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#850351 05/31/21 1:44 pm
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gunner Offline OP
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After deliberating for ages I've decided to strip the engine on my 72 Norton Commando Combat.

I bought the bike as a wreck but the engine was complete and apparently in good condition, I believe the mileage to be around 17k based on the documentation which came with the bike.

After straightening the frame, adding a new rim, rebuilding the gearbox, and rewiring, I've been riding the bike for a couple of years. I made the mistake of adding an oil pressure gauge to check the condition of the engine which was untouched. I found the pressure could barely keep above 10psi at 3k rpm and then decided to check the rocker oil hole alignment. After re-orientating the rocker holes there was a slight improvement so then I added a new AN oil pump.

Despite all of the work above the oil pressure still well under 15psi and seemed to keep dropping the further I rode and the hotter the engine got, so I decided to strip the engine.

I've removed the head and barrels and found the following:-
- the pistons are STD and are of the slotted type, there is scoring evident but there was excellent cylinder pressure and no oil burning. As can be seen, the RH piston has markings from the exhaust valve, looks like some kind of failure occurred in the past.
- one of the cam followers shows an odd broken area around where the pushrod fits, looks like a pushrod came loose at some time and possibly got stuck keeping the valve open, hence the marks on the crown of the piston.
- the barrels also show signs of scoring and dark areas which I assume is rust from where the engine hasn't been run for years
- the cam followers show wear but pass the fingernail test, I can see a line but can't feel it
- the camshaft doesn't look too bad but will have to wait for further inspection when the cases are split.

The biggie is the amount of rock on the conrods, I haven't measured it exactly yet but seems like a few mm side to side and I'm not confusing it with side play. I've tried the up and down test and there does seem to be a barely perceptible amount of play. I imagine that the big ends are worn badly and are causing oil pressure loss.

A couple of other things I've noted are that the cam follower retaining screws are lock wired, there are a couple of other places where lock wire has been used like the rocker cover screws. I wondering if someone has tried racing this engine or at least revved the nuts off it?

I'm now wondering how far to go on the rebuild, I can see that I will at least need the crank reground, new big end shells, and new oversize pistons.

Any opinions on the cam and followers, can these be refaced and reused, or are new ones needed?

I haven't got to the main bearings yet, but hopefully, these are in good condition, how long do they usually last? I don't mind splashing out for a new set but just wondered what their life expectancy is.

The cases look like they are original combat, the breather is at the rear and there is a small sump drain plug, engine no is in the 220* series and the head has the "C" stamp on top. I will probably do the oil way and breather mods as most others do.

Thoughts and comments welcome on what I've found so far and how to proceed.

Piston with markings from the exhaust valve
[Linked Image from i.imgur.com]

STD slotted pistons

[Linked Image from i.imgur.com]

Cam followers wear.
[Linked Image from i.imgur.com]

Camshaft lobes

[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

Barrel condition

[Linked Image from i.imgur.com]

Crankshaft and Rods

[Linked Image from i.imgur.com]


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There are slotted pistons and slotted pistons, one type has an open groove behind the oil control ring in its groove front and back joining the slots, these are the ones to avoid. Second type has a series of drilled holes under the oil control ring between the slots, these are ok.

Ignore any rock on the rods, if you can just feel some perceptible up and down play then you have 3 thou clearance (human feel minimum clearance is normally 3 thou) and the shells are worn, you need to check the journals as if you have an oil filter they last a long time and new shells may be enough. Only measuring the journals will tell you, avoid Wassells shells as for other engines they seem to be undersize so stick with Andover Norton or RGM if you want new, they will be trimetallic so prone to wear, NOS Glacier will be bimetallic Alum/Tin and wear slower in a road bike but needs an oil filter.

The last Combat was 211110, your may have a Combat head but its not a factory Combat so the camshaft may not be the Combat cam.

220 series should be superblend mains, so if they are really superblends and good I would run them again. I also thought 220 series was a 73 model year not 72 but of course could have been built and registered in 72, 73 model year had a Hi compression head as an option but with a std cam..

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Heads, note the RH number which will be stamped on the head, RH3 is Combat, RH6 is 73 750 Hi compression.

