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I have a genset 4bt that Im trying to collect parts for to swap into my scout. My next challenge is getting the correct fan pulley. The one it came with doesn't fit on the hub correctly. The PN for the pulley is 3914462 and my hub assembly is 3901364. The bolt holes for the pulley dont line up with the bolt holes on the hub. Any advise on the correct pulley I need? Also, I plan on using the top mount v-belt driven vacuum pump which I have the pulley for. Does that just bolt over top of the fan pulley?
 

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I'm a bit puzzled as to what you have. 3901364 in not a hub assembly. It is a part shown in the photo below. Frankly, I don't remember ever seeing that part on an engine. Believe that part is a Cummins/Onan part; That piece has a recess in the center where normal fan hubs have a 1" pilot that sticks out front. Not counting the Dodge Cummins hubs, the regular hubs came in 2 bolt patterns and probably more than a dozen different styles. They either had 4 8mm bolts in a 50mm circle or 4 10mm bolts in a 60mm circle. The normal fan hub assembly used on road 4bts is like the one in the diagram below and used pulley 3914463. This is the one with the 50mm bolt pattern. There is an identical assembly in the industrial engines 3911203 that has the 60mm bolt pattern and used pulley 3929269. The pulley you have I believe is the one typically found in the high mount industrial engines which has the 60mm bolt pattern. Below is a photo of your pulley. Do you have a photo of your hub assembly? Do you plan on a mechanical fan? You may need to source a different hub assembly than that Onan. Those parts can be quite expensive.

128071
 

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Very good photo. That is a standard high mount industrial fan hub often found on Onan engines. Is the engine ID plate still on the side of the gear housing? If you'll give me the engine serial number I'll look up the parts on Quick Serve. That will show the OEM pulley part number. That hub assembly is part 3909888. That part 3901364 you gave is part of that assembly. That one has 8mm bolt holes and uses pulley 3914458. See photo below. That particular fan hub is a bit odd in the fact it does not have a ball bearing like all the others. I'd be a bit hesitant to use that one on a road vehicle. Your pulley 3914462 should fit hub assembly 3913433 which looks much the same as what you have but is a more robust design with larger bolts and a ball bearing. Mounts in the same spot as the one you have. See photo below.
 

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