Is my 77 Inline 6, 250 CID, 165 HP a GMC or a GM Chevy

[Cadmandu]

Can anyone tell me if these are two different engines or just a name mis-understanding?
I was told buy the guy that i bought the boat from that it was a GMC engine.
I like Chevy and Ford trucks but not any GMC stuff. When i was a kid in the 60's they sold Chevy trucks and GMC trucks they were different i thought.
My mercruiser is it a block made for cars or trucks?
Thanks

 

[ahicks]

They're the same block. And the dislike for GMC vs. Chev. would have to be because the GMC had 4 headlights vs. Chevy's 2. Rarely was there ever any other difference.

The GMC was ALWAYS been sold as a higher end/up scaled Chev. whether that was actually true or not....

A Chev. fan with a dislike of GMC makes no sense at all here....

 

[Cadmandu]

My block casting number is 366855 GM that refers to a Buick, Olds, Pontiac and Chevy
I dont think that GMC made a 250 cid inline.
If they did would the block say GMC on it.

 

[Rick Stephens]

Each Chevy motor those days made in just a couple GM lines. At manufacturing level saying GMC or Chevy is saying the same thing. Those differences are all marketing from the same corporation.

 

[Cadmandu]

Thanks for all the great input. Outside of the motor mounts is there a difference between a 250 i6 Chevy motor for a car and a truck. What years would be a perfect fit for my 77 if I were looking for a replacement engine.

 

[kenny nunez]

All GM truck engines have the extended “ear” on the starboard side which is used for the front engine mount. I have seen auto style engines in boats where a bracket was fabbed up. These engines are getting harder to find and if you cannot locate a truck engine then with some work a bracket can be used. If you should be really lucky and find a 292 truck engine it will have the “ear” for the front mount. One way to identify a 292 is where the fuel pump is mounted near the middle of the block. All of the 250 hardware mounts to the 292. A new fuel line an extension to the alternator adjustment bracket and a piece of tubing in the exhaust hose because the 292 is about an 1&1/2“ taller. You will have to drill out the coupler and flywheel of the 250 to 1/2” as the 250 studs are 7/16”.I have seen “stepped” studs that were made and if you have a machine shop friend then that will help.

 

[Fun Times]

The following link might contain some info too regarding a GM engine and your Marine engine by Mercruiser, Chapter Six.PDF (wvu.edu)

 

[Cadmandu]

Love that PDFs thanks a lot

 

[KD4UPL]

GMC and Chevy trucks are made in the same factory on the same assembly line with the same parts. They make one "brand" one day and the other "brand" the next. I once worked for a company that had a 90's truck with a Chevy badge on one fender and a GMC badge on the other fender. They said they bought it brand new that way. It must have been the first one down the line on a new day.

 

[matt167]

Gmc until the early 70s had their own engines. And through the 70s had bigger engines for big trucks. Gmcs first v8 was a Pontiac.. GMC engines were never used in boats

 

[Cadmandu]

Gmc until the early 70s had their own engines. And through the 70s had bigger engines for big trucks. Gmcs first v8 was a Pontiac.. GMC engines were never used in boats

 

[Cadmandu]

Info getting better all the time. Did GMC block casting numbers say GMC on them

 

[matt167]

I don't think brand is stamped on any engine from the GM umbrella. When Chevy was using the 250 straight 6 in the trucks, GMC had a 305 V6 and a 351 V6. Stopped in the early 70s. Before the V6 they had the Jimmy straight 6 that was 302 cid or 265 iirc. They did v12 engines as well which were 2 V6 engines cast as one. 2 distributors, 2 intake manifolds and I think 4 exhaust manifolds

 

[Cadmandu]

This is really getting deep

 

[Scott06]

This is really getting deep

And starting to stink...

 

[matt167]

And starting to stink...

Some interesting information on the GMC engines https://en.wikipedia.org/wiki/GMC_V6_engine

of course, none of this is actually boat related

 

[Scott06]

of course, none of this is actually boat related

that was the point trying to clarify info for the OP not confuse him ...he has a straight 6 not a V6

 

[Scott Danforth]

GMC motors differed from Chevrolet motors prior to 1955

The V6 and V12 GMC motors were in the 50s

The 250 cubic inch corporate GM motor was in every brand except Cadillac. The new inliners (4 and 6 cylinders) came out in 1962 model year

1976 the integral intake and head casting came out on some Chevrolet vehicles, but not all. Avoid this turd

Your mercruiser motor uses same block, head, flywheel, damper, crank and pistons of the truck. Different cam, head gasket, and other marine specific stuff (fuel and electrics) are what mahes it a marine motor

Kenny covered the Truck/Industrial/marine vs car

 

[matt167]

GMC used the V engines until 1974..
The 250 integral head was used after 1975 on most trucks. If you needed a good short block, it would work in a pinch. separate intake heads are not too difficult to find.