Dominator conversion to 750cc

Hello - In one word 'NO'. The crank will have to have been from a Atlas with 89mm stroke and the crankcase couldn't take the strain of the extra power. To me it's a non-starter? Don't go there!

Les Emery did say in conversation some time ago that it is possible to machine and weld 88/99 crankcases to accept a 89mm stroke crank. I have never tried this. What I did to my 99 was to fit a complete Atlas engine, oil tank (they are different) and a 21T engine sprocket. I also went for a single carb manifold but that's just me. Can't be bothered with the hassle of twin carbs. The reason for doing the swap was that my original 99 engine blew up catastrophically at about the same time as my son wrote off his Altas (but the engine survived the crash). An engine transplant seemed like a good idea. I am happy with the result. Son now rides a Kawasaki...

The big question is "Why would you want to do this?". The 99 should be powerful enough for most purposes, and is a relatively smooth running engine, unlike the Atlas engine which vibrates horribly, and was the main reason for designing the Commando with its rubber mountings.

The Atlas was designed mainly for the American market. I'm sure Jeremy Clarkson would make a comment about how the extra capacity was needed to haul obese American riders around, but Americans just like to brag about bigger engines, and probably bigger everything else...

If you look at road tests from the time, you'll see that the performance of the Atlas was down compared to the 650SS, and fairly similar to the 99SS.

Colin,

I laughed out loud at your American comments! I agree about "why would you want to do this?" But seriously, all americans are not obese (like the UK cooking show 2 fat cooks on a Triumph), I think due to the different landscape & riding style, it seems the Brits do a lot more "cafe" style riding: fast,short bursts, more abouthandlingthan top speed . Many (not all) American riders have a long straight road ahed of them that they like to be able to wind it up to 100mph & have the gearbox in 5th gear and drop the rpm's to just over idle (Harley Davidson big twin). And sure many american guys have a testosterone contest to see who has the fastest,biggest, shineyest, (etc, etc, ) bike!

But I swear, we are not all obese Harley Riders

I am planning on visiting your fine land in June for the national, I'll have to look you up & see if you wear a derby & amonocle with your pinky finger up in the air when having low tea! Its funny how our perceptions dont always meet with reality. About a year and a half ago I sold a Matchless on ebay to a Brit whose name was Harley (who didnt like Harley bikes by the way), he came to the USA to get it , I invited him & his son into my home for lunch. The son was overweight, Mr. Harley was obese & all he did was complain about Americans & America! Wish I never invited him in! On the other hand, I have had many dealings via this club & on ebay with people from all over the UK & have found everyone veryhelpful& friendly!!! Its a good thing I didn't judge everyone in the UK by my visit from Mr. Harley!

I cant wait to come over! Alan Throssell, Chris Grimmett, Tim Harrison, & Linda Harrison wereinstrumentalin my decision to make the big trip! They were all English members of the NOC at the USA rally last July & made quite an impression on me. Its a good thing the club assisted in the travel costs for them to visit, or I never would have met all of them & probablywouldn'tbe making the investment to come over! I have also been encouraged to visit by Paul Gibbons who made my model 77 restoration possible because of his constant email replies to my silly request of him to measure this, send me a picture of that, etc. He was suppose to come over for the Empire rally as well, but work got in the way. Sorry to get off on a tangent, but you know howloonywe Americans are

Skip Brolund

Great reply Skip!

Why go from a 99 to an Atlas engine? Easy- you can get new con rods for an Atlas or 650. To the best of my knowledge you can't for an 88/99 - except carillo of course. Again, speaking to Les E. about it, he reckoned that the rebalancing required would have an averse effect on the poor old 99 crankcases in time with cracks appearing. I have had the odd 99 engine break a rod over the last 40 years. Not a great experience all in all. What makes a 99 engine so exciting is that you don't know just how far through their fatigue life the rods are. Is is 50% or is it 99.99%? Hard to tell... I value reliability above everything in a bike, so fitting an Atlas engine with not many miles on it seemed like a very good idea. I have no regrets at all - and the vibration isn't anything like as bad as people make out. Yes it vibrates but certainly not to excess. I can ride it all day quite happily.

