1973 850 Rebuild

Mofosheee said:
Figured it would be a guy like Jim to conduct such analysis. Thank you
Click to expand...
As you live down the road from Jim, why not ask him? Tell him what you want the bike for and he’ll advise everything you need in probably two sentences !

You haven’t told us what you want to use it for? Without that it’s like asking ’how long is a piece of string’! You can go high lift, high comp, 920cc, 1007cc, 5 speed box, 6 speed box... the world is your oyster ...

Assuming you want a reliable, usable, street bike suitable for modern traffic, if it were me, I’d do something like this:

A good head job with Jim’s hardware is a good start.
Get the 3 threads in the head machined for 3/8“ UNC inserts and get the studs from cNw or JS.
Get Jim to check your pistons and bores and do whatever he says.
Yes, one of Jim’s reed valve breathers is an excellent mod.
As is Jim’s head steady.
The stock cam is a good profile and perfectly fine for most uses, but others are available if you want more / different performance. As already posted, some Commandos had soft cams and / or followers. Asking Jim to check your exsiting cam and followers is a good idea.
Stock crank and rods are fine for most uses, new bolts from AN are wise. Ask Jim to check your big ends, polish the rods to a blemish free finish (to avoid stress risers and a broken rod).
Playing with thinner head gaskets and no base gasket will raise the CR a bit, debatable gains, but a good exercise IMHO.
Composite head gaskets work perfectly IF the tedious re torquing process is followed. They’ll fail easily if it’s not! They’re less susceptible to minor oil weeps than copper. Copper has the advantage of offering different thicknesses and, if they do fail, they are less likely to blow out totally and leave you stranded.
A new oil pump is inexpensive and a complete must IMHO.
A rebuilt, or new, pressure release valve too.
The stock clutch can work fine and give long service. But it can slip and is very heavy. A belt drive and Barnet clutch pack gives a good slip free clutch and a takes a ton of weight off of the long suffering gearbox main shaft.
Other than the above, the stock gearbox will be fine with a good rebuild and new bearings inside etc. Some Commandos had layshaft bearings that can fail in a dangerous way, so a thorough rebuild is essential.
Oil? Read Jim’s study, it’s worthy of a phd and states it as it is.
Ign is the next hot topic, several good EI choices out there, all with there fans. I like Tri Spark. cNw sell Tri Spark and a well proven, compatible, high output double coil.
Stainless head fixings etc? There is stainless and there is stainless! cNw sell fabulous, top drawer, ARP stainless head and barrel fixing kits with 12 point heads which make tightening a breeze. The narrowed shanks provide stretch which helps better head gasket sealing too. JS sells similar kits.
Carbs? Your old carbs may be fine, have a good look at them. But chances are they’ll be worn and gummed up and generally no good. Old carbs can provide hours of fun if you like problem solving! New Premiers are very good carbs, but need stripping and checking for cleanliness and correct jets etc before use, read the for sale section for a brilliant US source of new Premiers. FCRs are available as a plug n play kit and are awesome, if you like that kinda thing (I do)!
There are many choices of gasket goo. Each having their fans. I like Wellseal. Some newer versions, like Locktite, are designed to be gasket REPLACEMENT, these should not be used WITH gaskets as they stick too well and make future gasket removal a complete nightmare, incurring risk of damaging the gasket faces.
The isolastics should be looked at whilst down this far. I’d buy new ones from AN if replacements are needed.

But, as already mentioned, please pick Jim’s brains. Also study the workshop manual. And join the NOC even if it’s only for one year so you can buy the Mick Hemmings engine and gearbox rebuild DVDs. And, peruse this forum, there’s loads of brilliant info on here, I only bought my Commando a few years ago and found this forum invaluable. There’s a rebuild section with loads of info and pictures that will help you. It’s also really informative to read threads about other people’s problems and how they solved them etc.

Ultimately, if your fairly familiar with spannering (wrenching) and have a decent shed (shop) you’ll be fine with a Commando, they’re very simple and easy to work on. You’ll enjoy it. Then when you’re done you‘ll enjoy riding it at least as much.

