Grease Cap Threads

[Willy]

Is there anywhere I can get information of the size of taps needed for chasing DB caps? I have a couple of nice caps, but the threads look to be partially stripped. I don't have them here, but they have a number like "J225", or something close. They have the small opening, which don't fit my 1918 DB Touring. And where would I find such large taps?

[platt-deutsch]

A machinist could chuck the cap in a lathe and set the feed for the threads per inch. Use a sharp threading cutter and should work fine..

[dwollam]

You might try a thread file.

[TonyAus]

I think you will find that, according to the relevant SAE standard, the thread on your hubs will be 20 tpi. Forget using the solid cast caps. By the sound of it you probably don't have a lathe and will need to pay someone to machine them out to size - a fairly time consuming job.

It may be more cost efective to purchase Australian made reproduction caps which are available in the US through Myers.

[MikeC5]

If there is an area with enough good threads you may be able to get a thread pitch gauge in there. The idea is you try the ones that look about right until you find the one that fits perfectly. Then measure the ID of the cap. This should get you close enough to find the standard size tap. A set of pitch gauges shouldn't cost too much.

[Willy]

Thanks for the info. I have a lathe, but the threading attachment has a broken aluminum housing. I do have and will try my thread files. Didn't think of that. I have pitch guages, so I'll check that. I was hoping there was a tap that would fit it (like a plumer's dies). I'm getting lazy in my old age!

[dwollam]

If this is for the typical 4 cyl DB I am fairly sure they are 2 7/16 x 16 tpi.