Crank seal behind alternator - seal codes?

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Oct 16, 2006
Hi all

I need to replace the crank seal behind the alternator (again - see below), I have a seal which I think I may have bought for this purpose some time ago, can you help me identify it, please. (Bike is a 1972 750)

The codes on it are D451 4 SC 175 118 18 TTO.

I'd be interested in knowing how the coding system works for future reference.

The long story here is that I had an ongoing problem with oil leaks, specifically from behind the alternator. I fitted a crank case breather valve which I pulled off a Volvo brake booster in a scrapyard and that did seem to cure the oil leaks. Yesterday evening I went to the Hagbourne bike meet, and on the way back the road was empty, so I took full advantage and had the old girl up to 90mph for about half an was running so well. Anyway, as I pushed it back into the garage, full of Norton happiness at how well the bike had gone, drip drip drip.... the old problem has returned. I guess that the Volvo part is not up to the job of sustained high speeds so until I have come up with a better valve I'll have to keep it under 70mph.

If you can help with the part number it will be appreciated.

Hi there,
your code is nearly the same as the special oil seal Steve Maney supplies, I have a Maney one under my eyes and the code is D451 2 SC 175 118 18 TTO , and after playing with my calculator and caliper , found that they are the dimensions of the seal: 1.75 " = 44.45 mm OD, 1.18 = 29.97 mm ID and 0.18 "= 4.57 mm width, at that time , no clue about the signification of the other terms, so when I gave those info to my local dealer (France !!) he could only supply me with :
ERIK 8 / R / 28.57 /44.45/ 6.35 /3, so the difference is about the width ( 6.35 mm instead of 4.57), and the ID ( 28.57mm instead of 29.97mm), as I had been a pain in the a.. with him for only 2 seals , I had taken and paid for them , but the will be difficult to use in my drive side due to the ID beeing too small by nearly 1.5 mm, and the width too big of 1.8 mm !!
So yours looks more the part than mine.......
Not easy to found in France !!!!!!!!!!
Missed Hagbourne but made it to Cassington. Sorry to hear about yr troubles. Am I guessing you have the Combat breather and front return pickup system? I went through all that and ended up doing the Mick Hemmings mod (see Old Britts site). Motor later blew for another reason but I couldn't go over 4000 revs without emptying the tank into the cases and that was with a Norvil breather. I now have some 73 850 cases and the oil return system that works. The result is a totally dry motor!
These seals are in inches, 118 = 1.1875" bore
175 = 1.7500" OD
18 = 0.1875" wide
The other nos are the seal mnfr and p/no. so you do have a crank seal but not sure if the TTO has a steel or rubber outer. Steels do not seal as well but stay in place better.

Metric seals are easier to fathom 30 40 8 = bore,OD,width

If you get bored here are some pics of my money pot since 98 ... 86531186.1
Thanks to marinatlas and Keith1069 for unlocking these codes for me.

I suspect that this seal has been leaking for donkeys years but only when I put a belt drive kit on did it become apparent.

The reason I think it has leaked for a long time is that the seal is not a tight fit in its housing. Perhaps too many changes of steel cased seals have worn the softer metal away.

So, taking this further, to make the seal a better fit, is there any reason why I shouldn't use a metric seal 45mm outside diameter, 30mm bore (which I think would be a tight fit around the crank end) and 5mm thick?

Thanks again


PS good turn out at Hagbourne...

in some ways it is better than Cassington...KTM and Enfield dealers, little kids riding their trials bikes all over skips and scrap cars, a smattering of classic cars - I wonder if it has become a victim of its own success though because the site is quite tight whereas with Cassington the bikes just take over the whole village!!! Perhaps next year for Cassington we should get all the Commandos in one place with lesser bikes arranged around them.
Hi Alex.
just my two cents , but I had fitted the XS 650 breather valve on my Cdo(got it from germany, XS performance shop)) , it was cheap enough and seems to be good. I had trouble too with my belt (oil passing through inside the case), that's why I had ordered from Steve Maney the superior oil seal , and plus fitted this crank vent , just only few miles from then and apparently no leaks , but still running in the Cdo......wait and see.On my Atlas with a belt too , I had fitted the Maney's seal also, and relocated the breather at the timing side (cause it was early Cdo cases , with Boyer at the camshaft end), no problem, from the beginning , but it is a soft tune engine, and I do not rev it hard.That 's why I was looking for the equivalent here in france, cause it's seems more simple for me because of shipping cost (and I have good discount on those parts!!!)Pierre
That's an idea. If yr housing is worn, the extra 0.55mm might be the answer and I can't see what harm 0.16 on the bore would do. The only issue might be that 5.0mm against 4.76 for the imperial could rub on the front pulley. With my Norvil belt drive it has touched in the past.
I did go to Hagbourne 2006 and yes I thought it was a better night than Cassington. Will stay in touch for next year. Are you doing Calne on Sat?

thanks for the advice, I will try the metric one, it can't be any worse than the one fitted now.

Calne looks unlikely for me because of family commitments...I'll drop you a line if things change.

I did calne in 2006 and thought it was brilliant, helped no doubt by perfect riding weather. Tried to get there last year but could not get past the floods in the Swindon area...highlight of the day was trying to ford a flood...I reckoned the water would be just below the exhaust. It was, until a twat in a 4 x 4 came the other way showing off, bow wave flooded me, had to get off the bike and push it about 50 yards to dry land...very wet. I took off the points cover, wiped the Boyer kit with a rag, kicked it over and she started first time. I had an audience of about twenty onlookers by this time and they all gave a great cheer...I jumped on and rode off into rain...not exactly Ted Simon but exciting anyway.

All the best

The poor old seal gets blamed every time, but sometimes, just sometimes, it's the studs that screw into the crank cases. Give them a good looking at while you're in there.

Cash is right, it's worth checking those three bolts and sealing them with plumbers tape or silicone. Tape is better if the threads cannot be degreased easily. Silicone under the tab washers is also good insurance. Managed to get mine totally dry this winter as it always leaked a little engine oil into the primary. Check the crank seal surface also. may be a rough spot there that wears the seal more.
The metric seal ( if one is made), should work . The crank is 30mm at that point, not 1 3/16th" as the standard seal is.
You can tell this because it is supported by a metric bearing.
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