GM 8.1 Rod Length?

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DaveMcLain
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GM 8.1 Rod Length?

Post by DaveMcLain » Wed Jul 19, 2017 6:01 pm

I was just wondering what are the dimensions of the rods used in the GM 8.1 Vortec engine?

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Re: GM 8.1 Rod Length?

Post by ProPower engines » Wed Jul 19, 2017 8:09 pm

DaveMcLain wrote:I was just wondering what are the dimensions of the rods used in the GM 8.1 Vortec engine?

If you are referring to the GM 8.1/496 engine they measure 6.700""
I have one in the shop at present and I am replacing the rods to get rid of the stock fasteners that hold the cap to the rod.
They are a unique nut that are out of stock nation wide to the supplier according to GM dealerships here but a 10mm 12 point nut like a ARP Honda head stud nut will work.
I did that on the last engine I had in here now I just change the rods
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Re: GM 8.1 Rod Length?

Post by Newold1 » Wed Jul 19, 2017 8:09 pm

It's really a POS connecting rod from a bad design team. It's metric but the length is 6.693" long and uses a 1.05" diameter pin press fit in early 2001 thru 2003 engine and floating fit with no bushing in 2004-2009 versions. The section modulus at the base of the I-beam is smaller than the early 396, 427 chevy Gen 4 engines and the weak breaking point. Rod bolts are #10 metric with torque to yield spec. Suggestion to anyone rebuilding an 8.1L especially if they want to build for more power is to loose the pistons and rods, (really POS hypereutectic heche in Mexico pistons!) Buy a decent 6.700" h-beam rod and use a forged piston with a 1.340" CH with a .990 pin. Wiseco makes and stocks some nice lightweight forged units. The block is a tall deck but metric and the deck height is about 10.220" not 10.200" like other BBC tall decks. Thus the 1.340 CH piston size.

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Re: GM 8.1 Rod Length?

Post by ProPower engines » Wed Jul 19, 2017 8:12 pm

the marine version uses a forged piston by JE.
They use an unconventional ring package but they seem to last as long as the stupid rod don't break #-o
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Re: GM 8.1 Rod Length?

Post by DaveMcLain » Wed Jul 19, 2017 8:56 pm

Ok thanks. I have a customer with one of those in some sort of rock crawling contraption and he was wanting to do a few upgrades to the engine. I was curious about the rod length with the 4.375 stroke and it seems like finding solid information on the engine isn't easy for some reason.

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Re: GM 8.1 Rod Length?

Post by jred » Thu Jul 20, 2017 1:18 am

just got done doing one for a marine app used scat rods and wiseco pistons kit with rings had this engine in the shop for over a year was going to have custom pistons made then wiseco came out with pis/ring kit checked block for 0 deck and only had to mill .003 or .005 off block so be-careful as there's not a lot for clean up..

also arp really likes to price out their metric stuff for this engine..

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Re: GM 8.1 Rod Length?

Post by ProPower engines » Thu Jul 20, 2017 1:39 am

FYI
JE has a +.005 piston available if you run into this again
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Re: GM 8.1 Rod Length?

Post by Newold1 » Thu Jul 20, 2017 7:08 am

The factory stock piston is a hypereutectic coated skirt piston with a metric ring pack. The weakness in this piston is not the hypereutectic construction necessarily but the ring pack was moved up for emissions reasons pretty close to the top of the piston and with the intake valve relief the edge of this depression is very close to the actual top ring groove and a little detonation in the engine tends to break a portion of the top ring land off, it comes up in the chamber and pounds the brittle piston quickly into pieces. Then the big pin and rod lock up in the cylinder and usually push themselves through the thin non-siameased cylinder wall. WHOLA- totally junk engine! Be careful using this engine for big power upgrades with large over bores or hypereuctectic pistons and weak rods! Go forged with new forged pistons and H-beam rods and you can build a nice BBC Gen 7 engine.

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Re: GM 8.1 Rod Length?

Post by Walter R. Malik » Thu Jul 20, 2017 9:15 am

I have used several sets in other stroker type, NOT high RPM engines.
Just like a normal big block Chevy OEM production rod, just metric bolts and a bit longer with a different pin size.
The pin end is a press-fit 1.041" Ford size and bushings are readily available from a lot of vendors for using a .990" floating pin.
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Re: GM 8.1 Rod Length?

Post by PFC1 » Tue Oct 10, 2017 10:31 am

ProPower engines wrote:
Wed Jul 19, 2017 8:12 pm
the marine version uses a forged piston by JE.
They use an unconventional ring package but they seem to last as long as the stupid rod don't break #-o
I've rebuilt a lot of these marine 8.1s both Volvo and Mercruiser and never seen a forged piston in one yet. Now all the 502s have JE forged pistons.
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