Crankcase mods for timing chest breathing

… as your pistons descend, there is 750cc that needs to go somewhere, or get compressed.

If your reed valve is attached to the timing side of the crankcase, you have opened up significantly more volume to play with, so there is less pressure on the seals as the compression is not as great.

This will lessen the pressure on the drive side crankshaft seal, the tacho drive and the three alternator mounting studs, but to be of value it does rely on you removing the bottle neck between piston side and timing side.

I am personally a big fan of going the other way, and sealing up the holes between piston side and timing side and having a reed breather on the piston side instead.

Plus, a much lower oil drain hole to reduce the amount of aeration the oil gets.

The combat service sheet was okay at the time, but was a huge compromise half geared toward a fix that could be performed in a small workshop without having to strip the engine right down.

Plus of course, there was no talk of reed breathers at that time.

The Norton engine was originally designed to have oil and oil vapour entering the timing cover to help lubricate the gears, pinions and chains.

Of interest here being the very first Model 7 engines only had a single hole plus the crankshaft disc in the timing side crankcase to allow this to happen. The single hole having a one way valve fitted. This valve was dropped in the first year and other holes added to direct vapour at specific chains etc and help drain excessive oil back to the sump.

The later Commando engines only had a single chain to drive the camshaft sprocket but even this needs a supply of vapour to keep it working correctly. Plus don't forget the Oil Pressure Release Valve kicks off very early with a cold engine start-up. This in turn needs a means of getting back to the sump for recycling. 

If you wanted to go the sump breather route, this works well on my 69 Commando. 

Sump reed valve breather - JS Motorsport

Quick edit... The tube (snorkel) in the centre of the sump breather allows for some residual oil to remain in the crankcase for the oil pump (return pickup?) I use an anti wet sump valve (RGM) on the inlet side rubber pipe between the oil tank and the feed to the oil pump. The down side for the early oil tanks (Greg Marsh from the US pointed out) is that on some versions the breather inlet pipe into the oil tank from the sump sits opposite the breather outlet pipe from the oil tank to the back of the air cleaner. Not a problem with the weak factory breather But if you do start the bike with oil above the sump reed valve breather tube you will get a jet of oil across and down into the air cleaner.  He recommends adding a new separate breather inlet pipe into the oil tank. So far my anti wet sump valve keeps oil below the level of the snorkel. I'll soon know when it doesn't! Hope that makes sense.

Regards Eddie

I like the idea of the piston movement being used to help clear the sump of oil.

However......on my 650 and 750 Nortons, the large drain plug filtered out a lot of the crankcase rubbish and then sent the cleaner oil up an internal tunnel to the return side of the oil pump.

I know that the sump plug disappeared from some Commando models and so presume that the return oil pick-up point moved to somewhere else inside the cases. My question being, by having two outlets from inside the crankcase does the oil pump still get its fair share of the black gold???

The sump plug breathers have a ‘snorkel’ which makes sure that the oil pump isn’t starved of oil.

The Colorado Norton Works/Jim Comstock crankcase breather mounts in the same location as the 72 Commando breather.

Both can evacuate a wetsumped crankcase in a few slow kicks, but importantly leave enough to keep the oil pump fed.

Obviously, the 850 breather location on the ‘magneto ear’ and following the Combat service notes does not give you this benefit.

I’m really interested in the various views on pros/cons of different breather arrangements, since I’ve got a ‘72 Combat engine which had a breather from the old magneto location and oil seemed to build up in the timing case at high revs, improved by restoring the original combat breather location at the back of the crankcase, together with a breather valve to promote lower case pressure. The case hadn’t got enlarged/extra holes into the timing case (at the moment!). Unfortunately I have not got a clear view of what is being compared above (2 paths for return, potential for starving the oil pump?). If anyone could provide some sketches illustrating the points it would be much appreciated.

Hello all,

the sump plug breather will not fit on a 1970 Commando because of the main stand cross tube

under the engine which is in the way. I also drilled holes in the timing sinde crankcase and fitted a Yamaha XS 650 breather to the cover on the rear of the timing side crankcase.

I connected the hose from this breather with a t-connector to the hose coming from the timed breather at the left front side of the engine.