someone said 2.5 thou clearance...for high boost use.

any comments?

thanks, ian

 

It should have said on the info with the pistons ( Have you bought them? )
In another engine we had running with them, at +0.5mm, they reccomended 2 thou.

 

yeah got the pistons today, but there was no info sheet in the box. all pistons and pins are there, but the seal had been opened. i suspect it may have been purposely removed by RC DEVELOPMENTS as i had to kep moaning at them to get the parts here, about 10days after they said they would!!

i guess theyve removed it to try as p*ss my off...

 

0.5mm is a little too much for that type of piston, in the sense that it wont expand much but use oil!

 

Why dont u ring RC Development up and ask them.

 

What I said/meant was they are +0.5mm bore size, running at 2 thou clearance.
ie, the pistons are oversize, not std. The block was rebored to give a 2 thou clearance.

 

Stevie,

.5mm....20thou clearance

Know you corrected it, just made me laugh though.

Right, lable removed, Cossie pistons, typical of GGR, who also happen to be the man supplier of said pistons.

They have actually done you a favour, because the box says 4thou (iirc) and that's FAR too much for our type of application, even running high boost. Run 4thou, and they will rattle like a rattly thing. 2~2.5thou max'.

They will still be a little slappy when cold, but will quieten down when hot.

The main problem, is that the Cossie pistons have a fairly heavy taper on them.

Mark.

 

when you say 2 thou, is that 0.2mm?

 

He means 0.002 inches which is 0.051mm (rounded)
JohnD

 

you probably know this already, but as per taper.

measure pistons at base of skirt at 90' to pin. crown may also be a little oval depending on design

ANyone remember David vizzard writing about knurling old pistons to make them a little bigger? not for slipper pistons though..

and while were on the subject of taper and ovality, cosworth used to grind oval bearings on the rods of the F1 engines to compensate for stretch. Can't rememeber if it was oval rods or crank journals.

back to work...

paul

 

Thanks, John

<Sarcastic mode on> Didnt the UK adopt the metric system 5 years ago? <sarcastic mode off>

 

260 million americans can't be wrong, FACT.

 

thanks for all the replies
stevie, they are 0.25 oversize mate, and will be rebored allowing an additional 2.5 thou clearance.
mark, good job they did rip the label off then eh! it would've sound like an old ford 2wd cossie engine slapping about!

cheers
ian

just gotta wait till monday to get the block back, then i will start my 'engine rebuild' thread

 

Mark, yeah you got it, although rc developments supplied them, they got them from ggr

 

Events in recent history seem to disprove that

You said that before, didnt you? What about the 500 million people who believed that the earth is a disc, then?

And hey, what's this got to do with Cosworth piston clearance anyway??

 

 

I didn't realise that the American weights and measures standardisation organisation had 260 Million members... wow... imagine the logistics involved in voting on a descision... now hat explains why they still haven't pulled themeselves into the metric age!

Moray

 

LOL!

 

Taper?? There is hardly any skirt on the damn things, never mind a taper.
The info that came with the pistons, from GGR did state 2 thou running clearance.

 

Ian as mark says .0025-003 about right for aroad car or it will rattle when cold, on a racer you would be looking at .001 per 1 inch bore size 92mm / 3.622 so .0036 skirt to bore clearance. Whe boring a block if i have any dout i will heat the piston to about
80 90 degrees and measure the skirt as steve said 90 degrees from pin just below pin, then measure expansion. expansion rate of alloy 1.7 of cast iron (cyl bore). Check your ring gaps, top ring min .015" would hope for .018-.020 max .025. As a point of interest if your checking used pistons and you have no taper on the skirt piston is scrap.
Steve.

 

Much as Mark and Steve say. I am running with 2thou clearance which gives piston slap until the engine is warmed through. Think this is what to aim for on a road engine.

 

If you're using a Cossie piston, you just have to accept that they will be a little slappy, when cold. I know someone running a 1.5~1.7 thou clearance with them, and he say he still gets a little slap when cold. Personally, I wouldn't run that close a tollerance.

As an example, I use JE pistons, with a 3.5~4 thou bore clearance, and they are less slappy than any Cossie pistoned engine I have heard (so far).

If you are running high boost, and intending to run the car hard, the track for example, you must allow enough for the pistons to expand.

Mark.