Combat crankcases: designer mistake?

Combat cases are the only Commando cases without a sump filter (only the small sump plug is present) prone to premature oil pump wear and failure. Whatever reason could possibly have crossed the designer’s mind?

Maybe they thought the magnetic sump plug was a better idea? It wasn't.

The oil pick up for that scavenge is poorly designed too. Only saving grace is that lots of returning oil goes up the breather pipe & bypasses the correct return path.

Maybe the apprentice designer was given the job?

Ian

Maybe he had a note on the desk that there was a sump filter screen breakup reported and he wanted to try something different.

Acutually this is a very serious problem in my Combat cases as the swarf from a blow up get sucked into the internal drain while its swollen with heat then contracts and traps stuff in there blowing or even wires can't dislodge. Otherwise Norton did provide a great back up oil return in the big ole breather hose. There is fairly simple way to move the sump drain to rear that's easier than grinding big chunks out of the cases. A screen adapted would be a good thing and magnetic plugs are sold for Combats.

nortonspeed said:

Combat cases are the only Commando cases without a sump filter (only the small sump plug is present)

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1972 and 1973 750 Commando cases have no sump filter, not exclusively "Combat".

The magnetic plugs for these were a later addition. Mine was just a simple threaded plug (bolt).

L.A.B. said:

nortonspeed said:

Combat cases are the only Commando cases without a sump filter (only the small sump plug is present)

Click to expand...

1972 and 1973 750 Commando cases have no sump filter, not exclusively "Combat".

Click to expand...

True! I should have said 'Combat breather type cases'. Still it remains a mystery to me.

nortonspeed said:

L.A.B. said:

nortonspeed said:

Combat cases are the only Commando cases without a sump filter (only the small sump plug is present)

Click to expand...

1972 and 1973 750 Commando cases have no sump filter, not exclusively "Combat".

Click to expand...

True! I should have said 'Combat breather type cases'. Still it remains a mystery to me.

Click to expand...

Yup - me too - please see my thread 'another rescue'

I know I've read something years ago, maybe by Mick Hemmings or somebody else in a British bike mag. They said that someone in the engine design department thought that this was a change that needed to be done since the Commando engine tilted forward and in its Atlas featherbed days, it was a true vertical twin. Of course, if that was the reason, it had no real test time to speak of behind it to see how it worked and was changed back to a sump filter fairly quick.

ETA - I was talking about the decision to move the oil pickup to the front.

Ehk, only real Combat mistake off of good design crankcases was removal of the cam oil bath. Combats can't really wet sump that much as the big ole hose get excess oil forced in pencil thick jet stream back to tank I've seen, so most the mess outside a run to red line Combat is the crank sling forces making it an elastic with bolts stretching and parts shifting and seam separating. Similar to the supersonic Blackbird made with loose seams leaking fuel and oil till heat swollen at speeed. I do argee front sump is bad ideal as better for crank to have some puddle to throw on cam on kick off dry no oil flow to lifters starts but heck should be enough oil over night to do that anyway.

A reed valve on the breather hose of the '72-'73 750 cases ensures crankcase oil evacuation.

In actual real life the real mistake was the breather baffle in the way to remove gear box w/o having to half remove engine to tip it out the way, ugh. The breather exit back there was not a bad idea otherwise. Oh OH yeah, there is another manufacturing mistake in at least Combat cases if not others, the machined faces of the through bolt bosses are a sharp corner stress riser. Dyno Dave found crack in one of mine so I ground them all out and had welder melt in shockingly even deeper to discover cracks in most of them not yet worked to surface. Ugh and another thing cases tend to break through crank shaft hole and 2nd bolt behind barrels. A plate across the bolt bosses on DS was tradtional fix but I did Peel on both sides which allowed adding an 11th clamp bolt right where the dang case seam length tends to dribble oil. Last but not least the through bolts are too thin so tend to work bores more and more and more, enough so a really hard run Combat can twist the cases on the locating dowels to bind like welded together, ughhhhhhhhhhhhh. So another good upgrade is to bore to almost interference fit 7/16" and use hi grade bolts. A fully fettered Commando is quite a nebulous temporal concept.