 for example is the preferred alloy for high power engines and expands more with heat and hence requires more piston to bore clearance (I ran around 0.0045" on an 85.5 mm bore in a 4G63 with a 2618 JE piston for example). 4031 is the other popular alloy and has a lower thermal expansion rate and hence can run a tighter piston to bore clearance. I have no idea what alloy HKS uses so you would need to find this out.
I'd start by finding the HKS information regarding the pistons you have as this will provide advice on the required clearance. You certainly don't want to get this wrong!
Thank you both for sharing your knowledge.
Just to update this post... I have opted for pistons of a known material and advanced design and precise manufacturer clearance specs.
Those are asymmetrical Wiseco pistons, pics attached, with thicker pin and larger / deeper pockets, CR 9.6:1. This new design is rated to withstand 1600 bhp, so plenty of safety margin here.