I decided to replace the leaking oil seal in the distributor to my 1990 740 wagon with a B230F non-turbo engine. I studied the various posts on the board, the FAQ etc. and ordered up the Honda 12 x 20 x 5 seal that was the "first choice" in the discussions on rebuilding.

I just got the distributor apart and the old seal was not only hard as a rock, but the entire seal was spinning in its bore, scoring and abrading the aluminum wall of its bore. I removed the old seal and to my chagrin, the seal is marked 12.5 x 20 x 5. I measured the shaft diameter and it measures 12.436 mm (.4896"). The shaft diameter appears to be original and not worn down since it measures the same throughout its length -- even in non-contact areas. The original seal design for the distrubutor was for a 12.5 mm seal over a slightly undersized (12.5 mm = 0.4921")shaft. Seal pressure would have been on the low side of nominal. My shaft had light scoring where the original seal ran on the shaft. I rounded off its sharp edges and polished it out with some wet 800 grit and then 1500 grit wet-or-dry sandpaper with the shaft turning in my drill press.

Because the 12 mm Honda Seal is designed to run on a 12 mm (.472") shaft, it is a tight fit on the distributor shaft which is .018" larger than nominal.
There is no question that a 12 mm seal will stop the oil from leaking.
My concern that the significantly tighter fit will prematurely wear the seal and more aggressively groove the shaft than the original 12.5 mm seal.

I have been able to locate very little data on oil seals on the web. I did find one hint that it will be OK to use the 12 mm seal on the oversized shaft. SKF makes a speedi-sleeve (99049)for its 12 mm seal. The wall thickness of the speedi-sleeve is .28 mm (0.011")which would result in a shaft diameter of 12.56 mm when applied over a 12 mm shaft. This is even bigger than the distributor shaft, so it seems likely that the 12 mm seal will not be over stretched.

I found out after I ordered the 12 mm Honda seal and the o-rings that both FCPGroton and eEuroparts.com now sell a distributor seal kit (969330K) for $3 that contains both green o-rings and a new single lip, metal OD replacement oil seal. Unfortunately, the seal has no markings on it to determine what size it is really.

For those who have replaced the original oil seal in your distributor with 12 mm seals, how many miles have gone by since you did it? Still holding up?
Anyone have to take their distributor apart a second time and noticed increased shaft wear from the smaller seal? Anyone installed the metal OD seal from the aftermarket 969330K)distributor seal kit?

This thing is way too hard to take apart and reassemble for me to do it twice.

Thanks to all who have helped me get my car staightened out.