Norton Commando CYLINDER HEADS
Identification
Number Part
Number Capacity Compression
Ratio Inlet Port Remarks

RH1 060988 750cc 9.0 : 1 30mm Standard up to 1972
RH2 061427 750cc 10.25 : 1 32mm AMA racer
RH3 063327 750cc 10 : 1 32mm 1972 Combat
RH4 064038 850cc 8.5 : 1 32mm 1973
RH5 064048 750cc 8.9 : 1 32mm 1973 low compression
RH6 064097 750cc 9.3 : 1 32mm 1973

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gunner Offline OP
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Thanks Kommando, I will check the head to see what RH number it is and also the type of slotted piston I have, although they look too scored to me to reuse.

I think you're right about the year and suspect its actually an early 73 bike although the head definitely has the "C" stamp, so maybe a mix and match of parts.

The engine is still in the frame, so the next step is getting it out and splitting the cases, stay tuned for further updates.

Given the trouble I had getting the head off, I can imagine that refitting the engine with the head and barrels fitted is going to be a lot easier.


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Originally Posted by gunner
The cases look like they are original combat, the breather is at the rear and there is a small sump drain plug,

That doesn't conclusively identify the cases as 'Combat', but can be any 200000+ series 750, both '72 and '73, Standard or Combat, however...


Originally Posted by gunner
engine no is in the 220* series and the head has the "C" stamp on top.

...The Combat series officially ended at 211110.

220xxx is 750 MkV (or '73 if you like). No Combat parts are listed in the 212278 parts book. Note that the "High compression RH6 head" isn't the Combat 'C' head.
http://www.classicbike.biz/Norton/Parts/1970s/Commando_Parts_73-74.pdf


Pistons
https://www.norvilmotorcycle.co.uk/techtalk34.htm

Originally Posted by gunner
I will check the head to see what RH number it is

Edit: Although the Combat head is RH3 you may not find that stamp on a 'C' head.

Last edited by L.A.B.; 05/31/21 4:11 pm.
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gunner Offline OP
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Thanks LAB, there is definitely a "C" stamp on the head, I will check tomorrow for the RH number and post more images.

I'm away from home at present so can't check the piston type or cylinder head.

I was wondering if it's possible to determine the piston type by the numbering stamped on the head? I can see a 18964 stamping together with ID13, STD, "A" as well as the RH and EX (right and exhaust) markings.

I'm guessing the ID13 indicates a particular piston to cylinder fit and the STD means standard bore but I don't know whether 18964 is a specific part no.

Either way the pistons are probably past their best and need oversize new ones.


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Given the trouble I had getting the head off, I can imagine that refitting the engine with the head and barrels fitted is going to be a lot easier.

Only if you leave the front iso mount off before fitting the engine, support it with a jack until the front iso mount is fitted and plenty of wrapping to protect the paint on the frame tubes. I would still leave the head off but then I have taken the head off a few times, as long as you get those pushrods well up inside the head then its a lot easier.

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Originally Posted by gunner
I was wondering if it's possible to determine the piston type by the numbering stamped on the head? I can see a 18964 stamping together with ID13, STD, "A" as well as the RH and EX (right and exhaust) markings.

I'm guessing the ID13 indicates a particular piston to cylinder fit and the STD means standard bore but I don't know whether 18964 is a specific part no.

'18964' is the Hepolite number.
https://www.ebay.com/itm/Norton-750CC-Commando-STD-PISTON-RINGS-PIN-AE-HEPOLITE-18964A-/383239077396

'STD' is standard bore and the 'A' denotes the factory bore grade size or "fit" so very likely to be the original pistons or at least original for those cylinder barrels (look for 'A' on the cylinder).

'ID13' I believe identifies the piston as being '750 Commando' and with a 220xxx serial, therefore, should be the later, stronger type without the slots behind the rings.

Last edited by L.A.B.; 05/31/21 5:11 pm.
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gunner Offline OP
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Quote
'ID13' I believe identifies the piston as being '750 Commando' and with a 220xxx serial, therefore, should be the later, stronger type without the slots behind the rings.

Very interesting, looks like they are probably the original pistons and I suspect the bottom end has never been opened up.

I will get the piston skirt to cylinder dimension measured and also ring groove clearance. If they are within tolerance then I guess there's a possibility I could get away with reusing them together with a new set of rings and a rehone, though somehow I doubt it and a rebore is a more likely outcome.