I am sure you will have a great time when you come over here Skip - can you be tempted to ride some of the Scottish roads? We have some rather good ones.

Gordon.

P.S. One does indeed drink tea with a raised pinky. There are standards to maintain.

Thanks Gordon! I am trying to work out details as to the length of my stay & have been offered bikes by 2 members to ride (mostexcellent!). I would love to make it toScotland, having done a bit of trials riding I have always wanted to have a look at some of the Scotish six days course.

I am restoring a model 7 (1950) should I have the rods checked by aprofessionalfor cracks & fatigue?

Sorry for the image below (note pinky finger), couldn't help myself!

Skip Brolund

My pleasure! For absolute peace of mind I would suggest X ray or similar checking. If you are going to be considerate to your Model 7 (trundling along at 50 - 60 mph) you really shouldn't have any worries. I have always used my 99 reasonably hard - well, cruising at 75 mph, sometimes a lot more if in a hurry and have also raced it. No wonder the rods eventually cried enough. It all depends on what previous owners have done... In my case, 100,000 miles before I got the beast and then started abusing it and adding another 100,000+ miles.

Nowadays I am no longer a slave to the 9 to 5 (i.e. a doddery old retiree) so the 99 and its friends have an easier life. Mind you, when I am out in it I do take full advantage of its performance! It's that kind of bike, brings out the hidden hooligan in me.

Give me a shout if you do manage to head North.

All the best,

Gordon. www.scothebs.co.uk

Well, Eugene and Gordon, first I must point out that having lived as an ex-pat Brit in the USA since 1988, I am somewhat familiar with American riders, and I know not all of them are fat. The comment was more a reference to Jeremy Clarkson of the "Top Gear" program on BBC Television who makes a lot of comments of this nature.

I see the point about conrods no longer being available for 88s and 99s, and my response to this would be to make the bike into a 650 and not a 750. The 650 is a smoother bike.

I was also at the Empire Rally in NY last year, and might have met both of you there. I took my stock looking 750 S Type Commando and my Manxman based cafe racer to the rally. I am seriously considering going to the International Rally in Yorkshire this year, but sans bike, since shipping would be a little expensive.

Tea in America is a bit of a joke - see what Ginger Baker of Cream had to say:

Hi.

Rob Sewell used to race a Dommi back in the 70s that was bored and stroked out to 750cc and itpretty well demolished all opposition. I met him and asked how he got round the VMCC regs ot the time and he said he'd manage to get a 750 crank into 600 cases. Comparing the cases I havethere is no difference in the basic dimensions - i.e. the two different halves fit together. The two main differences are that the Commando cases are 'relieved' at the rear to allow clearance for the longer stroke crank (just where the cases crack if you're - er - a bit over-enthusiastic) and the oil drain hole that leads from the head through the barrels is in a different position. The latter was re-positioned (I think) because it became too close to the bore when the barrels were bored out. How Rob got around that I don't know. As many others have said, it seems a lot of hassle for little reward. Robs bike was heavily modified with thru bolts holding down the head etc. By the time you've had all the machining done, cases, barrels, head etcit might be cheaper to pick up a 650SS/Atlas/Commando motor and slot that in.

I've had one set rods go on my 600 but it was the tops of the (pattern?)con-rod bolts that 'let go' rather than the rod itself. When re-built we put the old original (50s/60s)bolts back in 1982 and the bike was thrashed mercilesslyuntil 2006 when I took it off the road to be restored (hopefully next year). Has anyone had experience with Carillo Dommi rods? It seems strange that the Dommi should be anymore prone to crankcase cracking than a (pre 72) Commando.

I did take a few pictures to demonstrate the above points but shrinking them down to 100Kb makes them pretty useless (and I can't be bothered with the hassle) and I can't seem to just copy & paste into this post!!!! Apologies.

Regards.

Ian.

