Tightening alternator belt: M35B with standard 50A alternator

[Breakin Away]

Hi all, I think my belt needs tightening. I don't hear any squealing and don't see excessive rubber dust, but I do smell a faint burning rubber. IIRC, my alternator measured 144F when running at crusing speed, but everything around it was around 160F, so I don't think it's excessive. (Tell me if it was.) I did see a little in-out vibration of the belt in the long run from crankcase to alternator.

I see a lot written about tightening the belts for the smaller (older) motors, but not the M35B. From what I can tell, the alternator is in a very nice position to loosen the top bolt and pull it outward. Is it that simple? Is there another bolt that's the pivot point? Does it need to be loosened, and is it hard to access.

I'd appreciate any advice from someone who has done this on the M35B. I have the standard 50A alternator, not the big 100A upgrade.

[Jim Hardesty]

From what I can tell, the alternator is in a very nice position to loosen the top bolt and pull it outward. Is it that simple? Is there another bolt that's the pivot point? Does it need to be loosened, and is it hard to access.

Yes, there is a pivot bolt that needs to be loosened. It's in an attachment on the engine block.  All straight forward but access to the rear is a pain.  I loosen my belt at winter storage and readjust in the spring.  Think it keeps the belt from taking a set.
I take a very good look at the belt when changing the raw water impeller.  If my memory is correct the belt is behind the hoses and the hoses need to be removed to change the belt.  Not something I want to do in a rush.  So for the small cost of a belt, I change at any signs of age/wear.
Jim

[KWKloeber]

See pg 21 of the 25b/35b Ops Manual.

[Ron Hill]

Breaking : Your alternator is on top of the pivot point, so just push down on the alternator case to the correct tension and then tighten that pivot bolt.

A thought

[Jon W]

FYI- To check for proper belt tension after changing belts, I bought a belt tension gauge for ribbed v-belts from NAPA auto parts. There's others brands available online as well.

[KWKloeber]

See pg 21 of the 25b/35b Ops Manual.

PS:  suggest you change to a Dayco "Top Cog" model belt if you're not currently using that one.

[Breakin Away]

I know this is an old thread, but I think it's a good place to post my question.

When tightening the alternator belt, how tight is too tight? I know the over-tightening can damage the shaft bearings, alternator, and other components, but how tight is too tight? (Yes, I've read the service manual and know that they say ⅜-½" of deflection with 22 lb of force, but I find that very tricky to measure.)

I like to have the belt tight enough that there's no "wobble" when it's running. When I do see wobble, it's always in the long run between the crankshaft pully and the alternator. After many adjustments using my Krikit belt tension gauge, I'm pretty certain that tension needs to be at least 60 lb to get rid of the wobble. But if I adjust to 60 lb, I have to re-adjust every few weeks since it's right "at the edge of a cliff". So I typically adjust to 70-80 lb so I can get through a few months before I have to re-adjust again. For those who have tension gauges, is 70-80 lb excessive? Am I being too anal about avoiding any wobble on all three sides of the triangle?

I've recently noticed that it seems that the engine vibrates a little more when the belt is wobbling. It's hard to imagine that a lightweight little belt could vibrate the whole engine, and the vibration is not terribly severe, but I do notice a difference if I put my hand on the transmission or stuffing box while the engine is running. (I've been tempted to buy a vibrometer to quantify this, but don't want to spend the $$$.)

One reason that I am asking is because my belt seems to be wearing a little faster than the previous one did. It's been in use for about 2½ years, and I'm starting to run out of room on my M35B's adjustment arm. According to the previous owner's records, the prior belt lasted about 5 years. Maybe he let it wobble a little bit, but I don't like the vibration that seems to cause.

[Ron Hill]

Breaking : One of the keys for the best belt wear is to buy a good brand name belt!!  I've always stood by "Gates" belts which I've gotten from NAPA stores.

On the tension - I used the "Old Farmers Almanac" belt tension method of - "On the longest belt length be able to finger twist the belt 90 degrees"!!  Works for me!!

A few thoughts

[Breakin Away]

I'd love to get a better quality belt, and had seen some references in other posts. However, I have not been able to find what length I should buy for my M35B with standard 50A alternator. If anyone here could recommend the right one for an M35B I'd appreciate it.

[Jim Hardesty]

I have not been able to find what length I should buy for my M35B with standard 50A alternator.

You can take the belt off and take it to a NAPA store, or any good auto parts store, to match it. 
FWIW I think it's a spare worth having.
Jim

[Ron Hill]

Guys : I overtightened the drive belt on my OEM M25XP engine.  The first bearings to go were in the engine water pump - not the alternator!! 

A thought

[Jon W]

If you take Jim's advice, while your at your local NAPA store ask about their belt tension gauge. It's called the Krikit. Part numbers used to be NBH KR1 and NBH KR2. I have the KR1. Pricey for what it is, but makes the belt tightening simple and correct.

[KWKloeber]

I'd love to get a better quality belt, and had seen some references in other posts. However, I have not been able to find what length I should buy for my M35B with standard 50A alternator. If anyone here could recommend the right one for an M35B I'd appreciate it.

I don't see why it would be different than other B engines?  The front ends should be identical.  At least they are on the older non-Bs so I'd believe Kb would want to standardize parts.  Does the Wb part numbers match on the 35B and 25XPB?

[Ron Hill]

Jon : Ask Westerbeke the size of their OEM belt for your M35BC engine.  Any dealer should be able to tell you the OEM length.

Most of us C34 Cruisers with refrigeration have increased our Amp Hr. battery bank.  Most of us have changed to a hi -output alternator w/an external voltage regular to keep up the battery/ies charged. Then that different alternator probably requires a different OEM belt length depending on that alternator pully!!

A few thoughts

[Stu Jackson]

Jon : Ask Westerbeke the size of their OEM belt for your M35BC engine.  Any dealer should be able to tell you the OEM length.

Most of us C34 Cruisers with refrigeration have increased our Amp Hr. battery bank.  Most of us have changed to a hi -output alternator w/an external voltage regular to keep up the battery/ies charged. Then that different alternator probably requires a different OEM belt length depending on that alternator pulley!!

A few thoughts

I agree.  Ron's point, as I understand it and have experienced is just that.  I've written here about how I searched for belts even with my OEM alternator as far back as 1999.  At that time I had to match what my PO left on the alternator, and took it off to go shopping for a new one at NAPA.  When I later installed my 100A Blue Circle, things, of course, changed, as one would expect.

Because of the range of adjustment of the arm on M25 engines with the "enhanced" bracket like the XPs, which seems large but is actually quite limited, even with a similar sized case for the alternators, there is always a "sweet spot" on belt sizing.

With my new alternator, I went to start with a 7390, tried a 7400 and finally realized they made a 7395 which is just perfect.  I bought a half dozen and still have a few left.  That was almost 20 years ago.

I believe that even with an OEM alternator and whatever engine you have, you're the only person that can test, right there in the field, how well whatever recommendations are made by the engine builder.  I'd bet most of their recommendations are right on, so you have a good starting point.

Changes?  Ha!  Your boat, your choice.