Leak Down Test (Don't Laugh!!)

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Not at all, that’s a good question, and one that I asked myself as soon as I’d posted!

I don’t know if there’s any real advantage, but basically, having extra steps in the head and then steps in the piston to miss the head all adds undesirable shapes and edges to the combustion chamber.

It MAY make the motor more inclined to ‘knock’ as the charge ignites prematurely due to poor flame path and / or those pointy edges getting over heated.

Then again, it MAY make no difference!

But as Pete will have everything needed, it’s not a big job for him to do any such machining.

I would discuss it with Pete and go with his advice.
 
Fast Eddie said:
Not at all, that’s a good question, and one that I asked myself as soon as I’d posted!

I don’t know if there’s any real advantage, but basically, having extra steps in the head and then steps in the piston to miss the head all adds undesirable shapes and edges to the combustion chamber.

It MAY make the motor more inclined to ‘knock’ as the charge ignites prematurely due to poor flame path and / or those pointy edges getting over heated.

Then again, it MAY make no difference!

But as Pete will have everything needed, it’s not a big job for him to do any such machining.

I would discuss it with Pete and go with his advice.
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Appreciate the explanation.

The only disadvantage I see to doing this is if I plan, at some stage in the future, to bring the bike back to a standard 850 I would not be able to use the head if I get it machined to the same diameter as the 920 bore. Considering good RH4 heads are difficult to come across I'd like to keep this option open but I'll chat with Pete about doing this, thanks for the suggestion.
 
click said:
Appreciate the explanation.

The only disadvantage I see to doing this is if I plan, at some stage in the future, to bring the bike back to a standard 850 I would not be able to use the head if I get it machined to the same diameter as the 920 bore. Considering good RH4 heads are difficult to come across I'd like to keep this option open but I'll chat with Pete about doing this, thanks for the suggestion.
Click to expand...
If you are planning on going back to an 850 eventually why not bite the bullet and do it now?.
Your engine is half way stripped you'd be better off sourcing some new/used con rods rather bushing the ones you have,get the crank re balanced it may actually not need re balancing but while it's out get it done and find yourself some 850 barrels and pistons and as fast Eddie says a good 850 is better than a bad 920 cheers
 
Are there any high mileage 920s out there?

Note Jim Cs roadbike is at 880, not 920. Even there, I seem to recall that he reported some top end wear issues.
I'll have to reread that info to make sure I have it straight, the memory has been a little shakey lately.

I've also read on the NOC of some problems with a Maney 920 that went out of round after a few thousand miles, started blowing oil.The owner went back to a stock cast iron 850 setup.
There all of these options that give a bit more oomph for awhile, but for how long?

Starting to rethink the whole 920 idea myself.

Glen
 
Next time just use a smaller bead of pliobond around the bore, the pushrod tunnels and the oil return hole. No point in applying it outward past the bolt holes.
 
jseng1 said:
Next time just use a smaller bead of pliobond around the bore, the pushrod tunnels and the oil return hole. No point in applying it outward past the bolt holes.
Click to expand...

Yep, figured that one out myself, I gave a 'liberal' coating on the deck, both sides of the gasket and the head!!! Thought this head was staying on for a long time :rolleyes:
 
worntorn said:
Are there any high mileage 920s out there?

Note Jim Cs roadbike is at 880, not 920. Even there, I seem to recall that he reported some top end wear issues.
I'll have to reread that info to make sure I have it straight, the memory has been a little shakey lately.

I've also read on the NOC of some problems with a Maney 920 that went out of round after a few thousand miles, started blowing oil.The owner went back to a stock cast iron 850 setup.
There all of these options that give a bit more oomph for awhile, but for how long?

Starting to rethink the whole 920 idea myself.

Glen
Click to expand...

I appreciate all your comments about making the move to a Std 850 now. I don't want to, at this moment, turn this into a project. I don't have the skill or the resources to strip the bottom end so I'm looking at paying for each & every hour an experienced Norton guy has to spend doing this work.

Because I do very little mileage on my classic bikes (I have some modern bikes also) It might be 2-4 years before I even think about changing to a standard 850.

I'll ask Pete what his experience of the 920 conversion is, in terms of longevity!!

I obviously do not want to waste my money on work that has a very limited life span!! More research required!!
 
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Pete Lovell will have the answers.
When I was making enquiries about 920 kits 12 years ago his name came up as the man in the know for the UK.

Glen
 
Click, do keep us up dated as to what Pete says.

FWIW, I say keep it at 920, it’s less work and more fun!

There are people out there with good mileage on 920 motors. Reggie and Matchless (on this forum) both use their bikes properly, I know Reggie uses his for touring in Europe.
 
I'm learning more and liking it less the more I read/re-read this thread! Am I understanding correctly that the combustion chamber diameter at the head/barrel joint is larger than the barrel diameter? AND that the head gasket protrudes into the combustion chamber, thus providing "steps" in the combustion chamber? If so, I'll wager that with all else being equal, this engine may make less power than a properly assembled 850.

Classic hemi combustion chambers (like the Norton and, say the 392 and 426 Mopars) have one big advantage and one big disadvantage (and several smaller ones that I won't get into)...

The advantage is they can breathe much more efficiently than a wedge head and, therefore, make more power with the same engine displacement.

