M70A2 motor with M70B lower unit. Can it be done?

[Zwegerst]

Ok I have a '91 tohatsu 70 hp 2 cylinder and I have a lower unit for a newer tohatsu 70 hp that has a 3 cylinder motor. The only difference I see is that one lower unit has a 13 tooth prop shaft and the other has a 15 tooth prop shaft. Will the newer lower unit work on my 2 cylinder motor? I ran it at the boat ramp with out the prop on and it ran cool and pissed water no problem but I'm not sure if the gear ratio will be different and will it work with a prop on. I don't want to buy one if its not going to work. Please help. Thank you.

 

[Sea Rider]

Welcome to Iboats,

If both lower legs fits inside each with no sweat, water pump works ok, it's not big deal if with different gear ratios, Tohatsu usually uses 1:85 & 1:92 gear ratios. You can always maximize a prop to take all the ponies out of any engine. Just select a prop with same number of splines to match prop shaft splines.

Happy Boating

 

[Sea Rider]

Sorry, both gear reduction ratios for 70 HP should be same 2:33

Happy Boating

 

[Zwegerst]

Good to hear! Thank you! Do you think I should stay with the same pitch prop that used to have or do I need to change it? Also where could I find a thrust washer for it?

 

[Sea Rider]

To tell if you can stay with same prop pitch will need to make a wot test lightly loaded on flat calm no wind water cond and check achieved wot rpm, check if inside 5150-5800 rpm wot range for that engine. If around middle to full max wot you'll be fine. Must test with a healthy tach. For parts check Boatsdotnet under Tohatsu/model.

Happy Boating

 

[Petedacook]

Hi Zwegerst,

Were you able to get the new lower unit working on the M70A2? I also have a M70A2.

Thanks,

Pete