RGM 920

I am thinking of installing the RGM 920 kit on my nearly stock 75 MK111 Interstate. Has anyone who has used this kit done a before and after dyno?
I searched and read several threads on this general topic, but those discussions centred around bikes that had been highly modified.
I'm hopeful that adding 11 percent to the displacement and leaving all else alone would give a gain of maybe 6 or 8 percent to horsepower and torque.
My MK 111 has the RH4 32 MM port size, so I suspect the ports are already big enough for a bit more displacement. Exhaust is standard crossover pipes with straight thru peashooters. Ignition is Boyer.
I would plan on setting the compression of the RGM KIT at around where the bike is now as stock, 8.5 to one, or at most sneak it up to nine to one.
I'm looking for power below 6,000 RPM, just the way the bike is now.

920 Commando

Hi there - I had my 850 MkIIA engine rebuilt by RGM as a 920 and it's superb. I bought RGMs 'special parts book' previously and they go into a fair bit of detail ref the 920 kit and what's required if you fit it yourself - they do state that "it is possible to fit the 920 kit to a 750 bottom-end but it's not recommended" they don't go into much detail about this statement but I suspect its to do with the barrel fitting (barrel 'thru-bolts' on the 850 but only barrel-base bolts on the 750). RGM don't quote any power figures for their 920 but Norvil quote something like 68 BHP for their version.
My own bike is fitted with a single 36mm MkII Amal Concentric which seems to suit it well. The torque is great and I suspect that the fact that I also fitted a belt drive and uprated clutch helps prevent any clutch slip.

If you do fit the 920 Kit then I suspect that you'll need to fit larger carbs and also upgrade the clutch. I suggest that you call RGM and discuss all of this with them - I've always found them very helpful.

920 Commando

Further to my previous reply - I should have read your posting properly !! - I misread it as stating you had a 750 not a Mk3 850. In this case you shouldn't have any problems but I would suggest that you speak to RGM in any case ref work required to fit the 920 kit.

I have fitted 920 kits to two Commandos about 18 years ago. Both ran 4S cams (and are still running the same 920 hardware).

You will of course get more torque and power , but on the two that I did there wasn't any gain on top speed. This was running twin 32mm Amals. I didn't persue the higher top speed issue very much, as the more low down grunt that I got was what I and my friend had wanted in the main. The only problem that I had was with a liner which moved as it hadn't been seated properly, but that was easily fixed (by Pete Lovell).

If I remember correctly, this will mean that you will have to run a copper head gasket, and when I was doing this I had to modify them to suit the larger bore myself, I don't know if they come cut to size now?

Reggie, what compression ratio did you set the RGM kit at?
Roger tells me that without any machining the kit provides about 7.5 to one, however this can be raised somewhat by machining material from the cylinder base.

I would have guessed that two 32 MM Amals should provide lots of Carbeuration since many report good results with only a single 36 MM on 750s and 850s.
It would seem that the standard setup with almost twice the Carbeuration of the common single Mikuni conversion ought to work OK for a 920.

As far as I can remember it was 9:1ish. It was about 18 years ago and cannot be sure, but I don't remember it being as low as RGM are suggesting. I bought a burette especially for the purpose of doing the compresion ratio. I don't remember having any material machined from the cylinder base, unless it was done in advance as part of the "kit."

I got my kit from Fair Spares (I don't think they were called Norvil at that time). I understand that RGMs current 920 conversion kit has different, lighter pistons than Norvils and RGMs previous 920 conversions, and that the little end is left standard. The kit I used needed the little ends enlarging, and the exchange con rods came polished and crack tested.

Sorry I can't be more helpful but it was quite a long time ago. The extra torque of the 920 is nice. Enjoy

I ordered the 920 kit this AM. In one of the earlier threads it was stated that the RGM kit uses Cosworth pistons. Roger (RGM) tells me that years ago he did contact Cosworth to see if they could supply him pistons for a big bore kit, but their cost estimate was about $400 per piston.

He didn't tell me who supplies him with the 920 pistons , but he said they are not Cosworth. They are forged pistons though, which is quite amazing since forged pistons normally are a big ticket item and his entire kit is just 143 GBP with the VAT deducted.