All only IMHO of course.
 
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I do not want to disagree but will say.
There is little point going to a UNC thread at the three 3/8" head to cylinder/barrel studs unless you have to to.

As far as I know there is nothing wrong with the stock 20 TPI studs themselves (The stud thread into the head could be longer but not sure the UNC studs are any different there)

I was under the impression the JC Al/Bronze insert had a coarse thread on the OD so will never pull out of the head, the insert material strong enough the 20 TPI thread will not pull out of it either and recall a JC video that showed the stud tightened to the snapping point with no drama to the insert or cylinder head.
The stock 3/8" AN studs cost pennies.
 
Time Warp said:
I do not want to disagree but will say.
There is little point going to a UNC thread at the three 3/8" head to cylinder/barrel studs unless you have to to.

As far as I know there is nothing wrong with the stock 20 TPI studs themselves (The stud thread into the head could be longer but not sure the UNC studs are any different there)

I was under the impression the JC Al/Bronze insert had a coarse thread on the OD so will never pull out of the head, the insert material strong enough the 20 TPI thread will not pull out of it either and recall a JC video that showed the stud tightened to the snapping point with no drama to the insert or cylinder head.
The stock 3/8" AN studs cost pennies.
Click to expand...
You might be right. Can you post the video? Heck of a lot of work though if you’re wrong!

Jim C did my head, and thats how it came (with UNC thread inserts and studs).

Personally, if I was having the insert work done, fitting upgraded studs would just be what I’d do (or did do) automatically. But if there’s no need, there’s no need...
 
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I had to go find the video from 2015.
The stud necked not broke.
A much later video from late 2019 shows the Helicoil taking the win in that test. (which you could install at home which is important) showed the 16 TPI stud puling out at 420 in/lbs which is 5 ft/lbs over the factory torque of 30 ft/lbs / 360 in/lbs.
2015


2019.



A Helicoil although an insert is not like the Al/Bronze body insert which is far from a DIY at home option.
Not forgetting the posts about so called mentors stating, just crank the likes of crankshaft bolts up with no consideration to suggested torque so no wonder these head studs get a hard time with that thinking.

As far as I know (which is not much) there is no wire type (Helicoil ) in 3/8 - 20 to suit the stock stud thread or any other commercial insert, only the JC version in Al/Bronze.
 
Last edited:
Unless I am mistaken, that ‘necked’ stud is a fine UNF thread? Perhaps that’s why Jim does the UNC mod.

Interesting to see good ole helicoils perform OK in the second video, but as our OP already has his head with Mr C I’d be surprised if he intends to do DIY inserts. Again, unless I missed something, all studs in the second video are UNC? The videos therefore seem to support the use of UNC studs?
 
Time Warp said:
I had to go find the video from 2015.
The stud necked not broke.
A much later video from late 2019 shows the Helicoil taking the win in that test. (which you could install at home which is important) showed the 16 TPI stud puling out at 420 in/lbs which is 5 ft/lbs over the factory torque of 30 ft/lbs / 360 in/lbs.
2015


2019.



A Helicoil although an insert is not like the Al/Bronze body insert which is far from a DIY at home option.
Not forgetting the posts about so called mentors stating, just crank the likes of crankshaft bolts up with no consideration to suggested torque so no wonder these head studs get a hard time with that thinking.

As far as I know (which is not much) there is no wire type (Helicoil ) in 3/8 - 20 to suit the stock stud thread or any other commercial insert, only the JC version in Al/Bronze.
Click to expand...

Amazon product ASIN B00AC1H3W0
 
Fast Eddie said:
As you live down the road from Jim, why not ask him? Tell him what you want the bike for and he’ll advise everything you need in probably two sentences !

You haven’t told us what you want to use it for? Without that it’s like asking ’how long is a piece of string’! You can go high lift, high comp, 920cc, 1007cc, 5 speed box, 6 speed box... the world is your oyster ...

