Forged pistons bore clearance

Wolf66 · Post by **Wolf66** » June 7th, 2007, 4:53 pm

Hi, everyone! I'm new to this forum and i have some questions.

I have an Alfa Romeo 155 Q4 with Lancia Delta integrale 16v engine.
A couple of months ago i swapped my fairly standard engine with other engine with forged pistons, gasflowed head, new cams, custom made inlet manifold, 12mm head bolts, group A head gasket...
I bought already built block and head with crankshaft and forged pistons from a guy, who stopped building his project.

This engine ran fine with 2 bar boost for some 3000 km. Because i had some big Holset turbo, it didn't produce much boost until 6000 RPM, so i set the ignition cut to 7800 RPM. This was to high and main bearings failed.
I stripped the engine to change the bearings, but because i don't know if everything was built as it should be, i would rather check all the clearences, before i put the engine back in the car.

So my questions are:

What should be the cylinder bore clearance for forged pistons (CPS with 84.4 mm diameter) and ring gap?

Are the aftermarket trimetal main and rod bearings any better than standard?

Post by **Guy Croft** » June 7th, 2007, 5:30 pm

You say CPS - do you mean CP Pistons?

GC

Wolf66 · Post by **Wolf66** » June 7th, 2007, 5:38 pm

I'm told that these are CPS pistons from Italy (Costruzione Pistoni Speciali)

This is the picture of them prior the installation, which the guy that i bought the engine from, sent me.

Post by **Guy Croft** » June 8th, 2007, 5:38 am

OK, good photo.

Normal practice is that 84.4 marked on the piston is the bore size for those pistons - not the piston size. So you don't need to know the running clearance. Have a read of article at:

http://guy-croft.com/viewtopic.php?t=117

GC

Wolf66 · Post by **Wolf66** » June 8th, 2007, 8:29 am

Thank you for your info.

I will measure the piston and if it is smaller than 84.8 (at least 0.07 mm as this is standard bore clearence for this engine),
then bore diameter should be 84.4. But if piston diameter is 84.4, then bore should be at least 0.07 mm bigger,
but i don't know how much. I already emailed the manufacturer but i didn't get a reply.

Janez

Post by **Guy Croft** » June 8th, 2007, 8:50 am

Don't start guessing! Everyone does that, forget contacting the mfr and measure the piston at gauge height like in the article. I bet you will find that they are about 0.003" or so smaller than the 84.40 bore.

GC

Wolf66 · Post by **Wolf66** » June 8th, 2007, 9:11 am

I will do that and post the measurements. Thank you again.

Janez

Wolf66 · Post by **Wolf66** » June 9th, 2007, 10:07 am

I measured the pistons and you were right. Two pistons are 84.31 mm and other two are 84.32 mm in diameter. So the 84.4 mark which is stamped on the pistons is bore diameter.

Janez

Post by **Guy Croft** » June 9th, 2007, 11:18 am

Yup, now and again I get something right!

Well done, thanks for reporting in.

GC

Acki · Post by **Acki** » June 10th, 2007, 1:39 pm

Different diameters, is that ok Guy?

Post by **Guy Croft** » June 11th, 2007, 7:59 am

A difference of 0.01mm (0.0004") on a piston running 0.08mm (0.0032") - or so - skirt_to_bore clearance is definitely not significant, bearing in mind the pistons have already been run, it could easily be wear or user measurement error, I know that the dimension of skirt 'diameter' at gauge height is not that easy to measure with a micrometer.

However, good engine building practice might suggest checking the pistons in the bores with a feeler gauge down the skirt side.

GC

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Forged pistons bore clearance

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