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A rehone and new std pistons may work, measure the bore wear and allow for depth of grooves to see where a clean bore would come out at. 50k miles would be typical before needing a rebore.

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I was looking on the Norvil website and they list 3 types of 750 pistons as follows:-
- Commando & Dominator Hepolite pistons with 8.9:1 compression, these are £83
- Commando & Dominator (unknown make) pistons with 9:1 compression, these are £145
- Norvil - Race Use Powermax Type Skirt with 9:1/10.25:1, these are £186.90

Just wondering what the difference is between the first two. I note the Hepolite pistons are 8.9:1 compression vs 9:1 for the second ones listed. Surely the small difference in compression isn't really noticeable, however, the price difference is over £60.


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I never use Norvil for anything so can't comment on these pistons other than to say stick to other suppliers.

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Combat (200000+) builds are sometimes sloppy but yours seems to be a "hack attack" from a previous owner.
I saw no reference to the weight of oil you used for your evaluation of oil pressure! I always use 50W for spring summer and fall-40W winter
RH1 heads were never marked, RH3 were marked with "C" (yours is a "C" RH3) OK
Early RH5 and RH6 were not marked in a consistent fashion like 850 heads. Measure the port size and depth of squish band to verify the version.
Detecting the piston types requires additional internal investigation of the "meat" supporting the rings.

RGM Norvil and ANIL have all screwed me over the years, but Norvil got the bulk of my business and I still use them all.
Just new rod shell may boost the oil pressure. Measure the rod throws. My friend got 160,000 miles on a standard crank with an occasional shell replacement.
Curious of the rocker shaft orientation? lot of bad information and opinions coming out of NOC


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Thanks for the comments Dave,

I've now pulled the rings from the pistons and they are the later 'good' slotted type, see pics below, though they will be replaced with new ones.

Additionally I've checked over the head and cant see any evidence of a RH stamp although there is a "C" stamp.

The inlet ports measure 32 mm and the squish band which is most prominent towards the inlet ports 2mm (0.08") deep at the edge, the inlet valves measure 1.5" wide.

For info, testing the oil pressure was done using both 50W oil and 15W60, both types produced over 60psi at startup (new OPRV used and adjusted), but the pressure slowly dropped so that after a good 20 miles the pressure somewhere around 10 psi at 3k. I then checked the rocker oil holes with a paper clip and found them to be unobstructed, so I turned them 180 degrees to face away from the head, this lead to a small improvement. I then fitted a new AN pump and there was further slight improvement, now up to barely 15psi at 3k.

My thinking was then that the big ends are worn and maybe other issues as well, hence the strip down.

I'm still not sure what head I have but you can see for yourself what I've got below.

Cylinder head "C" stamp but no RM stamp:-
[Linked Image from i.imgur.com]

Cylinder head with BIRCO, and 06-0380 forging marks.
[Linked Image from i.imgur.com]

Inlet ports 32mm
[Linked Image from i.imgur.com]

Slotted piston
[Linked Image from i.imgur.com]

Last edited by gunner; 06/01/21 3:41 pm.

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Have been researching further and I now agree with Dave that the head is the RH3 Combat type.

This is based on my understanding is that standard squish band recess lip was 0.120 " deep from edge of head and that the RH3 Combat head had 0.042" machined off the cylinder face, resulting in squish band 0.078, which is virtually what I have.


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The pix you show do have the drain COMA ,s yes..... but do not show slits from coma to coma.
You unfortunately do have a typical combat head. fine for bragging but poor on the dyno and road. An RH1 is better on the road despite the slight lower CR.
No problem, use what you have and just make it mechanically clean and run it....
Std cam or 2S?
How are the jounals? for smooth and size?
Head running rich BLACK LOL !


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Agreed about the head running rich, I think thats just from running with the choke on for a few miles last week, normally the plugs show correct color after the choke is off. The bike came with a conversion to single Mikuni 34mm which seems to work well.

I don't know what cam if fitted but taking some basic measurements of the cam lobes with engine still fitted, it seems to match the 2S dimensions.

The plan at this stage is simply to inspect the big ends, fit new shells and/or regrind as required. I will do the breather mods whilst I'm in there as well.

Pistons will be replaced and I will strip the head for inspection.

