Swapping out Alpha 1 Gen 1 transom gimbal assembly with older generation (MC-I, etc.)

[Blueghost924]

I have a 1986 Renken 21 foot boat with the Alpha 1 Gen 1 gimbal transom assembly (gimbal housing, ring, bell housing, etc...) I have a 1982 Mercruiser 470 as the inboard. I have to replace my Alpha 1 gimbal housing due to corrosion. I looked at an older Mercruiser gimbal assembly (housing/ring/bell housing) for sale in which the serial number was worn off and was not present. The assembly was in very good shape. In looking through diagrams and serial numbers for different year ranges for Mercruiser gimbal assemblies, the match I see is that it's from the 470 year range from 1978-1982. I know that the exhaust pipe would have to be changed out (different o-ring and bolt hole pattern). Other than that, are the previous generation gimbal assemblies backwards compatable with the Alpha 1 Gen 1 that's currently on my boat? In other words, will I be able to use it? Looks like the previous generations were the MC-I/R/MR.

Hopefully I have enough information here....

 

[achris]

R and MR will 'straight swap' an Alpha One, but an MC-1 will not. The main issue is that the rear engine mount spacing is 10" on R, MR, Alpha One, but nearer 14" on MC-1. There are other issues which preclude you using one generation inner transom plate with the other gimbal housing, so no, you can't do that either.

1982 was the year of change, so we would need the serial number of the 470 to know which it is, or just measure the spacing of the rear mounts.

Chris....

 

[Blueghost924]

Thanks for the reply Chris. My 470 engine serial number is 6004274, which may put it in the 1892 range. Just for verification, I did a rough measurement of the 470 engine's rear mounts, and they appear to be spaced at the 14" mark. I know another "challenge" is that I have is the the current exhaust elbow that mounts to the gimbal housing is the Alpha 1 part 42421A5, I would have to swap it out with the older exhaust elbow 72082A2, which I do have a spare for....but still...who wants to do extra work? BTW - another wrinkle in all this is that my 470 is currently INSTALLED.

To add to the confusion, the 3.7L 470 isn't the original motor for my boat. My boat title lists the Mercruiser 170 serial number A409012, which matches up with the inline 4 Mercury 170.

I'm beginning to think the best thing to do is find a good Alpha 1 Gen 1.

 

[achris]

Thanks for the reply Chris. My 470 engine serial number is 6004274, which may put it in the 1892 range. Just for verification, I did a rough measurement of the 470 engine's rear mounts, and they appear to be spaced at the 14" mark.

That makes it an MC-1 motor... Both by serial number and rear mount spacing.

I know another "challenge" is that I have is the the current exhaust elbow that mounts to the gimbal housing is the Alpha 1 part 42421A5, I would have to swap it out with the older exhaust elbow 72082A2, which I do have a spare for....but still...who wants to do extra work? BTW - another wrinkle in all this is that my 470 is currently INSTALLED.

To replace the gimbal housing the engine must come out, no option on that.

To add to the confusion, the 3.7L 470 isn't the original motor for my boat. My boat title lists the Mercruiser 170 serial number A409012, which matches up with the inline 4 Mercury 170.

Same engine, different label. '470' is 4cylinder, 170hp.

I'm beginning to think the best thing to do is find a good Alpha 1 Gen 1.

I may be confused here. What gimbal housing is in the boat? Is it still the original? Do you have the serial number for it?

Chris......

 

[Blueghost924]

The serial number is really faded on the housing, but I can make out "1C.......". I think that serial range is an Alpha 1.

Engine has to come out...well it is what it is!

 

[achris]

So, how are you making an Alpha One transom assembly match up to an MC-1 engine? Someone has done some seriously dodgy work to make that happen....

If you want to put an Alpha One series engine with that Frankensteined transom assembly, you are going to need to replace the inner transom plate, and anything else that's been botched up to make that lot work...

As you have a really 'mix'n'match' set up it's really difficult to advice without seeing it. I have no idea what's been modified and how...

Chris......

 

[thumpar]

I think you need to figure out what you have before you move forward. The 470 is a 170 4 cylinder. It may be more original than you think.

 

[Blueghost924]

Well, I looked at the inner transom plate and it's definitely an Alpha 1 Gen 1. The older 470 that's in the engine bay is sitting on wooden blocks and not hooked up - now I know why. It currently can't! The engine runs very well, so I'm under the opinion that I have 2 ways to resolve it:

1. Try to hook up some dodgy bar fastening rig from the older 470 rear engine mounts to the Alpha 1 Gen 1 transom plate. This just sounds like a bad idea even if it could remotely work.

2. Obtain the older MC-1 inner transom plate and swap them out - I believe it would be Mercruiser part number 65545A12. Hopefully, the transom bolt pattern would match between the older MC-1 plate and the Alpha 1 Gen 1 plate. The other thing I would not cherish is pulling the motor. Unless you have an overhead crane (which I don't) it's not a fun affair. I do have an engine hoist, so if I would only need to lift and suspend the engine a foot or two up while within the boat it wouldn't be as bad...

Facepalm to this dilemma I have...

 

[Bondo]

Ayuh,.... The later 470s were used with the early Alphas I believe,.....

Ya need an flywheel cover for a 470/ Alpha drive,....

 

[Blueghost924]

Got it...but it looks like I have the earlier version of the 470 (MC-1) that has the 14" rear mount spacing. Darn thing runs like a champ too!

 

[achris]

Hi BG,

Sent you a PM (actually replied to yours), but for others here... I made an interesting find last time I was home. I visited a member who had pulled a couple of 470s out of a sunk boat (some of you may remember him)... While I was looking at the engines and gimbal housings, I did some measurements. We had all previously thought that Merc changed enough in 1983 to make the engine mounts and gimbal housings incompatible. And we were not entirely correct. In 1974 or 75 Merc changed the drill pattern for the gimbal housing (moved a couple of the middle holes up about an inch) , and opened the cut out up slightly in the middle. That made the earlier MC-1 (without gimbal ring mounted trim sensors) incompatible with the later MC-1. But the engine rear mounting spacing remained at 14"... When the R was introduced in 1983 the rear engine mounting spacing was reduced to 10" (hence a new inner transom plate), but the drill pattern and cut out shape remained the same. As far as the gimbal housing is concerned, the only thing that changed was the bell housing.

Upshot? A later MC-1 gimbal is compatible with a post '82 inner transom plate (hence engine flywheel cover) and visa versa. The only caveat is the drive yoke. The shorter yoke still needs to be used on the drive when using an early model in-line engine....

HTH (but I know will add to an already confusing area)

Chris.......