Any Transom experts here. I have a Transom Rebuilding Question.

[tpenfield]

I tend to agree with the comments that the transom may have been rebuilt (incorrectly) at some point. The spec's for transom thickness is 2", which we are not seeing from your pictures.

Maybe take a look at some other re-build threads on this forum as some are similar in design to your boat. Essentially, it is a notched ( or inset) transom, and it should consist of a center section ( which you have shown) and port/starboard outer sections.

 

[Scott06]

Agree with others - have never seen a fiberglass gasket. Wonder if it was cloth that resin never kicked off. Read the resto forum to see what others have done. Certainly given how you need to bond and sandwich the transom wood to the outer fiberglass there is certainly the opportunity for it to not bond right.

Also merc has installation drawings on their website that give the dimensions for transom thickness and flatness which is critical when you go back in.

re the lag bolt- as mentioned above they are wood screws that hold the motor mount into the engine mount stringer. Usually when the wood mount rots they just come right out. So if it took some azz to remove them it is a good sign that the stringer is not rotted. When I repowered the original ones were bedded in 5200 or similar sealant to keep water out. Would recommend doing the same to keep water out when you go back in

 

[Chris51280]

that looks like de-lamination to me. Like others have said, look up other re-builds and also the stickies on top of the forum.

 

[1985 Century Mustang]

I tend to agree with the comments that the transom may have been rebuilt (incorrectly) at some point. The spec's for transom thickness is 2", which we are not seeing from your pictures.

Maybe take a look at some other re-build threads on this forum as some are similar in design to your boat. Essentially, it is a notched ( or inset) transom, and it should consist of a center section ( which you have shown) and port/starboard outer sections.

The wood that's not rotted by the transom looks like it's almost 2" thick.

Question, what is the normal thickness of a transom on the avg. I have heard it's 1" and 1/2.

 

[tpenfield]

Mercruiser spec is 2" thick . . . 2-1/4" maximum.

 

[1985 Century Mustang]

Mercruiser spec is 2" thick . . . 2-1/4" maximum.

Thanks, Can I possibly get away with 3", the more the merrier I suppose?. I would use 4 - 3/4 pieces of marine plywood. I think it may be slightly less than 3".

 

[Scott06]

Thanks, Can I possibly get away with 3", the more the merrier I suppose?. I would use 4 - 3/4 pieces of marine plywood. I think it may be slightly less than 3".

In a word No ... use the merc thickness spec otherwise transom plate will not fit on and driveshaft will go that much less in coupler. 2 x 3/4 ply plus fiber glass on either side is fine as long as it is not rotted

 

[1985 Century Mustang]

In a word No ... use the merc thickness spec otherwise transom plate will not fit on and driveshaft will go that much less in coupler. 2 x 3/4 ply plus fiber glass on either side is fine as long as it is not rotted

Good Point,

 

[tpenfield]

Thanks, Can I possibly get away with 3", the more the merrier I suppose?. I would use 4 - 3/4 pieces of marine plywood. I think it may be slightly less than 3".

You probably should get a copy of the Mercruiser installation manual. You can get yourself into a whole world of trouble if you are not rebuilding the transom to Mercruiser spec's in terms of thickness and 'flatness'.

 

[Chris51280]

Thanks, Can I possibly get away with 3", the more the merrier I suppose?. I would use 4 - 3/4 pieces of marine plywood. I think it may be slightly less than 3".

if you do not follow the transom spec, nothing will fit. down pipe, bolts, steering arm, drive shaft etc. 90% of the people rebuilding use 2x3/4" plywood. (ACX or BCX is good) with plenty of resin-thickened and 1708 woven to get your thickness. Did you watch any of the rebuild videos or the instructions that are above this forum?

 

[matt167]

If you really want extra strength, the only way is to increase the amount of plies. You can do this by using more ‘sheets’ of thinner plywood. That will get you more plies for your target thickness. Going to 1/2” plywood would be as thin as you would want to go, and you would want to target 2” as that will give you 1/4” of leeway

 

[Scott06]

this is the closest I could find for transom specs - is for a gen 2 which will be the same cut out I believe for your gen 1

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://samerwebapp01apncus01.azureedge.net/mmgw-env-b/filer_public/c9/7d/c97d2d3e-43ff-4927-b6c0-5366cc9f4b8b/alphagenii_1992-2001.pdf

 

[1985 Century Mustang]

Thanks everyone, Ill see how 3 pieces of 3/4" inch ply will work out. It should be around 2 and 1/4" total. I'll do a dry test and take measurements.

 

[1985 Century Mustang]

this is the closest I could find for transom specs - is for a gen 2 which will be the same cut out I believe for your gen 1

chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://samerwebapp01apncus01.azureedge.net/mmgw-env-b/filer_public/c9/7d/c97d2d3e-43ff-4927-b6c0-5366cc9f4b8b/alphagenii_1992-2001.pdf

Can u resend the link, I can't click on it, its a dead link.

 

[Chris51280]

Thanks everyone, Ill see how 3 pieces of 3/4" inch ply will work out. It should be around 2 and 1/4" total. I'll do a dry test and take measurements.

only 2x 3/4" needed. the hull is 1/4 to 3/8 thick in that area. if you use the 2x 3/4 plywood (2x 23/32")+3/8" = with the hull will get you to approx. 1.8" you add 2-3 layers of 1708 and you are in the range that is needed. 3x 3/4" will be too much

 

[1985 Century Mustang]

only 2x 3/4" needed. the hull is 1/4 to 3/8 thick in that area. if you use the 2x 3/4 plywood (2x 23/32")+3/8" = with the hull will get you to approx. 1.8" you add 2-3 layers of 1708 and you are in the range that is needed. 3x 3/4" will be too much

I agree thanks. I'm going with 2- 3/4 pieces of plywood. I may go with pressure treated also. Or should I use another type of plywood?

 

[JASinIL2006]

I would not use pressure treated. Regular exterior grade plywood works fine.

Too many potential problems with PT wood, such as interaction with any fasteners/thru-hull fittings or problems with the resin sticking to the wood. Not worth the risk, IMO.

 

[Scott06]

I agree thanks. I'm going with 2- 3/4 pieces of plywood. I may go with pressure treated also. Or should I use another type of plywood?
View attachment 397023

The only upgrade I’ve seen folks do is use a synthetic material like Coosa board.

Of course this is pricey and the plywood lasted over 20 years …

 

[Chris51280]

pressure treated is not dry most of the time and the chemicals will not allow the resin to bond very well. Any exterior grade ply will work. ACX/ BCX. If you feel wealthy, use marine grade but i would keep that money for beer

 

[1985 Century Mustang]

pressure treated is not dry most of the time and the chemicals will not allow the resin to bond very well. Any exterior grade ply will work. ACX/ BCX. If you feel wealthy, use marine grade but i would keep that money for beer

I agree, especially if your resin coating and fiberglassing the plywood. FWIW, I don't plan on keeping this boat for more than 3 years or less anyhow. But who knows, the way prices of everything have gone up I just may keep it longer.