Not sure if the cam needs replacing or is good enough to reuse, doesn't look too bad so far. Cam followers will probably get resurfacing.

I don't plan on gaining any extra HP, just want decent oil pressure and a reliable bike.


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leave the breather where it is at. add a reed valve and all will be fine. CNW sells a nice one.


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add a reed valve and all will be fine.

Thanks for that advice, sounds like a good idea, and the CNW part looks well made so will research further.


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If you don’t put the reed valve breather on it I would make one suggestion: be sure to do the breather and oilway mods together. The ‘72 style oil pickup naturally sumps at rpm and the crank “paddlewheels” the oil to the back of the crankcase and up the stock breather and back into the oil tank. My experience was, if you move the breather and don’t alter the scavenge pickup things get worse on the “sumping at high rpm” front.

Last edited by linker48x; 06/12/21 12:13 am.
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Originally Posted by linker48x
If you don’t put the reed valve breather on it I would make one suggestion: be sure to do the breather and oilway mods together. The ‘72 style oil pickup naturally sumps at rpm and the crank “paddlewheels” the oil to the back of the crankcase and up the stock breather and back into the oil tank. My experience was, if you move the breather and don’t alter the scavenge pickup things get worse on the “sumping at high rpm” front.


IF you do the oil pick up mod PLEASE do not butcher the case as per INOA tech digest or the old britts mod.


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Today I got a chance to work on the engine and managed to remove it from the frame and split the cases.

I removed the timing side conrod and inspected the big end, I found the following:-
- the big end shell is worn and copper is visible, it looks like there was some kind of grey metal over the copper, but most of that seems to have worn off.
- the shells have the Vandervell marking and have part no 23255, additionally there is a 4444 stamp. I'm not sure if these are undersize of standard shells based on the 23255 marking.
- The big end journal is in good condition and when I measured it with a micrometer, I found it it to be 44.25mm in diameter. This suggests it may have been ground under size as I believe the standard size is 44.450/44.463 mm.
- The main bearings are in good condition and are made by SKF with a marking of 3nu06EC. I'm not sure if these are Superblend's or just a size that happened to fit.
- the camshaft has no oil flinger and no markings apart from a stamp which seems to be either "L" or "T", so I'm not sure what bike is intended for. Cam at full lift approx 0.322 inlet and 0.315 exhaust, maybe the lobes are worn?

Based on the above, it looks to me that the crank has been reground at some point and that new big end shells have been used, the shells are well worn hence the loss of oil pressure.

Big end shells
[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

[Linked Image from i.imgur.com]

Big end journal
[Linked Image from i.imgur.com]

Camshaft
[Linked Image from i.imgur.com]

Last edited by gunner; 06/14/21 7:48 am.

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The lead-Indium layer has almost entirely gone off those shells, they are worn !
They are worn so evenly though, and if the big end journals are not oval at all ?,
you should be able to fit new (undersize) shells and all will be good ?
Need to be measured that this is so, obviously.

Combat cams have more lift on the inlet than exhaust - unlike the std cam - that they could well be combat. ?
At least one of those lobes looks a little bit rough ?

NJ306E and NU306E are very similar bearings, but only the NJ306E is whimsically called 'Superblend'.
The difference is in how the shoulders are presented, and which side the shoulders are on ?
Whether this makes any difference in actual use/endfloat maybe someone could say ??
Probably not, since your motor is still hanging together after some miles ...

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I tried measuring the journals again and couldn't find any ovality, so I'm hoping to reuse them. I Will probably take the crank to a specialist for checking and see what they think.

Looking at the shells again I believe the 23255 stamp indicates STD size, whereas my crank looks like it's been ground undersize. I'm guessing that someone has previously been in the engine, added the SKF mains, and had the crank reground. Then they managed to fit STD shells on an undersize journal which would account for the low oil pressure and wear.

Not sure whether to get the cam reconditioned or just buy a new one, I will try and find the cam timing so I can get an idea of what it might be off.


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Those shells are well worn, also the engine has been run with dirty oil as the shells have pronounced extra wear in line with the oil hole. Check the radius in the corner of the journal, needs to be a minimum certain figure of 0.090" from memory but confirm from workshop Manual which also has the journal undersize figures. The rocker shafts being turned the wrong way round probably initiated this issue, wear was too far gone when you turned them back the right way.

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