901 ring and pinion markings

[helidoc]

Need help in figuring out my ring gear marking. I understand the 9552 matches the pinion for the pair but the numbers that should be for backlash, 0.75 and -0.05 don't make sense when the manual only allows 0.12 to 0.18 mm. I am getting a reading of 0.24 mm from a modified vw521/4 tool.

[Daugaard]

Its 0,15 and not 0,75. And the - 0,05 mean you goal is from 0,10 to 0,15 and not 0,12 - 0,18 as bentley says.

Daugaard

[helidoc]

Thanks Daugaard. Looks like i have some work to do. Wonder how much .10 mm equals in terms of ring gear shim change?

[kevingross]

Jon B posted a great couple of notes on the topic of early 901 markings here. He has a different meaning for the -0.05 "T-value."

Your "a" distance measuring the backlash looks awfully small to me. What I've observed is that Porsche (well, Getrag) specify measuring at the outer limit of the ring gear teeth. The teeth, not the base flange. Looking at the last 901 I did, a '66 "chilled casting" version, I measured the ring diameter at about 182 mm. So divide by two, subtract 16 mm for half the VW521 ATF dimension, and I would be measuring with an "a" value of 75 mm.

(I may have ID'd a VW521 series bushing that fits the early spigots. It's on order, we'll see what I get! Until then, this is how I dunnit:

[helidoc]

Looks good to me. The optics on my photo may make it look off but it is about the same 75 mm. I have my diff out now but will check it soon. I am in the process of making a tool to measure the pinion depth as I really don't want to spend the $6000 for the VW 385 tool set, not being able to find a used one or the original P258.

[oldie44]

You can modify a VW289d tool to make a P258. The castings are the same.
Need to cut down the spigots to take Porsche bearings and reduce the length of the housing to about 120mm. Also need to rework the setting block.
Picture shows one modified for the mag case 901/915. For an aluminum case, you need cut down the spigots even smaller and reduce the length of the housing to about 109mm.

[helidoc]

I think the VW 289d tool is as hard to find as the P258 so I have started making my own tool. I will call it the TW 258x1 as it may be an evolutionary effort.

[helidoc]

I started with some aluminum stock rod I had around from another project and cut it to about 11". I figured my pinion carrier length from my existing ring gear an bearing with the preload shims in place to cut the shoulders for the bearings, which I purchased for about $50 for the pair. I bought a used side case plate for about $125 and milled a couple of openings for which I may enlarge. I then drilled the holes for the gauge and set screw to hold it in place. I knurled the ends for the heck of it. I now need to do some measurements and get an extension for the dial gauge. Will let you know how it turns out.

As for the position of the measuring point for the back lash, there are a couple of pictures to show that it is about the outer edge of the clutch tab with the diameter of the ring gear close to 181.75 mm. This would put the measuring point just about inside of 90.875 mm from the center line axis of the differential shaft.