Outboard clamp screw replacement

[weustace]

In a second thread which perhaps could be titled "start-of-season woes", I have found (despite fairly frequent lubrication) one of the transom clamp screws on my Mariner 4hp outboard to be seized solid. I have steeped it in oil, poured litres of boiling water over it, attacked it with a blowtorch, and used progressively longer levers until the screw began to yield plastically at the top of the clamp. I have now hacksawed the top and bottom off, and intend to replace the screw with a piece of 316 stainless studding with an appropriate block on one end and a handle of some sort on the other... not too worried about these two details, but quite concerned about where to drill. The existing screw is 8mm roughly, and I am faced with replacing with M6 (and thus drilling largely into the stainless steel and hopefully tapping into same) or with M8 and drilling out all of the stainless + tapping aluminium.

My experience from suffering similar trials in the past suggests that the M6 option may be preferable if I can get a 5.5mm hole sufficiently straight in the screw... views?

Also, am I correct in assuming the new screw should be 316 stainless? Can't imagine an aluminium screw would be hard enough, but given how readily stainless steel work hardens, I was surprised by how much the existing screw yielded—though a back-of-the-envelope calculation suggests a max shear stress at yield of ~180MPa, which is about right for stainless steel and far higher than what one might expect for aluminium.

 

[steve66]

In a second thread which perhaps could be titled "start-of-season woes", I have found (despite fairly frequent lubrication) one of the transom clamp screws on my Mariner 4hp outboard to be seized solid. I have steeped it in oil, poured litres of boiling water over it, attacked it with a blowtorch, and used progressively longer levers until the screw began to yield plastically at the top of the clamp. I have now hacksawed the top and bottom off, and intend to replace the screw with a piece of 316 stainless studding with an appropriate block on one end and a handle of some sort on the other... not too worried about these two details, but quite concerned about where to drill. The existing screw is 8mm roughly, and I am faced with replacing with M6 (and thus drilling largely into the stainless steel and hopefully tapping into same) or with M8 and drilling out all of the stainless + tapping aluminium.

My experience from suffering similar trials in the past suggests that the M6 option may be preferable if I can get a 5.5mm hole sufficiently straight in the screw... views?

Also, am I correct in assuming the new screw should be 316 stainless? Can't imagine an aluminium screw would be hard enough, but given how readily stainless steel work hardens, I was surprised by how much the existing screw yielded—though a back-of-the-envelope calculation suggests a max shear stress at yield of ~180MPa, which is about right for stainless steel and far higher than what one might expect for aluminium.

I use a tool I made out of an old socket. It fits over the top of the thread and handle. I then heat the clamp and apply lots of oil to the thread. Then unscrew with an impact wrench. Seems to work well for me and never damaged a thumbscrew

 

[snowbird30ds]

Assuming the outboard is off and portable why not take it to a welder and weld a nut over the broken end, allow to cool fully then heat the hell out of the aly and try a spanner on it, it often helps if you drill into the broken bit as far as you can and weld the hole up again as it shrinks it a bit then weld the nut on.
Have got many broken threads out like this.
Don't heat it full of fuel......

 

[{151760}]

I've just replaced one of mine with stainless studding, but I was unable to drill a hole in the studding to accommodate a tommy-bar. I used Holt studding which is very very hard. Perhaps I work-hardened it by filing a flat on it. Maybe a cobalt drill would do it. Anyway I have used large nuts on each end as handle/end.

 

[Plum]

In a second thread which perhaps could be titled "start-of-season woes", I have found (despite fairly frequent lubrication) one of the transom clamp screws on my Mariner 4hp outboard to be seized solid. I have steeped it in oil, poured litres of boiling water over it, attacked it with a blowtorch, and used progressively longer levers until the screw began to yield plastically at the top of the clamp. I have now hacksawed the top and bottom off, and intend to replace the screw with a piece of 316 stainless studding with an appropriate block on one end and a handle of some sort on the other... not too worried about these two details, but quite concerned about where to drill. The existing screw is 8mm roughly, and I am faced with replacing with M6 (and thus drilling largely into the stainless steel and hopefully tapping into same) or with M8 and drilling out all of the stainless + tapping aluminium.

