Tohatsu / Mercury / Mariner (Nissan?) 5hp 2-stroke shaft conversion

[josephmoore]

I have a 2002 Mercury 5MH two-stroke sitting doing nothing which I feel like trying on my Hurley 22 as if anything it's quieter than the 8hp Suzuki on there at the moment. The catch is that its a short shaft so I'd need to do the conversion to make it a long shaft. Why not just sell it and buy a long shaft? Well, it's a decent light engine at a handy power so it'd be useful to keep it and have the option of putting it on any boat I might own.

Without disassembling it and taking a guess myself, has anyone done this before? Of course it will be possible as parts are interchangeable but which parts?

The leg is in two pieces so I could get hold of a long shaft mid-section, but does that have the drive shaft contained within it, or is it a separate item? Similarly the shift linkage would need extending - but does the long-shaft use an extender piece or is it an entirely different part?

There appears to be no difference between the two-stroke and the four-stroke mid-sections. Is that correct? I'm aware that the 4hp, 5hp and 6hp were all the same engine with changes to the prop and carburetor so a mid-section from any of these should be usable.

Any nuggets of experience gratefully received!

 

[C08]

I have had Tohatsu 25HP 2 stroke and 20 HP 4 stroke converted from long shaft to extra long shaft and I think the Mercury engines are very similar to Tohatsu.
Normally the leg extending exercise needs another drive leg section, a longer drive shaft which is quite separate from the leg, a longer water intake/transfer pipe (from the pump to the head and a longer gear shift rod. Longer studs will also be required to hold the legs through the extra leg section. It is a very easy job to do but the parts may be pricey although you do occasionally see parts for sale second hand from people who have converted from long to short!

 

[Lakesailor]

Far too expensive to do. Different drive shaft, gear linkage, cooling pipe plus the longer bottom casting. (I'm not even sure it's just a spacer like the Johnson).

Sell the one you have and buy the same model longshaft. Or buy a knackered longshaft and swap bits. As long as it's only the powerhead that is jiggered.

 

[josephmoore]

I think the larger engines have an extra 6" spacer piece which sits between the main section of the leg and the gearbox section, whereas on the smaller engines it is an entirely different casting for short shaft and long shaft which sits between the power head and the gearbox and has all the stuff like the mounting bracket on it.

Here's an image of the piece from a relatively new four-stroke. It's almost identical to the section on my older two-stroke bar a couple of minor changes (that thumbscrew on the back is one)

 

[josephmoore]

Far too expensive to do. Different drive shaft, gear linkage, cooling pipe plus the longer bottom casting. (I'm not even sure it's just a spacer like the Johnson).

Sell the one you have and buy the same model longshaft. Or buy a knackered longshaft and swap bits. As long as it's only the powerhead that is jiggered.

Thanks Lakesailor - if everything was integral to the mid-section then it might not have been too much of a big deal but if there's more than one or two parts then it's a right pain. Vague eye out on ebay for a knackered long shaft version it is, then!

 

[VicS]

Thanks Lakesailor - if everything was integral to the mid-section then it might not have been too much of a big deal but if there's more than one or two parts then it's a right pain. Vague eye out on ebay for a knackered long shaft version it is, then!

The only likely economical way to do it is with second hand parts. Otherwise as LS says too expensive.

I have not checked exactly what parts you will need but basically as Ls says. 5" spacer, longer gear shift rod, water tube and drive shaft. Thats assuming it can be done with a spacer, otherwise it might be a different gearbox or exhaust housing.

The cost of a drive shaft alone will probably rule out using new parts.

 

[josephmoore]

The only likely economical way to do it is with second hand parts. Otherwise as LS says too expensive.

I have not checked exactly what parts you will need but basically as Ls says. 5" spacer, longer gear shift rod, water tube and drive shaft. Thats assuming it can be done with a spacer, otherwise it might be a different gearbox or exhaust housing.

The cost of a drive shaft alone will probably rule out using new parts.

I'd only entertain doing it with second hand parts. New isn't a word my wallet can cope with!

It is definitely not a case of using a spacer - the entire mid-section of the leg has to be replaced between the head and the gearbox on this model.

The question was more related to the bits which sit inside the mid-section: shifter linkage and drive shafts etc. and if anyone knew whether they were part of the mid-section or separate in their own right. It seems they're all separate which means they get split up and I have more hassle tracking down suitable parts or pay over the odds for them individually, plus more hassle than just a few bolts if I wanted to switch the shafts over.

Bummer.

So unless a knackered one crops up cheap, it's a bit of a no-go financially and more sensible to just save up for another 2nd hand engine. Time to set the eBay saved search I guess....

If anyone has one mouldering away in their garage, do let me know!

 

[TSB240]

I have done the reverse of this but with a Johnson 4hp twin.

All the parts that I could not adapt were obtained at a very reasonable prices from Bill Higham. He was extremely helpful in making sure I had the correct parts for my particular model.

The hardest part to obtain was the drive shaft with the correct no and length of splines at each end and position of drive for impeller.

I was prepared to cut and weld the old one (there is no reason why a reasonable engineer could not do the reverse and add a few inches)


I even made my own power head to leg gasket from material bought from an engineering supplies company as I was sent the wrong one by an ebay supplier.

 

[VicS]

The question was more related to the bits which sit inside the mid-section: shifter linkage and drive shafts etc. and if anyone knew whether they were part of the mid-section or separate in their own right. It seems they're all separate !

Should have posted a link or two to parts lists.

You might find the exploded diagrams and parts lists at http://www.boats.net/ useful.

 

[GrahamM376]

I converted a Tohatsu 9.8 long shaft 2 stroke to short. Needed complete leg, drive shaft, 1 gear rod & gaskets. Parts new would have been around £350, managed to get second hand but still £200+. Quite a fun job persuading lots of stainless bolts in aluminium to shift after corroding in tight for 12 years.

 

[Lakesailor]

There you are then.
Flog the short shaft and buy a long shaft. Probably a lot cheaper to do anyway.