I'm still learning a bit about the newer cases and breathers

Can anyone recommend another link for more information?
I seem to have an early 72 case that someone has welded a reinforcment rib under the rear barrel area.
It has been rigged with a breather in the bolt on plate for the magneto.
I also understand that this case can have an oil scavenge problem, does this rear its head at all in non racing situations? I think I read this oil pickup problem on Old Brit.

Re: I'm still learning a bit about the newer cases and breat

Cookie said:

Can anyone recommend another link for more information?

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Try this: http://atlanticgreen.com/engcases.htm

Cookie said:

It has been rigged with a breather in the bolt on plate for the magneto.

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The cover plate on '72 model crankcases was actually a blanking plate fitted to cover the mounting hole for the abandoned electric starter drive that the factory intended to fit at the time!
However, they didn't succeed in getting the prototype starters to work more than a few times before the starter drive self-destructed, - often destroying the crankcases in the process, apparently!

Early and late model Commandos do not have the plate.

Cookie said:

I also understand that this case can have an oil scavenge problem, does this rear its head at all in non racing situations? I think I read this oil pickup problem on Old Brit.

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Yes the information on how to correct the problem is on the Old Britts website: http://www.oldbritts.com/ob_start.html

Thanks very much mate. I'm amazed at how little I know about this bike even though it is so similar to my former early Commandos.

Dont think that you HAVE to do any mods to enjoy your commando, it should work perfectly well as it is. My '71 750 with this style of crankcase certainly has for the 30 years that I have used & abused it.
OK, if the engine is stripped for other work you may consider the upgrade mentioned but I certainly would'nt bother a major re-build just for this reason.

Remember the old saying "Dont try & fix what aint broke"

Regards

Bob.

Thanks Bobolink. It seems like someone has already adressed the case cracking problem, and I'll bet with the rebuild they have upgraded the bearings. For that matter I think I was putting in the upgraded bearings quite a number of years ago, I think my dealer only stocked the upgrades.
It does seem silly to tear down an engine that is running so cleanly with only a few thousand miles on it. It doesn't even leak or smoke at all.

I'm suspecting that in normal use one would not run into the scavenging problem or everyone would be insisting you do it. I've been running this bike in the mountains over from San Mateo to Alice's resturant in Woodside. While you can hit 90 MPH on a straight you are then braking as hard as possible to get around the next corner. I have a hard time getting out of third gear around here.

Cookie,
I have a '73 750 from new and have never had a problem with scavenging. Others on the board have. I think it is mainly riding habits. If your riding habits are such that your engine is living most of its life at 4-6K rpm you probably will see the level in your oil tank disappear on these runs. I admit as I have collected other motorcycles over the years '73 gets less miles and gentler ones. This bike as over 30K miles on the original bottom end and a standard bore (just one set of rings). I think that speaks to the "imminent" destruction without the modification.

However, I have a second '73 750 that I have split the cases to replace the rod and main bearings as I did not know its history. While I had the cases apart I had the lip of the sump machined. I'll move the breather before I put it in the frame. This project is long term and I did the bottom end of the engine now out of convenience and opportunity. I would always take the opportunity to do this mod if the cases are split for another reason.

I've had my '72 combat since new. When I was young I used to ride home from work at 2:30am - 2 miles of it was straight on a brand new highway at 110 mph. Never had a scavenging problem and didn't even know there was supposed to be one until a couple years ago. I think it is one of those Internet tales that get repeated until it is considered fact. The Old Britts article makes sense to me and if I had my cases apart for a rebuild I would do the crankcase mod and move the breather but it has worked fine for me as is. (YMMV). I have 35000 miles on it now and have always used Kendall 20W50 motorcycle oil until the past couple years. (The original crank bearings went 4700 miles and were replaced by Superblends.)
Russ

batrider said:

I think it is one of those Internet tales that get repeated until it is considered fact.

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I doubt that it could really be an Internet tale, as the problem does seem to have been encountered by racers for some years (note the reference to the 1978 MCM article by Mick Hemmings in the Old Britts scavenging tech. page) but it doesn't appear to cause problems under normal road use - or the Norton factory would have had even more warranty claims than it did at that time? However they certainly didn't continue to use that crankcase scavenge system.
So it would only appear to occur under sustained high RPM, so I don't think 2 miles would be enough to show up any problems, unless an oil pressure gauge was fitted then a rider wouldn't actually be aware of it?

As, I'm sure, the majority of '72/'73 750 riders are a maybe just a little more restrained in their riding habits these days, I would think the crankcase modification probably isn't absolutely necessary for normal road use?