Late 1972 Engine Rebuild - Some Questions...

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northern750

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I have enjoyed over 20,000 trouble free (mostly) miles over the past ten riding seasons with my late 1972 (October on the headstock plate) 750. Everything was running fine but I'd always wondered if it had superblends in it and it also had over .030" end play on the crank - coming up on 40K miles so time to look inside.

I was quite pleased with the condition of the internals. If the bottom end had ever been apart, it was a long time ago given the hard, crusty remnants of case sealer.

I have come up with a couple of questions:

1. it had R&M 6MRJA30 main bearings in it (have the scrolled '6'). If 750s were fit with superblends at the factory beginning with s/n 211891, my bike being three hundred-odd numbers before that, are these possibly the original main bearings or have they been replaced at some point - I'm not sure what were considered to be superblends in 1972...

2. The bike had no heat insulating washers under the inlet side valve springs. Should I rebuild it with the washers? - The lack of washers there has had no apparent bad effect. If so, are there any modifications I'd need to do to the seat so the valve springs do not over compress?

3. When reassembling the crank, does anyone bother to center punch the end of the nuts like the factory did or will red loctite and proper torque suffice?

Thanks.
 
northern750 said:
1. it had R&M 6MRJA30 main bearings in it (have the scrolled '6'). If 750s were fit with superblends at the factory beginning with s/n 211891, my bike being three hundred-odd numbers before that, are these possibly the original main bearings or have they been replaced at some point - I'm not sure what were considered to be superblends in 1972...
Click to expand...

I was under the impression that is the 'Superblend bearing.

Does that mean all 1972 model bikes got that exact bearing in the end (replacement and off the build line later on.
The reason I ask that is, it seems every time superblend comes up, the F.A.G bearings in the 850 seem to come up.... In the 1973 750 ?

#
I use Loctite personally, the punched end unless it made quite a flare might stop a loose nut departing at best.
 
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northern750 said:
1. it had R&M 6MRJA30 main bearings in it (have the scrolled '6'). If 750s were fit with superblends at the factory beginning with s/n 211891, my bike being three hundred-odd numbers before that, are these possibly the original main bearings or have they been replaced at some point - I'm not sure what were considered to be superblends in 1972...
Click to expand...


The 6MRJA30 is what the factory called 'Superblend'. The fact that your Commando's serial number is three hundred or so before 211891 suggests they might be the original bearings.

https://www.nortonownersclub.org/sites/default/files/Main-Bearings-v1.1.compressed.pdf

northern750 said:
The bike had no heat insulating washers under the inlet side valve springs. Should I rebuild it with the washers?
Click to expand...

Is it a Combat ('C') cylinder head?
Edit: Does it have a Combat camshaft (should be too late to be an actual Combat model but who knows?).
 
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L.A.B. said:
Is it a Combat ('C') cylinder head?
Edit: Does it have a Combat camshaft (should be too late to be an actual Combat model but who knows?).
Click to expand...

It's not a Combat, no 'C' on the head. It has 32mm carbs and inlet but not the high compression. Is there a way to visually tell if the cam is a 2S - I'm fairly certain it isn't - an actual Combat I rode years ago felt like a two stroke, nothing like the bike I have...
 
Late 72 bikes are a mix of parts. Mine had Superblends, unstamped lower compression head with 32 mm carbs and standard unmarked cam.
 
Is there any stamp on the head just above the right exhaust valve cover? ‘73 750 heads were RH5 and RH6.
Not all combat heads had a “C” stamp (under the head steady). Look at the fin spacing between the top cylinder fin and bottom head fin. A combat head is noticeably closer here than the others.
As above, Norton was probably scrambling for modifications during this time due to the warranty claims from the combats.
 
It seems there is also some mythology existing as to how many bikes may have been "fixed" before leaving dealerships. I'm not sure this has ever been authenticated nor busted as its hard to know what an individual dealer might have done with bikes that had shipped but not sold.
 
