Need help with custom rod dimensions for 3.6L 993 motor build

[lightbulb8817]

I'm piecing together lots of very helpful info from many threads and various engine builders; however, I figured I would make a thread about it. I could potentially make a $1500+ mistake if I order the wrong custom rods and would appreciate input from very knowledgeable people that participate in this forum...

The motor is a '95 993 motor that will be running a 76.4mm stroke 996 GT3 crank (along with GT3 oil pump).

I would like to run a 130.0mm length (cup car) rod to improve rod-length to stroke ratio since i need to order custom rods & pistons anyways.

What dimensions & tolerances should I spec for a custom rod? I believe here are the blanks I need to fill in but please let me know if there's something I'm missing or overlooking:

Rod length: 130.00mm (replacing my factory 127.0 mm length rods to improve length-stroke ratio, inspired by porsche cup cars)
Big end diameter: is it 56.01 or 56.02mm? (original 993 motor was originally 58.01mm I believe)
Big end width: 21.74mm? (original 993 crank was 18.75mm I believe)
Small end diameter (wrist pin): 22.0 or 23.0mm ?
Small end width: 23.00mm??

I will have to spec custom pistons as well but want to get the bottom end (rods) sorted first...

Thanks in advance!

[dannobee]

Porsche Rods - Pauter

Stock compression height is 31.8mm for the 76.4mm. You'll need to subtract 3mm from that for the longer rod, and subtract whatever deck height you want. I'm sure others will chime in.

[Mixed76]

I think your big end dimensions are mislabeled

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[lightbulb8817]

thx, yes they were

[Ollies930]

FWIW, depending on what bearings you use, I would size the big end dimension accordingly. For instance, if you use the Automotive Associates coated bearings, you will see approx .003" clearance with 56.00mm big end dimension.

[dannobee]

Have you already bought the GT3 crankshaft? If it was me, I'd send my 74.4mm crank out and have it offset ground. If you're going with smaller rod bearings anyway (not a bad idea for high rpm), you could increase the stroke by twice the amount of offset, ie a 78.4mm crank, in standard 53mm size journal. I'm sure that there are plenty of crank grinders in the LA area that would perform that work for you.

Then pick out your custom rods and pistons.

[RedCoupe]

Originally Posted by dannobee

Have you already bought the GT3 crankshaft? If it was me, I'd send my 74.4mm crank out and have it offset ground. If you're going with smaller rod bearings anyway (not a bad idea for high rpm), you could increase the stroke by twice the amount of offset, ie a 78.4mm crank, in standard 53mm size journal. I'm sure that there are plenty of crank grinders in the LA area that would perform that work for you.

Then pick out your custom rods and pistons.

The stock crank in a 993 is 76.4.

[dannobee]

Sorry. I was thinking 964 for some reason.

Still, offset grind it 2mm and you can go to 80.4mm.