Leak Down Test (Don't Laugh!!)

I’m pretty sure PL will have the correct liners.

Comnoz and Splatt do make strong cases for fitting new liners.

MichaelB said:

This has been a very interesting read. Even if most of it is above my pay grade. First I think you have mentioned anti-drain sump valve. I have some negative experience with those and have a theory. The damage/ seizing started prior to owning it with lack of oil from the anti drain valve. Possibly from the initial start up. What was the reason for the initial head work? Oil burning? Just a layman’s theiry...

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Depends on which anti drain ball valve is fitted and if the oil pipe is PRIMED before use and is air tight and a clear plastic oil pipe is used and checked that there is oil in the pipe before start up, also RGM use a 3/8 diameter ball with a very weak watch spring-just like the Velo Venom used –in fact the RGM is an exact copy!

Dances with Shrapnel said:

If I read this correctly, you really need to be taking the measurement on the piston skirt at right angles to the gudgeon pin, not "in line with".
I like hobot's assessment on the pistons. They really don't look too bad other than the scuffing but I would give them a good cleaning and inspect the ring lands with a jewelers loop to see that they are not damaged or scratched if you should choose to take that path. From your assessment of the bores (lack of feeling a ridge where the vertical scuff marks are) you may be able to clean them up with a light proper hone.

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a light hone????
more like a very heavy hone!!!!!

splatt said:

A waste of time trying to measure those, the pistons will have collapsed, wouldn't be surprised if the liners have distorted off the barrel in the middle.
Don't reckon they would have done 12000miles like that ,( just read only 5000miles)
Very clean ring grooves with next to no carbon in them, like they have been washed out, horrible marks in oil groove, ring appears to be bottomed out in groove
I'm guessing you've used something to clean the piston tops and bores and washed all the oil out of every where, mexicomikes famous dry start,
Personally I wouldn't trust the liners now, they will most likely have voids ,if money and time are a bit tight take up GRUMPY KNOBS offer but get the barrels resleeved, that also may not be that easy if the available sleeves are smaller than what's in there now,dont get to hung up on gudgeon pin fit alloy rods going to expand anyway

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I never used anything in the engine other than petrol in the petrol tank & oil in the engine, no additives used

The springy bit of the oil ring, the bit that sits against the wall of the piston groove, not the wavey bit in the middle, this was broken in both pistons. The rh piston had about a quarter broken off & the Lh piston had a very small bit broken off, about 10mm. Both ‘bits’ were still in place held by oil stiction. Not sure how significant these breaks are?

Bernhard said:

Depends on which anti drain ball valve is fitted and if the oil pipe is PRIMED before use and is air tight and a clear plastic oil pipe is used and checked that there is oil in the pipe before start up, also RGM use a 3/8 diameter ball with a very weak watch spring-just like the Velo Venom used –in fact the RGM is an exact copy!

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Now I know why SRM removed this valve

Fast Eddie said:

I’m pretty sure PL will have the correct liners.

Comnoz and Splatt do make strong cases for fitting new liners.

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Looks like I’m heading in that direction

Viewing the more detailed Pics, there was insufficient clearance. Nothing else that I know of can cause that sort of wear.

The correct place to measure piston to bore clearance is at the bottom of the piston skirt on the thrust side of the piston. The clearance anywhere else is unimportant because it will be greater in all other locations. Also, pistons are oval shaped, not round so the diameter measured from the thrust side to the opposite side is greater than the diameter measured 90 degrees from that point. SO...lets say the correct clearance for a certain piston per the manufacturer is .004". If the clearance is checked at the non-thrust side of the piston or not at the skirt and found to be "within spec" it's NOT and the actual clearance will be too tight and cause the sort of damage shown in the pics.

IMPORTANT: you cannot measure piston/bore clearance with a feeler gauge! Well, you can but it's like measuring millimeters with a Home Depot yardstick! You must measure the piston diameter across the thrust portion with a caliper and measure the bore with a bore caliper; the difference is the clearance. Pistons usually come from the manufacturer with specific instructions of where to measure. For example, one manufacturer might state that the measurement is taken x thousandths from the bottom of the piston skirt, another might say At the bottom of the skirt. No good machine shop will bore/hone cylinders without having the pistons on hand that will be installed in the engine.

Clearly, whoever assembled this engine either did NOT check the bore/piston clearance or if he/she did, it was done incorrectly.

OK lads here's were I am.

