72 crankcase mod to add sump filter

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.

That is an impressive job for sure.

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.

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?

elefantrider said:

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?

Click to expand...

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.

elefantrider said:

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?

Click to expand...

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.

Isn’t it fun to malign the combat 750s? There sure are a lot of them still running.

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.

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.

kommando said:

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

Click to expand...

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

kommando said:

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.

View attachment 77591

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.

View attachment 77592


View attachment 77593

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.

View attachment 77594

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.

View attachment 77595

Finally the sump plug with magnet is screwed in.

View attachment 77596

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

View attachment 77597

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

View attachment 77598

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.

Click to expand...

Nice work, it would have been even nicer if you hadn't had to do it.

I only said they were 'to be avoided'....I owned a set once, but I still avoided them!

Thanks kommando for posting this. This stock 207742 crankcase pushes oil out the tach cable. I don't know if it is from oil pump cavitation and pressure build up or what, I will follow this thread. My first step is to replace the stock breather with a reed breather. Next, if necessary, I will modify the crankcase as per these ideas.

Very clever. I do like an elegant solution. If I ever have the cases apart again on my Combat 750, I will be very tempted to copy your design.

Ken

BERT said:

Thanks kommando for posting this. This stock 207742 crankcase pushes oil out the tach cable. I don't know if it is from oil pump cavitation and pressure build up or what, I will follow this thread. My first step is to replace the stock breather with a reed breather. Next, if necessary, I will modify the crankcase as per these ideas.'

Click to expand...

I have 2023XX - I installed one of Don Pender's '72-'73 750 reed breathers, and it is very effective.

One thing about the rear breather is it will return excess oil as well. I have never had an issue with my Combat breathing, leaking, or returning oil to the tank. I do agree that the rear filter is nice and added protection, but I have not felt the need to tear into my motor to add it either.

My $0.02 - FWIW

Seems to me it isn't a debate as to whether or not oil quits flowing at high RPM but rather how long you can get away with it. Most of us don't hold our RPMs above 5k long enough to worry about it, which is why I'm riding around on an unmodified Combat.

However this project addresses keeping crap out of the oil pump, which is really a different issue. Keeping bits of your motor out of the oil pump could be the difference between a rebuild and the scrap heap.

Just heading out for the day, will post dimensions and where I got the parts later.

Square base is a 15mm length cut from 1.5" square alloy bar. I ordered a 50mm length off ebay UK and you got 1 free cut which saved me setting up the 4 jaw chuck on the lathe and doing a part off cut followed by milling flat both sides.

Large hex is 1.3" across flats (think I got that for Norton or BSA fork top nuts and its a left over piece), total length is 30mm and its threaded for M27 1.5mm for 16mm.

M27mm Dowty washer.

Mesh is from a CG125 engine sump filter pack


Sump plug and magnet is generic M12 1.25 sump plug.