When my Combat's drive side main bearing went at 4700 miles my local US dealer would not cover it because the bike was purchased in England. This supposedly came down from the East coast US distributor, Berliner. I got to know the mechanic and co-owner of the shop pretty well and I don't remember them working on any new Combats before sale.
 
Measured the cam lobes, spot-on 0.330" both inlet and exhaust so definitely not a Combat cam.

As for the head, it has '139973' stamped into it under the head steady, no other markings.

cash said:
Fitted under the exhaust only. If fitted under the inlet the washer might restrict the oil return hole ?
Dave
Click to expand...

The heat insulating washer is smaller diameter than the steel valve spring seat so I doubt it would interfere with the oil hole.

Brings me back to my question - any problems fitting the washers under the inlet spring seats and can this occur without the need for any modifications to the head?

Thanks.
 
cash said:
Fitted under the exhaust only. If fitted under the inlet the washer might restrict the oil return hole ?
Click to expand...

According to the parts books they were fitted to both inlet and exhaust except for the Combat and '73 RH1 (750) cylinder heads.
 
northern750 said:
Brings me back to my question - any problems fitting the washers under the inlet spring seats and can this occur without the need for any modifications to the head?
Click to expand...

As long as the valve springs do not become coil-bound* then it shouldn't be a problem.

*(Which is apparently why the insulating washers were not fitted to Combat head inlets due to the higher-lift of the Combat camshaft).
 
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cash said:
Fitted under the exhaust only. If fitted under the inlet the washer might restrict the oil return hole ?
Dave
Click to expand...

I recently finished assembling a head for a '71, 750; heat insulating washers (NM23392), according to Old Britts, are used under the intake spring collars (NMT2073), as well as the exhaust spring collars. Is this information in error? I acknowledge that your comment came with a question mark, so I'm hoping that one of the master gurus will chime in with the definitive answer. In my case the head is not yet bolted on and would be relatively easy to correct, if the heat insulating washers are not needed/required for the intakes.

Best.
 
As for the head, it has '139973' stamped into it under the head steady, no other markings.

I doubt this is a stamping done at the factory, but who knows during this time. Some on the machine shops have stamped their reference number on parts. My ‘73 750 had all the moving parts in the lower engine weighed with crank balanced. They put a stamped number on the crank flywheel. When I needed new piston they looked up the file number and machined the new pistons tp balance again.
 
My 850 RH10 head did not have insulators on the intakes and advice from this forum was that is the correct situation.
Btw you may want better intake seals than stock, which crumbled to bits while removing. I went with Kibblewhite Viton rubber seals and these are working well.
 
The early combat tune engines (2S cam+) for a while...they were delivered with FLAT bottom spring purches. Being wider where they seated on the head, and therefore NOT conforming with the machined pocket. Under these conditions they theoretically correctly removed the insulating washer but the springs were not really allowed to bottom out in the pocket and when the cam pushed the valve open they did indeed become coil bound and is what I suspect bent my original low mileage 2S cam. A stock 1S cam 06-1084 can/should have insulation washers on both in and ex of 750 heads. I noticed 850 are apparently slightly different heads...
IIRC there is a service note or production change calling for the spring perch correction. This is also the basis for the update made in the INOA tech digest.
 
Thanks all for the advice. Given the head on my bike has 32mm carbs and inlets, I suspect it's a RH5 or RH6 and should be okay with insulating washers under the inlet valves - I'll try and look out for coil binding under static test before starting the engine.
 
rvich said:
It seems there is also some mythology existing as to how many bikes may have been "fixed" before leaving dealerships. I'm not sure this has ever been authenticated nor busted as its hard to know what an individual dealer might have done with bikes that had shipped but not sold.
Click to expand...

Years ago I talked with T.C. (Hog Slayer) Christiansen at Sunset Motors Kenosha WI about the Superblend Saga. He told me that once the Combat problems were identified, he was replacing the bearings in his new unsold bikes IN THE CRATE. He claimed he got so good at it he could do one in a couple hours.
 
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