Piston - Bore gap?

[AndyP]

I spoke to Aelred the other day and he told me to run mine at 0.003" with JE pistons.

[Jamo]

I spoke to Aelred the other day and he told me to run mine at 0.003" with JE pistons.

Are you going to run at this then Andy?

[S2central.net]

And with what rings ?

[A80Avant]

a person who recently used JE pistons on his Fiat(turbocharged which now produces 350whp@rototest with GT30R) has enormous blow by in cold starts and basically he has to add oil quite often.He used the correct bore sizes for his JE pistons and the rings came with the pistons,he did nothing there.
He claims that the bore on the cylinders was excactly as JE states WITH the honing done,and he runs at 2bar steady boost.

Now since i dont know the specs as i wasnt around to see the numbers for that engine,i woudlnt want to have all that blow by on my car with those pistons no matter how good they are.

On my engine i have no rattle in the morning,no blowby,and as far as oil concumption,i think i might have added around 200ml of oil in the last 7000Kms i have done.So spot on!

As for the braking in,i used 10W40 oil for the first 15min or so,and then 10W60 castrol after that.

[caterpillar]

0.003" are a nice clearance , if the piston rattle in the syl, the pistonringland are worn out quite fast....
more commen are too tight ringgap.... and have a rebore again....

[S2central.net]

Any advances on .003" then - this is almost getting microscopic

Paul

[mcandmar]

Jamo, i'm going to be running around ~.004", it a little more than i wanted but as i don't really have a choice without another block or pistons to work with so i'm going to stick with it.

As i understand it going to as much as .005" will still work fine with the side effects of a loose fit when cold, slight rattling, and possibly more oil consumption until everything has warmed up. Going too tight however is a total disaster situation, the more power you produce, the more heat you generate, and the more the piston will expand. If i was aiming for 500hp like yourself i would rather have that little bit of headroom just in case.

It does seem wrong when you hold a piston in the bore, you have twice the clearance that the stock pistons ran with which is quite a difference. But as everybody has advised me gapping the pistons rings correctly is far more important for a good seal to stop blow by and oil consumption than the actual piston to wall clearance.

[S2central.net]

Yes - the ring gaps are crucial here...

If MarkH is happy at .004" (ignoring the cold start tinkles) then that is one very trusty guinea-pig... You couldn't do badly and copy his selection of rings and gap dimensions to remove that doubt as well.

[Mark Halligan]

Just be sure that if you are getting a little slap and rattle when the engine is cold that you don't give it too much stick until the pistons quieten down a little.

I know that the 2618 alloy pistons are strong but I still whince when I hear that rattle, as it is after all caused by mechanical contact.

Its all a trade off really and I doubt that any engine with loose bore clearances is going to end up doing starship mileage before it needs a rebuild.

Another thing to remember is that any machined clearances are going to be dictated by the measurement taken of the pistons. If your pistons skirts come coated it will be hard to get an accurate measurement, and you will have to trust the manufacturers dimensions supplied on the job ticket with the pistons.

If I ever build another I would definitely go to 0.0035" or even as Aelred has said 0.003", he has built more I5's than anyone here and knows his stuff.

Just pay attention and time to your ring gaps, as you will cause a lot of damage if they are too tight and the ends butt up under high temps.

I have a proper rotary ring gapping tool if anybody needs it, just pay the postage and its yours to borrow.

My opinion only

Cheers

[GazzaG60]

best way to do it is to have the bores done to each piston. they are eall slightly different in size. thats way you dont get any tight or loose ones

send the pistons in with the block and if you are fitting them have the pistons labelled for which bore they are for

[aelred]

I must say as mentioned I use .003" clearance, I had no problems with 600+ bhp.
With rightly gapped rings at that sort of BHP and boost you do get a lot of blow by, you will need a good catch tank.

[Mark Halligan]

Its not just us that have these discussions over this subject, I remembered this Speed Talk Thread the other day and searched it out this morning.

I remembered it for the interesting comments these guys made about the reasons they believed the main piston manufacturers had 'upped' the P to W clearances on their pistons. Its all speculation but it does make interesting reading.

[Jamo]

Its not just us that have these discussions over this subject, I remembered this Speed Talk Thread the other day and searched it out this morning.

I remembered it for the interesting comments these guys made about the reasons they believed the main piston manufacturers had 'upped' the P to W clearances on their pistons. Its all speculation but it does make interesting reading.

Interesting read, I must say I was bewildered with the choice of pistons on the market, it came down to CP, JE & Wiseco in that preferred order for me.

Went with the tried and trusted JE in the end.

Think i'll go 0.003" AndyP, Terrybullwon are running this as recommended by Aelred

[Thuppu]

So the 0.003" clearance has been proven good?

My JE's seems to be 82,43mm measured from the skirt with a digital slide caliper. Is it the right spot to measure the pistons? On the box it says "BORE 3,248" = 82,4992mm (82,50mm). So the clearance would be with that bore approx 0,07mm(0.00276"). Does it sound right?

Tomorrow I carry out the block for boring...

[Jamo]

Thats what Aelread advised. He did mine to 0.0035 because I had a stroker