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Old 21-05-2022, 05:50   #1
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Cummins QSB 6.7 tensioner pulley

Hey all,

This little devil is giving me a hard time. Weve replaced the part twice already and the replacements wear out within a thousand hrs of operation or worse start chattering right away when we install them. Ive isolated it down to the bearing as it seems to go away when i hit the hub with McLube but it comes back after a few minutes or as the rpms go up. Anyone have any thoughts or similar experience?
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Old 21-05-2022, 06:01   #2
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Re: Cummins QSB 6.7 tensioner pulley

Check out the Cummins marine engines forum:
QSB 6.7 belt tension ➥ https://www.sbmar.com/community/topi...-belt-tension/
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Old 21-05-2022, 07:32   #3
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Re: Cummins QSB 6.7 tensioner pulley

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Originally Posted by GordMay View Post
Check out the Cummins marine engines forum:
QSB 6.7 belt tension ➥ https://www.sbmar.com/community/topi...-belt-tension/
Thanks Gods, Id read that and didnt find it super useful. Did I miss something? Ive got an ace of a mechanic whos been looking after this engine for 8200 hrs of operation and who also did the install. Hes fiddled with belt tensioner and Id say 1/4 deflection at 80lbs sounds about where its at.
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Old 21-05-2022, 11:38   #4
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Re: Cummins QSB 6.7 tensioner pulley

The Seaboard website has a forum where you could ask a specific question to be answered by a real Cummins guru. (Tony Athens)
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Old 22-05-2022, 16:46   #5
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Re: Cummins QSB 6.7 tensioner pulley

I have similar idler pulleys on my Beta 50 (Kubota). Like your installation their purpose is to redirect the path of the belt.

I have the same problem as you. The bearings in the pulley only last about two years. My theory is that the bearings are spinning faster than they are designed to. If you look at the ratio between the crankshaft pulley and the idler wheel you will see what I mean.

If you could increase the diameter of the idler puller it would reduce it's RPM. In my case there is no clearance to do this.

I had the Beta factory provide a crankshaft pulley large enough to spin my second alternator at a speed that would provide full output at idle. This was a unique modification and neither Beta or I thought about the idler pulley speed.
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Old 22-05-2022, 17:21   #6
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Re: Cummins QSB 6.7 tensioner pulley

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Originally Posted by stormalong View Post
I have similar idler pulleys on my Beta 50 (Kubota). Like your installation their purpose is to redirect the path of the belt.

I have the same problem as you. The bearings in the pulley only last about two years. My theory is that the bearings are spinning faster than they are designed to. If you look at the ratio between the crankshaft pulley and the idler wheel you will see what I mean.

If you could increase the diameter of the idler puller it would reduce it's RPM. In my case there is no clearance to do this.

I had the Beta factory provide a crankshaft pulley large enough to spin my second alternator at a speed that would provide full output at idle. This was a unique modification and neither Beta or I thought about the idler pulley speed.


I have a Beta 50 - and would like an idler pulley. How/where did you get it?
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Old 22-05-2022, 18:18   #7
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Re: Cummins QSB 6.7 tensioner pulley

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I have a Beta 50 - and would like an idler pulley. How/where did you get it?
This was a factory mod. They have a bracket made to attach an auxiliary device such as a second alternator, refrigeration compressor, etc. You can see what this bracket looks like by viewing the keel cooled version of your 50 on the Beta UK website.

So, the keel cooled engine does not have a water pump">raw water pump in the way so if they fit this bracket to a conventional engine with a raw water pump the belt for the second alternator needs to be directed clear of the pump. That is why my engine has two idler pulleys. They are bolted to the block into what I assume are pre-existing bolt holes. Fairly large shoulder bolts go through a spacer to place the pulleys the proper distance from the block. The bolts go through a pair of bearings that the pulley turns on.

So, back to your question I suggest that you contact Beta UK or Beta North Carolina and discuss what you need. IMO Beta UK has much better support than North Carolina. Phone rates are so low these days that it is worth it to talk to them.
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Old 22-05-2022, 22:02   #8
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Re: Cummins QSB 6.7 tensioner pulley

Check water pump bearing. They chatter before they collapse. That is usually the cause--but it is not the only possibility. Some impeller pumps have replaceable seals and bearings, and need a press to fit them, others require the whole pump to be replaced.
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Old 23-05-2022, 07:39   #9
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Re: Cummins QSB 6.7 Water Pump Pulley

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Check water pump bearing. They chatter before they collapse. That is usually the cause--but it is not the only possibility. Some impeller pumps have replaceable seals and bearings, and need a press to fit them, others require the whole pump to be replaced.
Yeah, this is a coolant pump pulley, not a tensioner pulley. I've revised the thread title. Sounds like it is just wearing out and I need to replace it.
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