The disadvantage is that, compared to a wedge or a more modern multi-valve head, combustion is more difficult to light off and, if not done well, flame propagation is less efficient, negating any advantage to better breathing. This is why some race/performance applications of Hemi heads use twin spark plugs. A hemi combustion chamber with mismatching head/cylinders and head gasket that intrudes into the combustion chamber just adds to the difficulty and makes an efficient burn almost impossible. (note: the combustion chambers of modern engines claiming to be "Hemi's" have a totally different shape, NOT the classic shape; no major maker uses that shape nowadays. Though it was superior to a wedge in the day, it is not nearly as efficient as modern shape cyl heads with multi valves.

Obviously, if the 920 head (combustion chamber diameter) is properly matched to the cyl diameter and the head gasket is also correct for the bore, AND the ignition system is capable of lighting the mixture in the larger combustion chamber, then, of course the engine could produce more noticeably more power than an 850.

In any case, as noted, it sounds as if you have found a person/shop that knows what they are doing so it should end up as a nice motor!
 
When I rebuilt my commando the first time, I had the balls of an elephant but not a lot of experience. I was young enough to think if other people can do it then so can I, but old enough not to think it would be easy. So,... just like you're saying, I did my research. In my case what I found out was that my model had the early crank bearings which norton changed to superblends, so I felt like I had to get my original bearings changed out which meant I had to refresh the bottom too. I was scared to death to do it, but also scared that if I didn't do it I would be sorry later...

I think you have the right question for your machinst guy,... Is the version of the 920 which you currently have a viable version or not? Once you know that answer you can go about plotting the bikes return path. From following along with you thread the thing I don't like is the connecting rods with the larger diameter wrist pins. That part is keeping you from having more piston and barrel size choices. As you are well aware changing those con rods means tearing apart the bottom... If it were my bike, I'd want those con rods out of my bike, so If this 920 set up proves to be a headache, I could go back to 850 pistons and barrels sometime in the future.

Good luck going forward, I think many of us have been at the crossroad you're at now. I think you're asking all the right questions in order to make good decisions. I look forward to hearing that you're put back together and enjoying it.
 
Thanks for all your suggestions & comments , I'm going to take a few days to ponder life & all things Norton!!

I'll come back once I talk to Pete & seek some comments from a few others.
 
Hi Mexico, no the head combustion chamber on the common 920 is the same as the 850, but the piston of 81mm -(so 920) had been machined , just like a squish band to marry with that chamber head , plus the gasket is particular to 920 so no gasket protruding , it was not at that time a bodge job , but was thought as an "improvment" made by people who think about , and Pete Lovell was one of them ........the only problem was the piston with their big pin ( at that time !), now we could have normal pistons with normal std pins ( Pete had made one 920 engine for me 20 years ago, and it was the dogs bollocks in those days, much used here in Europe for scramble side cars), nobody was playing with goodies as we do now such as alloy jugs , belt drive and so on ......we were young , and believed we were the kings ! now we have access ( and Access forum) to lot more knowledge , we have more money , BUT we are older :(
 
You might find the pictures of my old 920 race engine teardown of interest here

https://www.accessnorton.com/NortonCommando/920-race-engine-teardown.17644/

The pistons are the old RGM style with the larger wrist pins, with the stock alloy rods honed out to suit. I machined the tops for the pistons for higher CR and for larger valve notches to accommodate the race cam and bigger valves. I ran this engine in my roadracer for several years, then fitted it into our landspeed racer streamliner for several years, and finally into my featherbed race bike for more landspeed events. I had no problems with it until I started using nitrous oxide injection for more power.

I'm not saying you should or shouldn't stick with the 920, with either the old style or new style pistons. This is just another data point for you to consider.

FWIW, although the pistons were originally from Fair Spares, and the cylinder liners were done here in the US, the end result is very close to what RGM sells.

Ken
 
lcrken said:
You might find the pictures of my old 920 race engine teardown of interest here

https://www.accessnorton.com/NortonCommando/920-race-engine-teardown.17644/

The pistons are the old RGM style with the larger wrist pins, with the stock alloy rods honed out to suit. I machined the tops for the pistons for higher CR and for larger valve notches to accommodate the race cam and bigger valves. I ran this engine in my roadracer for several years, then fitted it into our landspeed racer streamliner for several years, and finally into my featherbed race bike for more landspeed events. I had no problems with it until I started using nitrous oxide injection for more power.

I'm not saying you should or shouldn't stick with the 920, with either the old style or new style pistons. This is just another data point for you to consider.

FWIW, although the pistons were originally from Fair Spares, and the cylinder liners were done here in the US, the end result is very close to what RGM sells.

Ken
Click to expand...

Hi Ken,

I did read this already. Amazing that the 920 was used in so many high stress situations. I'd be more than happy to put your 'left over bits' into my bike!!
It's a testimony to your engine building skills that the bike lasted so long :)
 
baz said:
If you are planning on going back to an 850 eventually why not bite the bullet and do it now?.
Your engine is half way stripped you'd be better off sourcing some new/used con rods rather bushing the ones you have,get the crank re balanced it may actually not need re balancing but while it's out get it done and find yourself some 850 barrels and pistons and as fast Eddie says a good 850 is better than a bad 920 cheers
Click to expand...

Does anyone with crank balancing experience have any comments regarding the difference in weight of the pistons? I`ve also included a picture of the small end modification.

http://s628.photobucket.com/user/sk...[user]=100741386&filters[recent]=1&sort=1&o=0

http://s628.photobucket.com/user/sk...[user]=100741386&filters[recent]=1&sort=1&o=1

http://s628.photobucket.com/user/sk...[user]=100741386&filters[recent]=1&sort=1&o=2
 
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