Mine died a few months ago. I took it to an auto electrician who stocks hundreds of alternators and he was able to match it pretty closely, just needed to trim the output terminal which was too long and fouled the exhaust.
There are numerous measurements you need to consider; saddle spacing, mount offsets, pulley offsets, terminal locations etc. I looked at buying one online but am glad I didn't, there are just too many variables.
My engine originally had a 35 amp alternator. It had a 50 amp when I bought the boat and we fitted a 60 amp one.
I am quite poor on electrics but, talking to the electrician who sold me the alternator and the one who installed it, I don't see the point of going any higher as the output is only at the maximum if your batteries are very flat or you have very high loads. The charge will drop down to below 10 amps pretty quickly as the batteries charge. Even with a fridge, tiller pilot and lights taking say 10 amps if they are all on, the 60 amp alternator will have sufficient output to provide a full battery charge and run the systems at the same time.
My batteries are well charged from a 40 watt solar panel. When we flashed up the new alternator it started at 31 amps, dropped to 10 after 30 seconds to a minute and steadied at 5-6 after a few minutes. Unless you have extreme electrical loads what is the point of fitting an 80 amp one?
You also need o consider the ability of your engine, wiring and fan belt to cope with the loads of high charging alternators. For example, although the alternator on my engine had been upgraded to 50 amp, the output wire hadn't been so I wasn't getting the benefit of the bigger alternator anyway. My electrician had to fit thicker wire to the charging circuit.

cisco said..

aus005 said..
hi guys
my alternator has failed on my 2qm20 yanmar i have it off and have been looking for a replacement but nothing jumping out at me do you guys know of a place to get one or can i get one as best guess and modify the bracket i dont know how many amps it is but wouldnt mind getting something to charge a bit quicker.
would a car one do the trick?
any ideas thanks

Your alternator would be a Hitachi with 80 mm between the mounting lugs. A near match will be a Bosch from a Nissan or a Subaru.

Take your alternator to a wrecking yard and see if they can match one up to it.
You should be able to find a 60 or 80 amp unit which should be a better option than your current 40 amper.

Usually you would pay $30 to $40 from the wreckers. When you have found a physical match, take it to an auto sparky for a bench test.

I considered a second hand one. Reliability versus cost was the issue.
The sparky was going to charge me about an hours labour at $95 to replace the bearings, inspect the internals and test it. Plus the $40-50 for the alternator (don't get much in Sydney for $30). The new one cost me $290 with the modification for the tacho feed and a 12 month warranty.
If you are not going too far in your boat a failed alternator isn't too much of an issue. If you are four days from port with a fridge, lights and auto pilot reliability is a bit more important.
Another factor for me is there are no wreckers near me, that probably would have swung me to go 2nd hand.

www.balmar.net
basically there are around three mounting profiles on alternators the above link shows the three. Cars are much the same




www.amazon.com/ALTERNATOR-YANMAR-MARINE-2QM20H-121370-77200/dp/B007OX3AI0

I usually choose one with as built in reg and if you run a tacho ask for one with a terminal for that also
( cant remember the terminal name right now old age maybe ).
Older engines had lower charge amp alternators fitted ( there was less need for more amps back then) you can up the amps out put but remember you only have a single belt driving it. so don't go overboard a 20 Hp should easily run a 60 to 80 amp alternator off the original belt

thanks for your help guys amazing the knowledge on this forum.
yes it is a hitachi thats all it says i have taken it to auto parts wholesaler whom i know and he is sourcing a new one for me i will wait for the price and then deceide on second hand or new.

thanks guys fitted new alternator $180 from my mate exactly the same as old one 40 amps i think will be just fine
thanks for all the help