Tohatsu o/b shear pin keeps shearing

[tony_brighton]

The 4yr old Tohatsu 3.5hp has started behaving oddly. Twice its fired up lovely but wont engage gear. On closer inspection the shear pin on the prop shaft has let go. This would be a good feature if the prop had actually hit something but is annoying when it hasn't. Any suggestions as to possible cause?

 

[Guest]

Check right size of pin and also hole not worn, if there is play then the start/stop of drive will gradually cut through by sheering action.

 

[tony_brighton]

Thx for the suggestion. Last pin lasted 20 mins of actual motoring.

 

[Guest]

Re: Your\'e not the onlly one

I also have a Tohatsu 3.5, and strangely also suffer broken shear pins for no reason whatsoever. Would be interested to know if you find it out to be a design fault.

 

[toad]

I have had one of these engines for 4 or 5 years and had this and other problems. The reason that the shear pin breaks is that you are putting it into gear at too high revs or the tickover speed is too high. The answer is to adjust the tickover through the hole in the side of the plastic casing but the tendancy is still to shove it into gear as soon as it starts. My solution to this problem is to replace the shear pin with a stainless steel pin. One of the other small engines on the market has stainless shear pins, I believe that it is Suzuki, they are normally sold in chandlers amongst the little plastic bags containing half a dozen fuses and odd screws and bolts for about £2 or £3. Obviously, if you whack the bottom hard, you are likely to damage the propeller or gear train although the way that I use the engione, this is unlikely. If you still wish to use brass shear pins then buy a length of brass rod from a model shop and make 30 or 40 which should last you two years. The other problem with these engines and their Mercury and Mariner clones is that they are prone to silting up in the water passages. This is an easy job to do every two years by removing the cylinder head which is a flat plate held on with four 10mm bolts and scraping and washing the silt out. Also note that the head gasket which I have re-used twice now, has a small hole which the water has to flow through (about eighth inch) which is easily blocked will cause the engine to overheat. I would suggest that after four years you dismantle all the lower stainless bolts on the leg and reassemble them with waterproof grease, also the gearshift starts tightening up after a few years and the actual plastic lever wants removing and the detent ball and spring needs cleaning and repacking with grease

 

[tony_brighton]

Sounds like sensible advice - I'll give it a go.

Thx

Tony