Idler shaft and bearings

[stihlsawer]

A question for you all. I have a '95 model 510 that has the single bearing with a center groove with the new style split eccentric. Have any of you all replaced the original single bearing with two 6903 2RS bearings? Is the new idler shaft the same diameter as mine? Thank you.

[edma194]

I only replaced the idler bearings in the headstock I upgraded to a PowerPro, and a used PowerPro that the bearing assembly we wrecked on. I'd have to check the other headstocks I have but my 510 headstock is from the same time period as yours and I think has the water pump bearing that you describe. If you don't have a PowerPro you might as well stick with the water pump bearing unless it needs replacing. The shaft should be the same on both.

[DLB]

I've never disassembled the classic version we refer to as a water pump bearing, but I believe it uses a true 5/8" shaft. If that is true, the 6903-2RS bearings won't fit due to the 17mm ID. I only have the PowerPro version of the 'new' Idler shaft, but I would expect the conventional headstock version to be exactly the same as yours outboard of the bearings on both sides. The OD for the bearings and the space between them will be different. The PP version is 17mm OD for the bearings and 19mm between them.

The advantage of the new style shaft is that you can (probably) replace the bearings. The advantage of the old style is that the bearings rarely fail.

- David

[stihlsawer]

When I get home next week I'll measure the shaft. The small bearing and button seem to be in great shape. I will play around with the idea of 2RS bearings with the correct ID and OD. Thank you.

[dusty]

When I get home next week I'll measure the shaft. The small bearing and button seem to be in great shape. I will play around with the idea of 2RS bearings with the correct ID and OD. Thank you.

The shaft is 5/8" in diameter and is 6" long. The bearing is 1 3/16" in diameter (same as the Shopsmith bearing) and is 1 9/16" long.

[stihlsawer]

Thank you Dusty. I'll have time for measurements tonight, I think.

[DLB]

There are, IMO, at least two significant reasons why the water pump style shaft bearings exhibit superior service life. One is that the seals seem to be excellent, I have a 1957 Idler and the bearing grease is still on the inside and still feels like grease. I think that is relatively common. I have two PowerPro Idlers of the newer style (shielded, not sealed) both less than five years old and when I took the headstocks apart a couple of years ago to swap some parts both Idlers felt dry (free spinning), though both are still working. Second reason is that the integrated shaft bearings are effectively much larger bearings than a typical ball bearing with an inner race. If I understand the construction correctly the inner groove is machined into the shaft, allowing for much larger balls than a typical 30 mm OD bearing like the 6903. In short, the shaft bearing is effectively a larger diameter permanently lubricated sealed bearing system.

Note: Comparing PowerPro Idler to Conventional Idler is not a great direct comparison. The PP can run at higher speeds, depending on how the headstock is used. And the PP Idler generally runs hotter under comparable conditions for a variety of reasons. But at one time Bill Mayo did some testing/evaluation and found the integrated shaft bearing superior even at the higher speeds. Jacob Anderson also favored the old style bearings and when he was still selling parts he carried a new Conventional Idler Shaft with water pump bearings.

This attachment is for no purpose other than to show a cutaway mechanical drawing of an integrated shaft bearing for reference: (https://www.amazon.com/NMD-885586-Integ ... 0529&psc=1).

- David

[stihlsawer]

Thank you. I plan on working on it tomorrow, Lord willing.