Talked to Norman White, Steve Maney, Mick Hemmings, Geoff in SRM & Pete Lovell.

They ALL said if I was looking for a reliable motor do not go down the Norvil 920 route!!!!! or any 920 route, nuts

Pete explained he raced the Norvil 920 conversion & was very successful BUT he was stripping the engine after every race & checking everything was OK, not something you can do with a road bike. Norman didn't even take a breath before he said no to the 920 option. Mick had similar reservations and also felt my best option was to go back to an 850. SRM have had no experience with the Norvil 920 conversion so did not have an opinion one way or the other.

So, I have some thinking to do, if I give the engine to one of the guys above they obviously want the engine to be 100% which means they will want to replace EVERYTHING apart from some of the major components, which is fair enough as they are putting their name to the engine & don't want issues due to a used part being reused, potentially destroying the engine!

This option comes at an eye watering price, I got an engine rebuilt by SRM about 5 years ago so I'm aware of that particular finincial black hole!

I have one other option in Ireland. There's a chap I've dealt with who is an old school mechanic, even though he's not that old! He's THE only guy I trust with my bikes. He has built his own Norton Cafe racer and rebuilt a beautiful BSA A65 a while back. I'm going to drop into him tomorrow & have a chat about getting the work done with him.

Really thought I'd have it all sorted today but I'd be a fool not to listen to some of the World's best known Norton experts, including a lot of others on this forum who have suggested to go back to a standard 850.

I know some of you guys were routing for the 920 refresh but even Pete was saying that the 920 pushes the limits of what's possible with the Norton Commando & is very much on the edge of 'just possible' leaving you very little headroom for problems, as I well know! Maybe some guys have this 920 conversion & have gone around the World twice with no problems but I can't ignore the advice I've been given, even though it was not what I wanted to hear!!!

More Anon

I might suggest sleeving it to an 880. That is the largest bore that has the potential to last very long on the street.
I will have to check on piston availability as I haven't done one for a couple years.

comnoz said:

I might suggest sleeving it to an 880. That is the largest bore that has the potential to last very long on the street.
I will have to check on piston availability as I haven't done one for a couple years.

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Jim, interesting option!, I was not aware you could get 880 sleeves!!! I presume the pistons use the standard size gudgeon pin?

I didn't re-read all the posts to verify what was done to this engine to make it a 920.

IOW, what parts other than pistons/piston pins (+ rings, of course) and cylinder liners are different from the 850? is that that's all a 920 kit consists of. SO...if that is the case, all you need are pistons/rings/pins and liners to convert it back to an 850. Obviously the liners have to be properly inserted in the cylinders which requires a machine shop but otherwise it seems to be to be a fairly straightforward rebuild. If you didn't before, you now have a good bit of experience with the work you've done. I suspect you could easily do it yourself - other than the liner/barrel fitment. Some price examples that don't seem at all bad to me:

Pistons: http://www.mapcycle.com/categories/...342-e-grp-norton-850-commando-piston-set.html

BUT, there may be more to your specific 920 conversion than I am aware since I have no experience with Norton engines other than OEM 750/850 and their standard oversizes.

worntorn said:

I calculate 906 cc s actual displacement with an 80.5 mm bore.
The new type RGM kit measures the same.
The decals still say 920 tho.
I might just put the decals on the side panels and forgo the rest of the 920/906 exercise.
In 1965 I added an STP decal to my go kart and got an immediate 10 mph increase in top speed.

Glen

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Mike,
clicks conversion was not the RGM one, but a Norvil version.


Cheers,

cliffa

You could get someone like JE to make you any design of piston you want. Similar with liners, shop around and you can get anything you want.

It all depends how much ‘design’ work you want to do.

An 880 with a slightly smaller bore, and equally thicker liners would be great.

An 850 will no doubt do more reliable high mileage than a 920.

So both of the above are very good options.

But...

If I read your posts correctly, you aren’t planning any 5k across desert road trips on it! You don’t do 10k a year on it! And you don’t seem like the kinda guy who wants to put in lots of time and energy into ‘design engineering’.

I still say, that given what you’ve got, and given what you use it for, re sleeve it and keep it as a 920. It’s your cheapest and quickest way of getting a great bike back OTR and having some fun.

It’s gonna blow up or wear out again one day anyway !


All only IMHO of course.