Assuming you want a reliable, usable, street bike suitable for modern traffic, if it were me, I’d do something like this:

A good head job with Jim’s hardware is a good start.
Get the 3 threads in the head machined for 3/8“ UNC inserts and get the studs from cNw or JS.
Get Jim to check your pistons and bores and do whatever he says.
Yes, one of Jim’s reed valve breathers is an excellent mod.
As is Jim’s head steady.
The stock cam is a good profile and perfectly fine for most uses, but others are available if you want more / different performance. As already posted, some Commandos had soft cams and / or followers. Asking Jim to check your exsiting cam and followers is a good idea.
Stock crank and rods are fine for most uses, new bolts from AN are wise. Ask Jim to check your big ends, polish the rods to a blemish free finish (to avoid stress risers and a broken rod).
Playing with thinner head gaskets and no base gasket will raise the CR a bit, debatable gains, but a good exercise IMHO.
Composite head gaskets work perfectly IF the tedious re torquing process is followed. They’ll fail easily if it’s not! They’re less susceptible to minor oil weeps than copper. Copper has the advantage of offering different thicknesses and, if they do fail, they are less likely to blow out totally and leave you stranded.
A new oil pump is inexpensive and a complete must IMHO.
A rebuilt, or new, pressure release valve too.
The stock clutch can work fine and give long service. But it can slip and is very heavy. A belt drive and Barnet clutch pack gives a good slip free clutch and a takes a ton of weight off of the long suffering gearbox main shaft.
Other than the above, the stock gearbox will be fine with a good rebuild and new bearings inside etc. Some Commandos had layshaft bearings that can fail in a dangerous way, so a thorough rebuild is essential.
Oil? Read Jim’s study, it’s worthy of a phd and states it as it is.
Ign is the next hot topic, several good EI choices out there, all with there fans. I like Tri Spark. cNw sell Tri Spark and a well proven, compatible, high output double coil.
Stainless head fixings etc? There is stainless and there is stainless! cNw sell fabulous, top drawer, ARP stainless head and barrel fixing kits with 12 point heads which make tightening a breeze. The narrowed shanks provide stretch which helps better head gasket sealing too. JS sells similar kits.
Carbs? Your old carbs may be fine, have a good look at them. But chances are they’ll be worn and gummed up and generally no good. Old carbs can provide hours of fun if you like problem solving! New Premiers are very good carbs, but need stripping and checking for cleanliness and correct jets etc before use, read the for sale section for a brilliant US source of new Premiers. FCRs are available as a plug n play kit and are awesome, if you like that kinda thing (I do)!
There are many choices of gasket goo. Each having their fans. I like Wellseal. Some newer versions, like Locktite, are designed to be gasket REPLACEMENT, these should not be used WITH gaskets as they stick too well and make future gasket removal a complete nightmare, incurring risk of damaging the gasket faces.
The isolastics should be looked at whilst down this far. I’d buy new ones from AN if replacements are needed.

But, as already mentioned, please pick Jim’s brains. Also study the workshop manual. And join the NOC even if it’s only for one year so you can buy the Mick Hemmings engine and gearbox rebuild DVDs. And, peruse this forum, there’s loads of brilliant info on here, I only bought my Commando a few years ago and found this forum invaluable. There’s a rebuild section with loads of info and pictures that will help you. It’s also really informative to read threads about other people’s problems and how they solved them etc.

Ultimately, if your fairly familiar with spannering (wrenching) and have a decent shed (shop) you’ll be fine with a Commando, they’re very simple and easy to work on. You’ll enjoy it. Then when you’re done you‘ll enjoy riding it at least as much.

