idler arm bushing replacement

[burgermeister]

My idler arms and sprockets had somewhat worn bushings. Here is how I rebushed them without access to a machine shop. Obviously if you can turn custom replacements, do it!

17.5mm OD bushings are not available anywhere I looked, and I even searched google.de (figuring maybe the crazy germans have such stuff at home). No idea why Porsche chose such a wacky dimension.

But 15mm ID, 17mm OD, 15mm long bushings (Garlock or Bunting) are readily available from motionindustries.com and applied.com for a few dollars each. They're "DU" type bushings, which is a porous bronze, filled with a mix of lead and teflon, sintered onto a steel backing. Factory bushings are a bronze layer fused to a steel backing. The 15mm length is 1mm shorter than the factory 16mm long bushings. If I had to do it again, I'd order longer bushings and cut them to length for the idler sprockets.

First, mark where the worn part of the bushing is on the idler arm. The bushings are split, and I oriented the split to not be at the wear locations. Press out the factory bushing using appropriately sized sockets or other parts and a bench vise. It took quite a bit of force to crack the bushings loose, but they came out without problems or damage - I could have pressed them back in & reused them.



Obviously 17mm OD bushings don't work in a 17.5mm hole, but 0.010" brass shim stock is almost exactly 0.25mm thick and is carried by many hardware stores. It works for the press fit if the coefficients of friction are in your favor, so clean the idler arm bore & shim stock of all grease & oil.

I cut a 54mm long piece (17.25 * pi), and cut slightly wider than the idler arm bore is long. Using a 3/8 extension (1/2 might be even better), I rolled it as round as feasible & inserted it into the idler arm bore. If everything went according to plan, the split is touching or very close together but not overlapping. If not, fix it. I then "rolled" the edge over the chamfer on the idler arm bore with a 3/8" extension (anything round will work).



This shows the shim stock rolled & inserted in the idler arm bore. On the left are the 2 factory bushings - the wear spots can be seen clearly, too. On the right are the 2 replacement bushings.



I smeared engine assembly lube on the inside of the brass-lined hole, and on the outside of the bushing. I oriented the split in the bushing to not be in the same location as the split in the shim stock, or the wear location. Then, using the bench vise, I carefully pressed the bushing in - it was a perfect press fit. Careful not to catch the edge of the brass (this is why it needs to be cut a bit long & rolled over the chamfer), or it will crumple and require another try.

I used a socket with a washer hot-melt glued to it to press the bushings in.



On one idler I cut the shim stock a bit too long and it wasn't flush after pressing the bushing in, so I just filed the excess off very carefully. Remember to drill the oil hole through the shim stock. I deburred the inside of the oil hole with a dremel - it's tight, but can be done.

For the sprockets, if you use the 15mm bushings, they will need to be pressed about 0.7mm past flush to be centered in the idler. Also, the idler sprocket only has a chamfer on one side of its bore - you need to press the bushing in from that side.



Probably not up to the standards of the financially unlimited crowd, but it seems like a pretty reasonable solution to me.

[cgarr]

Looks good to me! Those bushings look very familiar ;-)

[burgermeister]

Originally Posted by cgarr

Looks good to me! Those bushings look very familiar ;-)

I got the idea from your rocker shaft posts

So, thanks twice!

[cgarr]

I wanted to use these bushings for this application too but because of the odd ball size I just made bronze bushings as replacements. Nothing wrong with using shim stock as long as its a good press fit, I will have to try it the next go around. I have had several reports of reduced oil temps on race engines using these bushings for the rocker arms, they seem to run a lot cooler than the bronze bushings. One more good area of the engine to now use them!

[Henry Schmidt]

Option two: Supertec idler arms.