Mercruiser 255 newbie

[Finnegan70]

I just bought a 1977 Gibson 36. The engine says 255 hp on one of the valve covers. It seems like everything I search on 255 hp brings up hits on Mercruiser. I am trying to identify the engine and the outdrive so I can learn more about what I have. I thought the 255 would help identify the make of the motor but it seems like it designates the outdrive because I keep seeing different engine makes (Ford 351, Chevy 350) with Mercruiser 255 hp. Can anyone explain how these engine/outdrive set-ups can be identified.

 

[HT32BSX115]

Howdy,

Welcome aboard!

Well, Mercruiser usually Identifies via serial number on the engine vs year model or hp rating.

So you would be looking for something like the following

250 (2 BBL.) GM 350 V-8 1977 - 4768000 THRU 4908849

Or​

​

888 (2 BBL.) FORD 302 V-8 1971-1977 - 3108723 AND UP

Now having said all that, someone in the last 40 odd years could have replaced that engine many times with a pick up engine etc. So you may not find any (Mercruiser) serial numbers at all.

BIG differences between chev vs FOMOCO is the Chev 305/350 has the distributor behind the carb. Ford 302/351 has the dist in front.

Cheers,

Rick

 

[Finnegan70]

That is very helpful because I kept seeing similar motors with the distributor up front but ours is behind the carb. Is this definitive of all Chevy inboard motors from that time period?

 

[GA_Boater]

You need to find out the brand. Knowing what motor (Ford, GM, Chrysler, etc.) isn't helping if you have an inboard set up with a transmission and a transmission problem. If you have a sterndrive, the coupler may have failed. In either case, maybe the prop came off.

What color is the motor painted and can you post some photos?

 

[Finnegan70]

the motor appears all black and the sterndrive is black as well. kicking myself for not taking pics. The actual problem is the stern drive or shift components. It seemed to pop out of forward gear after about 2 miles of driving fine. After that there was no forward or reverse. What is the coupler? We thought it might be an electric shift problem or sheer pin on the prop. We will be back down at the boat this weekend and get some pics and do some investigation with a little "google" knowledge on our side. LOL But let me ask a stupid question... Is the transmission part of the outdrive or sterndrive, and is an outdrive the same thing as a sterndrive. Just trying to learn the lingo.

 

[achris]

OK, the only '255' GM engine used by Merc with a sterndrive was a TR.

Here are pictures of the different drives.

This is a TR (Note the bolt-on cover on the back of the drive)

Here is a R/MR/Alpha drive. A lot smaller and lighter. (and no bolt-on rear cover).

And finally a Mercruiser-1 drive. (note the lifting eye on the top cover)

Alpha and 1-drives select the gears internally (on the propshaft). TR drives require a transmission behind the engine.

Chris.........

 

[Finnegan70]

Thanks for the pics achris. Mine definitely looks like the TR in the first pic. Except I don't think it has the "picking eye" at the top.?? My mom's going down to the boat tomorrow so I'm gonna have her taking all kinds of pictures and sending them to me. Also, does anybody know about converting the electric shifts over to a manual cable? Thanks for all your help!

 

[Finnegan70]

Okay, back down at the boat. We figured out we have a GM motor 255 hp with Mercruiser most likely TR sterndrive. It is electronic shift. We have 12 volts at all contacts and the shift actuator is physically shifting. We are afraid we have an internal problem but we do have a question. Does this prop have a shear pin. Because with the key on, we can shift it into gear and the we can still turn the prop by hand very easily, forward or reverse. Obviously w/o the motor running. We can also see it spin slowly when we first start the motor in gear. Will post pics later for help with identification. Thanks!