All only IMHO of course.
Click to expand...
Hi Eddie
You touched on most questions that I have, your input is greatly appreciated. Sincere thanks to the input of others who have responded to my questions. Dropped my cylinder head off at Jim's few months ago. Haven't heard from him, but I understand he takes time. Jim and I haven't had the discussion you suggested. While I have a mill and TIG welder, I do not have the specialized equipment, skills/knowledge/ jigs of the Norton that Jim has.
Respecting his privacy, I will say that his shop is rather impressive.
I'm familiar with Norton's having owned my first in the 70's. I hung my helmet up 40 years ago after being witness to an unfortunate bike incident.
Times have changed. Sealants, ignitions, drive belts, bearings, improvements the list goes on.

My goal is to reassemble this 850 to near stock condition AND incorporate many of the mods found on this forum. It's not to race, nor do I want to squeeze every bit of HP out of it that is possible. The Cafe look is not my style. Not a big fan of the isolastics mounted engine, (but will live with it) In the name of simplicity and reliability, the exhaust system and the Lucas Prince of Darkness electrical system are under review. Many parts ordered from Andover.
More questions to follow. For example, should I go the single carb Mikuni?, are main bearings from this un-named country a safe bet,

Thanks for being there!
sincerely,
jerry
 
Jerry, so long as you buy the main bearings from Andover or RGM or another trusted Norton specialist you’ll be fine.

Single carb versus twin is another one of those polarising topics!

A single carb strangles the engine above 5,000rpm, so if you’re intended use is below that, no problem. The issue isn’t really the carb per se, single carb Triumphs run very well. The issue on a Commando is the limited space for a good flowing 1:2 manifold. If you want to enjoy a wider usable range than that, twin carbs would be better.
 
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If one never needed to pass then it's easy to stay below 5 k
But when you've been following a string of cars and holiday campers for ten slow miles and suddenly there is that open uphill passing lane, twin carbs and 6500 rpm all the way!

Glen
 
Fast Eddie said:
Jerry, so long as you buy the main bearings from Andover or RGM or another trusted Norton specialist you’ll be fine.

Single carb versus twin is another one of those polarising topics!

A single carb strangles the engine above 5,000rpm, so if you’re intended use is below that, no problem. The issue isn’t really the carb per se, single carb Triumphs run very well. The issue on a Commando is the limited space for a good flowing 1:2 manifold. If you want to enjoy a wider usable range than that, twin carbs would be better.
Click to expand...
Then twin carbs it is. You guys have been down this road before. Thanks for the advice on the bearings
 
Never really liked the Norton exhaust systems. Came across an exhaust.............opinions please;
1973 850 Rebuild

Jerry
 
Last edited:
MichaelB said:
Do it, do it, do it............r
Click to expand...
Haven't looked very deeply into this. From my readings am I correct to understand that the crank case reed system is a check valve that prevents oil from gravity feeding into the case when the engine is shut down?
Thank you
 
Mofosheee said:
Haven't looked very deeply into this. From my readings am I correct to understand that the crank case reed system is a check valve that prevents oil from gravity feeding into the case when the engine is shut down?
Thank you
Click to expand...
No. It’s a one way breather valve that allows crank cases gases out but not back in, so your engine isn’t sucking 828cc of gasses in and out 6,000 times per minute! The main advantage is they prevent pressure build up and therefore help reduce oil leaks.

The other device you refer to is the anti wet sump valve. Now you’ve mentioned that I KNOW you’re just posting to try and cause a fight...!
 
Depends which one but it is to vent the crankcase, stopping or reducing wet sumping is something else.
The standard vent hose to oil tank on the 850 would be plugged and a hose from the reed valve run back to the oil tank instead.
 

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Hey...............I'm new here. I don't know what I don't know! Looks like a wise thing to do.
OK, a one way breather check valve :)
 
Time Warp said:
Depends which one but it is to vent the crankcase, stopping or reducing wet sumping is something else.
The standard vent hose to oil tank on the 850 would be plugged and a hose from the reed valve run back to the oil tank instead.
Click to expand...
And this is the part? I promise to study it. Thank you
 
Mofosheee said:
And this is the part? I promise to study it. Thank you
Click to expand...