Now I just have to deal with the dreaded exhaust port repair.
Has anyone tried the repair kit that Norvil sells? (not a helicoil)

I have had an exhaust port repair done by Norvil last year. They sent my head to Pete Lovell who I understand does a lot of the work for Norvil (Mr. Emery). Pete is a very nice bloke to deal with , and has many years experience repairing Nortons.

Are you thinking of shipping it to the UK for the repair? If so, you can go directly to Pete Lovell for advice and repair, and you can ask him to explain exactly what he does. As far as I can remember, it is a brass insert that is heat shrunk in. A very nice job, very neat and strong, but it is late and I am tired so best check with Pete himself in case my failing memory is misleading me :?: . There was quite a long wait when mine was done as he was very busy at the time he had my c/head.

Contact details are;

http://petelovelldevelopments.com/location.cfm

Enjoy the 920 :twisted:

Reggie said:

As far as I can remember, it is a brass insert that is heat shrunk in. A very nice job, very neat and strong, but it is late and I am tired so best check with Pete himself in case my failing memory is misleading me

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When I sent my cylinder head to Norvil it was returned fitted with two alloy inserts.

I have also had a head repaired by Norvil a few years ago and it was done the way Les describes, however Norvil do sell the part in their catalogue in the exhaust section as 'Insert - exhaust port repair' at 21.84 pounds each. I am thinking of buying a couple and having a local machine-shop do the job, rather than go through the trouble and expense of shipping heavy things half-way round the world. If anyone has any experience of doing it this way I would be grateful for any suggestions.

Here is what I learned about Norton exhaust port repair today.

1. Roger at RGM does them with a helicoil and says he has never had a return. This requires shipping the head to him since a helicoil kit of that dimension would be costly, about 300 GBP for the tap alone.

2 Les Emery told me that the Norvil bushing Dave refers to is alloy and he also claims it is a no problem fix. It is really much the same idea as the helicoil , with an inner and outer thread, except that being a bushing, it has a wall as well. Their bushing still requires machine shop work to install, but it can be done locally rather than requiring the head be shipped to Norvil and, as Reggie said, on to Pete Lovell.

Unfortunately my exhaust port has already been repaired with a weld-in alloy insert. The insert is still solidly in place, but the threads are stripped. The weld-in type can be a problem when trying to use the double threaded alloy insert that Norvil sells. Sometimes the hole in the port after the weld-in bushing is removed is already too big for the Norvil bushing. In cases like this, Les gets Pete Lovell to deal with the repair by making a larger thread in bushing as required to fit the larger hole.
He gave me Pete's phone number and told me to deal with him directly.

I also talked with a local British Bike dealer, Robyn of Robyn's classic motorcycles in Victoria BC. He informed me that there is a machinist on Vancouver Island who is all jigged up to do the Norton exhaust repair and has been doing them for twenty years or more. It turns out that this fellow is Chris Timberley, a friend of mine thru the Vincent club which I am Section Organizer for. Until today I had no idea that he worked on Nortons. or that he was an Commando Exhaust Port Repair Expert!

I talked with Chris and learned that he can deal with my weld in insert in much the same way that Pete Lovell would, by making the new insert as big as it needs to be. He charges $75 for the fix. He's on the Island and we are on the mainland, so the easiest thing is to ship the head via the bus for about $10 or 12 ea way, so all in all it is very reasonable.

Chris also told me that I should get bronze lockrings to replace the stock Chrome over steel lockrings I have now. He tells me that the bronze rings expand at about the same rate as the aluminium head, so they stay tight even when the engine gets hot. The steel ones get loose as the engine heats, so they will always be a problem. The last Norton head he did was refitted with chrome over bronze lockrings, so they are stock looking but solve the "loose when hot" problem.

Sure enough, I found the Bronze lockrings on the Norvil site, along with the note (Stays Tight), however they are only for unbalanced pipes. Mine are stock MK111 balanced pipes with the spherical cone seating rather than the crush washer setup as used on earlier bikes.
Anyone know if the earlier nuts will work with my MK 111 setup?

Mine are stock MK111 balanced pipes with the spherical cone seating rather than the crush washer setup as used on earlier bikes.
Anyone know if the earlier nuts will work with my MK 111 setup?

