72 crankcase mod to add sump filter

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The 72 cases were a backward step, the removal of the sump filter was the worst part of this. The current recommended mods cure the issue with the forward sump pickup not working at high revs by changing the location to the back. The lack of a sump filter can be partially covered by drilling a series of holes in the driveside dam that tries to keep the oil away from the rear breather, but the timing side case does not have the dam so debris can still get directly to the return side of the pump via that route.

If you compare cases with the factory sump filter to the 72 cases there is not enough thickness in the 72 cases for the sump filter, you could possibly weld a large lump of alloy onto the 72 cases but the danger of distortion from heat would be high.

Here is my attempt at installing a gauze filter that will stop large debris from entering the return side of the pump.

Step one is to mill a flat on the bottom of the driveside case, to minimise the amount of material removed I tipped the case 4 degrees as the taper for mould release was 3 to 5 degrees. The dot on the case mating surface is the centre of the hole on the timing side case for oil return, needed as a reference point.



Next step was to drill and tap 4 M4 holes and a central 19mm hole, the 19mm hole is not threaded that is just machining marks. The 19mm hole takes the filter mesh you can see sitting on the case.






Mesh is inserted, it has a rubber flange which fits neatly inside the block which goes over it and is held in place by the 4 M4 holes.



Next the large hex is screwed in, this nips up against the rubber flange on the gauze creating a seal plus it has a Dowty washer, there is a central hole to take a sump plug with magnet to filter out the finer ferrous debris.



Finally the sump plug with magnet is screwed in.



and this is the view inside of the case showing the mesh from inside the added oil gallery



The mesh fits the 19mm hole snuggly so all oil will have to pass through it.



I had to do a 3.5mm dog leg on the return gallery so that 1 of the M4 holes did not break through, this could be avoided by using a hexagonal mount instead of square. The large hex and sump plug could be dispensed with by just covering the gauze and rubber flange with a plate with recess.

I am still considering what to do with the dam.

1. Leave as is and let all the oil move to the timing side before then moving to the gauze and the oil return.

2. Drill with a series of 1.5mm holes.

3. Remove completely at the height of the gauze.

2 is my preferred option currently.
 
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I would leave it as is if your still running the rear breather. it helps stop the crank from slinging oil into the breather. also nice work.
 

elefantrider

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Looks good. A lot of work.

Is it generally considered that '72 cases with the small drain screw are not as good as '71 or '73 with the large plug?
Wasn't this the main reason many of the '72 Combats failed?
 

Derek Wilson

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Looks good. A lot of work.

Is it generally considered that '72 cases with the small drain screw are not as good as '71 or '73 with the large plug?
Wasn't this the main reason many of the '72 Combats failed?
Combats failed due to the installation of straight roller mains and the extra load created by the HP enhancements. Crank flex destroyed the bearings and sometimes the cases in very short order. Installing "SuperBlend" bearings was the fix, and it continued right through 850 production as well. All 750's after s/n 200000 share this same case design, combat or not.

While the scavenge system in these later 750 cases is not the greatest, it will not cause immediate engine failure if the engine is well maintained, has adequate breathing, and good oils / filters are used. I have put about 40,000 miles on my Combat after rebuild and it still runs strong with no ill effects. The rebuild was done long before the crank case mods became popular, so it is still stock.
 
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Is it generally considered that '72 cases with the small drain screw are not as good as '71 or '73 with the large plug?
Wasn't this the main reason many of the '72 Combats failed?

All large sump plug cases are better than the 72 cases.

I deliberately called these cases 72 cases but they are from a Combat engine that will be built as a Combat. 72 Combat engines failed more often that non Combat 72 cases and the resulting debris went into the pump but the case did not cause the Combat failures. The more frequent failures came from the higher horse power, lower gearing, tops of pistons coming off and main bearing failure.
 
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Nice work :)
The '72 cases certainly need modding if the bike is being ridden hard, IMHO.
The OldBritts article captured the issue perfectly - in race conditions the oil pressure dropped to zero, but backing off restored pressure.
The Combat failures I've seen have been of the rod-through-the-case variety, but it's only a small sample.
They didn't all blow up.... ;) but it's remarkable that Norton survived the effects of the '72 case design.
 
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These 72 Combat cases had a rod depart, you could see the gouges inside the cases which had been filled with epoxy, which also showed on the exterior in one place so a full hole inside to out. I removed all the epoxy earlier and then got it all welded before dressing it back myself.
 
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All large sump plug cases are better than the 72 cases.
Hog Slayer Twin engine dragster was 2- 20M3S cases.
The early 200000 cases were thinner and crack prone from the stress risers on the rear mount bosses. That was finally fixed but not the "thin" part. Followed by the 300000 series that were the best IMO.

K nice job/project :cool:
 
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