01GT4.6

ok, this is a very technical question, please chime in if you know the answer(s). Here's the deal, I had my motor built, but a machine shop took it apart, went through it and honed the block out and now I'm having major problems (piston slap). After 350 miles I pulled the motor back out and sent it to another machine shop (very reputable shop) for a tear down and inspection. They said among other things there was .007" piston to wall clearance and I should have about 1/3 of that.

Here are the "factory" specs (sandard bore) from the guy that originally built the motor.

Iron block grade 2 cylinders = 90.213-90.226 mm
AL block grade 2 cylinders = 90.213-90.226 mm <-----me

Hypereutectic piston grade 2 = 90.179-90.196 mm
2003 Cobra piston grade 2 = 90.178-90.193 mm <------me

I converted these numbers to inches, and with the smallest piston and biggest bore there would only be .0017" clearance.

my motor was originally built by engine design leader at Roush, so I believe it was right to begin with.

I sent the motor back to the 1st machine shop so he could mic it himself, I'm quite sure the number can be manipulated and he came up with .0045"- .005". He showed me a sheet for Manley pistons stating that the clearance should be .0035" and he said that with a 150 shot I'd be good at .0045"-.005". He said that if I could come up with the spec sheethe'd cover it, if it showed he overhoned it.

Everyone that seems to know is telling me that .002" is okay but what I'm at is way too big and I believe it because it sounded like a diesel until it heated up.

Here is a quote from the original builder about the pistons "They were machined on the same production line as the '03 Cobra pistons, with one modification....the dish volume, which is 5.5 cc's. Every other measurement is identical to the '03/04 Cobra piston, that I can guarantee." He also sent me the Ford's published drawings formy parts, which looks like a huge blueprint in .tif format that is too big for me to make sense of. I'm sure I have enough info to get the job done, just want more facts!

I "thought" they were Mahle's but could very well be wrong. Does anyone know eactly what brand of piston and what the piston to cylinder wall specs are?

Thanks!!
Mike

01GT4.6

anyone?

DEALBRk

I looked hard last night. Couldnt find anything. I want to say that spec is usually in the haynes manual.

Mark99GT

Are the pistons 4032 or 2618 aluminum?

01GT4.6

I'm not sure, they are forged if that helps. And thanks for looking DEALBRk

straitwangin

Id call brian at THR.

racer02GTstang

i think they are mahle... not 100% sure on that... how were they made on the same line if the pistons are only a 5.5cc dish? thats good for like 11-11.5 compression when the cobras have about a 22cc dish for 8.5 compression??

they should be forged from 4032 steel... my manley pistons are made from 2618 alloy my specs say but they are forged steel?

im trying to find the piston to wall clearances for you... maybe it can help out in some way?

01GT4.6

thanks, as for being made on the same line... I guess having some pull at Roush helps. They are aluminum BTW and the CR is probably close to 11:1, I'm sure the port work on the heads brought the CR down some.

Mark99GT

4032 and 2618 are aluminum forgings. You can usually run a tighter piston/wall clearance with 4032 pistons. Some 2618 pistons are known to have pistons slap on start up until warm because they expand more, therefore requiring a looser piston wall clearance.

OEM 03/04 cobra pistons are Mahle and I think theyre 4032 aluminum.

The problem with your conversions resulting in .0017 may be the rounding that takes place since were talking about thousandths of an inch. That .0035 for the manley pistons might be correct, but those are 2618 pistons. If you're running .007 as measured, or even .005 with 4032 pistons its probably too much. I also looked and couldn't find the ptw clearance for OEM cobra pistons.

I'd also call the guys at THR for some advice,they know their stuff.

Let us know how you make out.

01GT4.6

I called THR and talked to Bob, he gave me some info but told me to call back in an hour. I called Ford SVT and talked to them they said that the piston to bore clearance shoule be .010mm-.026mm. I coverted it to inches and came up with .00039" - .0010" (39/100000" - 1/1000"). I called back to double check and they said that is correct!