My experience from suffering similar trials in the past suggests that the M6 option may be preferable if I can get a 5.5mm hole sufficiently straight in the screw... views?

Also, am I correct in assuming the new screw should be 316 stainless? Can't imagine an aluminium screw would be hard enough, but given how readily stainless steel work hardens, I was surprised by how much the existing screw yielded—though a back-of-the-envelope calculation suggests a max shear stress at yield of ~180MPa, which is about right for stainless steel and far higher than what one might expect for aluminium.

I had to make a new clamp screw for my old 2.5hp Mariner as it was bent! Used a piece of 10mm SS studding and reused the plate and handle as in attached picture. However, I did not have your problem but if I had I would drill out the stub you now have stuck in the aluminium to completely remove all trace of the stainless and insert a helicoil into the aluminium like this https://www.ebay.co.uk/itm/M6-M8-M1...hash=item4b0ee955f0:m:m-SKt96LJcdnRUa8dqWUYOg so you can use M10 for the replacement. Hopefully you can borrow the Helicoil fitting kit from a hire shop of local car spares place.
Personally I would not use an M6 bolt.

www.solocoastalsailing.co.uk

 

[Topcat47]

+1 on not going down to a 6mm bolt. and helicoiling the clamp. The "bolts" are available as "spares". I've done it on a Mariner and a Johnson (and an old seagull).

 

[LadyInBed]

If you haven't already, remove the clamp assy from the engine to work on it.

 

[TQA]

Use more heat next time. I have never had to drill out a clamp bolt.

 

[weustace]

Thanks all for your advice; will have a play when next down on the boat and see what can be done.

 

[penberth3]

Thanks all for your advice; will have a play when next down on the boat and see what can be done.

Stud extractor kits like this sometimes work, sometimes not, but they do include bushes to help you drill a hole in the centre of the broken bolt - something that's hard to guess.

Drill progressively larger holes until the bolt is nearly all gone. Pick out the remains with an old scriber then run a tap through the existing threaded hole. If that fails you can think about heli-coils or a larger hole.

https://www.tooled-up.com/artwork/prodzoom/VS7233.jpg?w=1600&h=1600&404=default

 

[thinwater]

Use more heat next time. I have never had to drill out a clamp bolt.

Not just oil, but one specific to Aluminum. PB Blaster is famous here.

Then more heat. Specifically, you want to heat ONLY the aluminum and you want to heat it relatively quickly, so that it expands more than the bolt.

I've remove many stubborn clamps this way.

It REALLY helps to go to these methods before you fatigue the bolt. You only get so many tries.

 

[coopec]

I have no problem with what everyone else is saying and you seem to know what you are doing

When I have been in a similar predicament I have used a quality High Speed drill bit (correctly sharpened) to drill S/S. Then I have used a powerful very low speed (120 rpm) electric drill with as much pressure as I could exert. I have even put a piece of timber over the back of the drill as a lever to increase the force. (As soon as the drill stops cutting the s/s starts to work harden).

If the s/s has work hardened you can sharpen up a masonry drill and, with force, that will drill the hole.

Clive Cooper

 

[Ningaloo]

Anyone know where you can get replacement bolts for the transom clamp? I have a 4hp Mercury 2 stroke of unknown vintage and I am missing one of the clamp bolts. ideally I'd like a replacement assembly with the washer and rotating bar. I believe the bolt size is M10. I found plenty of 5/8" (16mm?) on ebay but no M10.

 

[Refueler]

My Mariner 20 had a locking bolt assembly replaced one clamp. The seller had lost the 'key' ... it wasn't stolen as he also gave me the registration doc (In latvia we can register the engine separately if we want ... yeh crazy !!) ...

So I had to cut the bolt and luckily it unscrewed with heat applied to the surround - taking care not to heat the bolt itself too much.

Stainless studding with double nut on the 'handle' end .... hard wood pad other ... I allowed the end to dig into the wood to give it secure anchorage. It needs a spanner to work it ... but unlike the OP's dinghy OB ... mine stays mounted most of the year.

I may see about getting a replacement original clamp bolt and pad .... one day !