I wonder about going back to standard 850 as well. As I understand it, your head is still configured for an 850? So two big questions would be, can you use the existing rods? What does the rebalanced crank do to the balance factor of an 850? I'm just wondering if you could pull this off without breaking open the cases.

How much did those 920 pistons weigh? It's probably in here somewhere, but figuring out that balance factor would be interesting.

MexicoMike said:

I didn't re-read all the posts to verify what was done to this engine to make it a 920.

IOW, what parts other than pistons/piston pins (+ rings, of course) and cylinder liners are different from the 850? My impression from this is that that's all a 920 kit consists of. SO...if that is the case, all you need are pistons/rings/pins and liners to convert it back to an 850. Obviously the liners have to be properly inserted in the cylinders which requires a machine shop but otherwise it seems to be to be a fairly straightforward rebuild. If you didn't before, you now have a good bit of experience with the work you've done. I suspect you could easily do it yourself - other than the liner/barrel fitment. Some price examples that don't seem at all bad to me:

Pistons: http://www.mapcycle.com/categories/...342-e-grp-norton-850-commando-piston-set.html

BUT, there may be more to your specific 920 conversion than I am aware since I have no experience with Norton engines other than OEM 750/850 and their standard oversizes.

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The stinker are the pins used in the Norvil 920 conversion, they are 13/16 in size, bigger than standard. If this was not the case I would already have it converted!!

This one difference means I need to get the bottom end stripped, crank out, rods off, crank re-balanced, new std rods put on & everything put back together, easy to type, expensive to get done!!

rvich said:

I wonder about going back to standard 850 as well. As I understand it, your head is still configured for an 850? So two big questions would be, can you use the existing rods? What does the rebalanced crank do to the balance factor of an 850? I'm just wondering if you could pull this off without breaking open the cases.

How much did those 920 pistons weigh? It's probably in here somewhere, but figuring out that balance factor would be interesting.

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Can't reuse the rods! 13/16 pins, larger than standard!

rvich said:

I wonder about going back to standard 850 as well. As I understand it, your head is still configured for an 850? So two big questions would be, can you use the existing rods? What does the rebalanced crank do to the balance factor of an 850? I'm just wondering if you could pull this off without breaking open the cases.

How much did those 920 pistons weigh? It's probably in here somewhere, but figuring out that balance factor would be interesting.

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Going to 850 would require new rods and thus a full engine strip down.

It would also require a crank re balance, and looking at the vast array of holes in the crank in some of the pics, I’d guess that re balancing that crank for lighter pistons would need some heavy metal plugs.

Of course, if you want a really sweet running 850 AND you gotta buy new pistons and rods to do so, you could do a lot worse than check out JS web site, an 850 with his lightweight pistons and longer steel rods is very sweet indeed.

That’s what my current 850 motor has in it. Here’s some pics:

Fast Eddie said:

You could get someone like JE to make you any design of piston you want. Similar with liners, shop around and you can get anything you want.

It all depends how much ‘design’ work you want to do.

An 880 with a slightly smaller bore, and equally thicker liners would be great.

An 850 will no doubt do more reliable high mileage than a 920.

So both of the above are very good options.

But...

If I read your posts correctly, you aren’t planning any 5k across desert road trips on it! You don’t do 10k a year on it! And you don’t seem like the kinda guy who wants to put in lots of time and energy into ‘design engineering’.

I still say, that given what you’ve got, and given what you use it for, re sleeve it and keep it as a 920. It’s your cheapest and quickest way of getting a great bike back OTR and having some fun.

It’s gonna blow up or wear out again one day anyway !


All only IMHO of course.

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That was my thinking but all the engineers I talked to said the issue I had is not uncommon with the Norvil conversion, they said a combination of oil control issues and sometimes the liner moves causing issues with getting the CH joint to seal, make this conversion unreliable. The liner actually breaks out of the barrel casting as it's so big, you can actually see parts of the liner from the outside of the barrels.

If I can get a competent mechanic, who has worked on & understands classic brit iron, to help me with the changeover to an 850 and is willing to work with me & be reasonable/sensable about using used but serviceable parts, I might be able to make this work for not that much more than getting the 920 reinstated, that's my thinking at the moment.

It would be daft to sleeve down your barrels to 850. Much better of buying another set of 850 barrels. You’re highly unlikely to find some that don’t need a rebore though, not that that’s any real issue.

I wonder if you could run those big pin Ford pistons successfully in a Maney barrel?