motor question - The 1947 - Present Chevrolet & GMC Truck Message Board Network

tommy217xxx · 09-10-2015, 06:28 PM

Hey everyone, I have 76 gmc and I had a question about the motor. The guy I got it from said he thought it had a 400 b/c It had a balanced flywheel. Does this sound right? Was a 400an option in these trucks? Is there any way by lookin at the motor to tell if it's a 400 or a 350?

Thanks for any advice. I don't know a ton about trucks and motors but I've been reading and learning as much as I can.

77 Big 10 · 09-10-2015, 09:45 PM

I think 1975 was the first year in was offered in the c10. The flywheel will be balanced on any engine, otherwise it would tear the engine apart. I believe it had a cut out in the harmonic balancer, in stead of smooth all the way around. And there is a casting mark on the block, drivers side rear, can't remember the number. Does it have three freeze plugs on the side of the block, some 400's had three, 350's had two.

**MikeB** · 09-10-2015, 10:07 PM

What is the block casting number? It can be found on the "shelf" just behind the drivers side cylinder head. These are the only two 400 block casting numbers I can find: 951509 and 3951511.

If it's a stock small block 400, it is externally balanced. That means it has an eccentric weight welded to the flywheel or flex plate. It also has a 8-inch OD damper that typically looks like this:

09-10-2015, 06:28 PM	#1
tommy217xxx Registered User Join Date: Oct 2014 Location: mi Posts: 359	motor question Hey everyone, I have 76 gmc and I had a question about the motor. The guy I got it from said he thought it had a 400 b/c It had a balanced flywheel. Does this sound right? Was a 400an option in these trucks? Is there any way by lookin at the motor to tell if it's a 400 or a 350? Thanks for any advice. I don't know a ton about trucks and motors but I've been reading and learning as much as I can. __________________ TJ 76 gmc 1500 swb patina finish Dropped 4/5, 350/4 speed 80 Chevy c10 lwb 305/350

09-10-2015, 09:45 PM	#2
77 Big 10 Registered User Join Date: Jul 2015 Location: La Quinta, CA. Posts: 131	Re: motor question I think 1975 was the first year in was offered in the c10. The flywheel will be balanced on any engine, otherwise it would tear the engine apart. I believe it had a cut out in the harmonic balancer, in stead of smooth all the way around. And there is a casting mark on the block, drivers side rear, can't remember the number. Does it have three freeze plugs on the side of the block, some 400's had three, 350's had two. __________________ Paul

09-10-2015, 10:07 PM	#3
MikeB Senior Member Join Date: Jul 2000 Location: North Texas Posts: 3,472	Re: motor question What is the block casting number? It can be found on the "shelf" just behind the drivers side cylinder head. These are the only two 400 block casting numbers I can find: 951509 and 3951511. If it's a stock small block 400, it is externally balanced. That means it has an eccentric weight welded to the flywheel or flex plate. It also has a 8-inch OD damper that typically looks like this: __________________ Mike 1969 C10 LWB -- owned for 35 years. 350/TH350, 3.08 posi, 1st Gen Vintage Air, AAW wiring harness, 5-lug conversion, 1985 spindles and brakes. 1982 C10 SWB -- sold 1981 C10 Silverado LWB -- sold, but wish I still had it! 1969 C10 (not the current one) that I bought in the early 1980s. Paid $1200; sold for $1500 a few years later. Just a hint at the appreciation that was coming. Retired as a factory automation products salesman. Worked part-time over the years for an engine builder and a classic car repair shop. Member here for 24 years! This is the very first car/truck Internet forum I joined. I still used a dial-up modem back then!



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