You could also add a reed unit from the likes of Mikes XS to the stock hose (I see they have gone up in price) instead of the case mounted version from JC.
Its only money but don't get caught up in the trap of having a bike with all the whizz bang parts that spends most of its life sitting in the shed ( IMHO)

Search results for: 'reed valve'

www.mikesxs.net www.mikesxs.net

That is why I mentioned the bronze stud inserts yesterday since the head is at the JC shop, they out perform all other options (it seems) with the stock fine pitch stud.
Unless you must have snazzy reduced or wasted shank UNC studs.
My point yesterday, whizz bang is not the be all.

Like you said, do it once, do it right.
 
Time Warp said:
Its only money but don't get caught up in the trap of having a bike with all the whizz bang parts that spends most of its life sitting in the shed ( IMHO)
Click to expand...
That's why I stay away from certain vendors who try to sell me things like $1500 swing arms :)
 
I am hesitant to give an opinion but here it is anyway.

UNC studs are a waste of time if JC is close by, get Al/bronze and fit original studs (You will never pull the insert out of the head and you will not pull the stud out of the insert)

Buy a Pazon ignition direct off Pazon, the exchange rate is in your favour and they have free shipping (iirc)
The Tri Spark has all the components inside the points cavity, I would not touch one based on that, I do not care what anyone else says, why is beyond logic.

Pazon Ignitions Limited

Overview of Digital & Analogue Ignition Systems. Classic bike & car electronic ignitions for road & racing.
www.pazon.com

Sure Fire or the Altair.

If JC is doing the head (I got the exhaust ports and three threads Al/Bronze inserted for stock studs along with new Kibblewhite guides and valves) that is done and dusted.
As long as the rockers and shafts/pins are good.

If you need pistons, JCC.
Copper head gaskets are OK if the surfaces are near perfect (imho) unless you want to plaster the lot with gasket agents.
At least the composite has a sealing ring.
You can use Kevlar thread around the pushrod holes (Grant Tiller inspired) over dental floss or Nana's wool.

Norton's blow up (That was my introduction going back to my first ride in 1978/9 (I forget being elderly) or throw rods, or blow head gaskets.
The India made main bearings as many have mentioned are about it, will they do 50000 miles, who knows, its about all there is besides NOS.

If the carburettors are worn get some new Premiers, fit them after going through them, consider a single cable throttle gantry from Don Pender (Madass140 here)

The crankshaft, ask JC about balancing (They came with all manner of drilled holes (number of) from the factory on the same size engine which says something.
Prep the rods, not with a mop polisher.
Rod bearings, you have RGM or AN shells, take your pick as far as those as off the shelf.

Go though the oil pump, it has narrow pressure side gears so is only ever going to be average, fit an oil pressure gauge.

If you fit the JC reed unit under the sump you will need to plug holes in the timing side crankcase wall which is where the stock breather hose was from (T/S chest)

A Commando without twin peashooters ? (No comment)

The gearbox, bearings as mentioned, my preference would be a roller layshaft on the 850 or either on the 750 (RGM ball or AN roller)
Replace the bearings and all springs (wound or hairclip) along with the kick start pawl, there is a good chance the shaft pawl housing will be cracked at the thin part, it probably doesn't matter.
The case itself might be cracked between the layshaft and output bearing bore at the thin part (Triumph used a bush then needle roller for good reason perhaps with small diameter gears)
You might ned to use locking agent on the smaller bearing if so.
The gears teeth wear but still go round.

If you end up with a reliable head gasket joint, valves that seat and seal along with the pistons, the crankshaft rotates with clean and filtered oil at a reliable pressure, all should be good.

There are plenty of other things if you have a mill and lathe.
Tapered roller steering head bearing retaining the stock spacer tube (shimmed)
Wider and 2RS sealed wheel bearings (off the shelf, no felts etc)

The first objective is usability, all the add on parts being just that an add on.
There is little wrong with the stock parts once prepped. (imho)

1973 850 Rebuild 1973 850 Rebuild
 
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