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The nuts (actually bolts) will work, but are longer and therefore a good 3/8" of thread will show unless you have them shortened. Check the fit of the collets into the nut also. These, of course, are the beveled ones for the MkIII. The ID of these 750 style may be too small for the collet to fit inside properly.

worntorn said:

I ordered the 920 kit this AM. In one of the earlier threads it was stated that the RGM kit uses Cosworth pistons. Roger (RGM) tells me that years ago he did contact Cosworth to see if they could supply him pistons for a big bore kit, but their cost estimate was about $400 per piston.

He didn't tell me who supplies him with the 920 pistons , but he said they are not Cosworth. They are forged pistons though, which is quite amazing since forged pistons normally are a big ticket item and his entire kit is just 143 GBP with the VAT deducted.

Now I just have to deal with the dreaded exhaust port repair.
Has anyone tried the repair kit that Norvil sells? (not a helicoil)

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Sending the head to CNW for fin or exhaust port repair would be far more cost effective than shipping that thing to England.

Just curious as to what is included in the RGM 920 kit? 143 GBP for a 920 conversion? I thought th alloy jugs alone were about $1000 or am I missing something here and those are not for the 920 kit?

Yesterday I found out that one of the local members of our Vincent club is all set up to do the Norton exhaust repair, so I can give the head to him when I see him or ship it on the bus for very little.

The 920 KIT includes forged pistons with rings (chrome top ring), wrist pins, and liners. The existing cylinders must be bored out to accept the liners. I have a local shop lined up to do that work.
Most importantly, also included are 920 decals for the side panels!



The Kit seems like pretty good value, especially if the bike being converted is ready for a rebuild. Mine isn't, so I suppose I am being somewhat wasteful here.

Coco said:

Just curious as to what is included in the RGM 920 kit? 143 GBP for a 920 conversion? I thought th alloy jugs alone were about $1000 or am I missing something here and those are not for the 920 kit?

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= Two pistons complete (RGM part 050163/4 £95.00) and two 920 cylinder liners (RGM 050016 @£27 ea. x 2 = £54.00) = £149 GBP.

Presumably £143.00 GBP price is with the inclusive 17.5% UK VAT deducted (- 14.8%) and postage added, unless their price has increased?

See their online catalogue: http://www.rgmmotors.co.uk/ (Engine Components - Top End)


Images from RGM catalogue:




Obviously there would be the added expense of having the liners fitted to the cylinder barrels.

The 143 GBP is with the VAT deducted. Total cost including shipping was 177 GBP (34GBP shipping to the west coast of Canada)
The Kit also includes a special 920 sized copper head gasket, plus those all important 920 side panel decals. :lol:

While searching for info on the 920 kit I read that Commando heads are between 50 and 55 CC in volume. Does anyone here know what volume the RH4 heads are?
I would guess they are at the top end of this range, 55 cc, since they are made to give relatively low compression on the bigger displacement (414cc per cylinder) engine.

Roger at RGM tells me that the 920 kit can be set as high as 9 to one comp by shaving the cylinder base (stock cams). Without shaving the cylinder, comp of the kit is around 7.5 to one.
I would have expected higher compression since with the 920 kit, 460 CCs of cylinder volume are compressed into the same head that previously had 414 CC compressed into it. Since stock comp is 8.5 to one on the 850 MK111, and stock MK111 pistons are flat tops like the kit, this should mean that if piston height is the same, the kit will give a 9.44 to one ratio (460/414 X 8.5 = 9.444). I can't recall how high the stock pistons go in relation to the cylinder top, but hopefully it should be possible to set the 920 pistons up in approximately the same way .

worntorn said:

Since stock comp is 8.5 to one on the 850 MK111,

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I believe the actual compression ratio was a bit lower than the quoted ratio? Apparently it was nearer to 7.9:1

That makes sense.
If the head is 55 cc and the stock piston rises to flush with the cylinder top then compression ought to be 414/55= 7.5 to one, so perhaps the head(including head gasket) is not quite 55 CC in volume, or the piston must rise above the cylinder a tiny bit.

The big bore kit should make it possible to have about 11% higher compression than stock with the same piston height, since an 11% larger cylinder volume is being compressed into the same size head.

So if the 850s are at about 7.9 stock, this should mean the kit will go to about 8.8 with the same piston height. It probably will require some cylinder base machining to get the same piston height though.

You have to take the cylinder cc + combustion chamber cc/combustion chamber cc which gives about 8.5:1.