Transom Bolt Size

[Dcaspino]

I’m restoring a 1989 Montauk 17 and a 1989 Yamaha 90 ELTF. The two top-bolts that go through the transom hold the motor bracket are there but the two bottom bolts are missing. These are bolts that seem to thread into the transom bracket itself. What is the length and thread pitch of these two lower bracket bolts? As always I am grateful for the help I receive here.

[NLA01]

What do you mean by transom bracket? Are you talking about the transom or an actual bracket?

Usually they are 1/2" diameter bolts. Not sure the thread.

When I had a Montauk I did not use the builtin lower bolt holes in the transom. Yours might not work with the built in one since the person before did not use it. There is probably reason they did not use them.

A way to not use the built in bottom holes is to install a manual jack plate and drill two new bottom holes higher in the transom in the splash well and drill matching higher bottom holes on the jack plate. Then mount the outboard on that.
I used a Bobs machine shop manual jack plate.

Good luck
Archie

[jimh]

...the two bottom bolts are missing. These are bolts that seem to thread into the transom bracket itself. What is the length and thread pitch of these two lower bracket bolts?

I think you are asking about the BLIND holes on the engine mounting bracket. If you have the actual engine, you can experiment to find the thread size of the actual blind holes on your actual engine.

Based on information given in an archived thread that discusses these bolts, the Yamaha blind holes may be tapped for 12-mm diameter x 1.25 pitch thread machine screws. See more at

Yamaha blind hole mounting bolts
http://continuouswave.com/ubb/Forum1/HTML/017033.html

The length of the machine screw will have to be determined by measurement of the transom thickness at the point where the holes are located. You may have to cut a stock machine screw to length in order to accommodate the dimension of your individual boat and engine mounting situation, depending on the availability of metric threaded cap screws in your locality.

Regarding a corollary topic not asked, for advice on mounting engines to the transoms of Boston Whaler boats with shallow splash wells (the "sink" style splash wells) see the FAQ at

Q8: How Does the Engine Mount to the Transom?
http://continuouswave.com/whaler/reference/FAQ/#Q8

Nota bene that using the blind holes locks the engine mounting height into a fixed position and no further adjustments can be made.

You will find a better solution presented in the FAQ as the fifth work-around to the problem. This approach is also recommended by Boston Whaler in their Boston Whaler Dealer Bulletin 10-84, which I have reproduced for you in an HTML version at

http://continuouswave.com/whaler/refere ... 84-10.html

The bulletin was apparently written when an engine manufacturer (Mercury) began to omit the blind holes in their engine mounts.

As always I am grateful for the help I receive here.

You are quite welcome. The advice provided here is usually well vetted and is accurate advice.

[Dcaspino]

You were absolutely correct and Thank you! Headache over.