Colin,

Wow, Ginger Bakers tea song was cool, although its getting better with all the little tea shops popping up now days. I have seen the UK Top Gear series & forgot about that reference. Seem the whole world is eithergettingfat , or are health nuts! I guess I am somewhere in the middle!

The 650conversion seems a good way to go, or pick up a later engine & keep the original hor historical purpose in case you ever want it "correct".

I bet we probably saw each other, or at least our bikes! here are my rally pics from the Empier rally:

http://s306.photobucket.com/albums/nn272/skipsoldbikes/Empire%20Rally%202011/

I would love to ship my model 77 over to the UK, but I cant seem to find a cheap way, so I have rigged it up so I can drive it across theAtlanticto the UK. (see pic below) Whats the closest port to the Rally? Hope they have hot tea at the dock when I arrive!

Skip Brolund

Eugene,

Liverpool would probably be the closest big port to the rally. You have a picture of my Commando, Page 5, second from the left on the top row.

Colin, I cant find any Commando's on page 5? Help!

It is there, exactly as I wrote. A red metalflake S Type, correct high exhausts on left side, seat, headlamp mount, etc. etc. Not absolutely correct, as wonderful American petrol, with ethanol, ate the original glass fibre tank, and I replaced it with a steel one off an 850 Roadster which I found on Ebay. Cleaning the gunge out of the carbs took nearly as long as painting the "New" tank . Oh, and I couldn't find "Fireflake red" paint, so I used a product from Metalflake paint company.

Previously wrote:

Colin, I cant find any Commando's on page 5? Help!

It is there, exactly as I wrote. A red metalflake S Type, correct high exhausts on left side, seat, headlamp mount, etc. etc. Not absolutely correct, as wonderful American petrol, with ethanol, ate the original glass fibre tank, and I replaced it with a steel one off an 850 Roadster which I found on Ebay. Cleaning the gunge out of the carbs took nearly as long as painting the "New" tank . Oh, and I couldn't find "Fireflake red" paint, so I used a product from Metalflake paint company.

Previously wrote:

Colin, I cant find any Commando's on page 5? Help!

Now I see it Colin, looks great! Paint color looks awesome! Yesterday page 5 had no commando's at all! Maybe it depends on my page set-up? Who knows, anyway I do remember your bike, as I have always liked the "s" & "ss" pipes! That pic must have been from the first day of the rally, as we ordered rain for all the Britts to feel more at home! I wished it hadn't got so hot the rest of the week though! The road trip to the caverns was nice though, nice and cool underground!!!

It is possible to convert an 88 or99 engine into a 750cc beast but without some serious strengthening and othermods to the crankcases, big problems will soon arise.

In recent years, the Classic Racing scene has seen a couple of such conversions and some good results have been achieved by the riders.

The key to enlarging the standard engine is to fit conrods of the Carillo kind, so that they clear the crankases at the bottom of each stroke. These rods can be mounted on an Atlas or Commando crankshaft which will fit in the standard bearings. Although, most people would change these for 'Superblend' bearings. A crankshaft rebalancemight be a good idea at thispoint.

Next you need some 750 barrels, pistons to match and a 750 cylinder head. Atlas or Commando will do as long as the head bolt holes are compatible.

The hard bit is next because the 88/99 or even 650 crankcases will need some milling to allow the barrel liners to fit inside. The old breather route will need a reposition job plus a rethink on the oil drain hole at the back of the barrel where it meets the crankcase.

The 750 engine has a bore of 73mm compared to the 99 engine,s 68mm. But this equates to only a 2.5mm greater circumference, or 1/10th of an inch. Which is roughly how much the 99 crankcase mouth would need to be opened.

The base flange of the barrels will needsome of the holes slightly moved but only by enlarging them a few mm. The studs could be repositioned instead but this is probably not the best solution.

Personally, to save all the above hassel, I would just buy a 650 or 750 motor and throw it into the frame.

However, back in the 1970s, a certain Mr Dunstall was selling an 810cc conversion for any Atlas or Commando owners who fancied giving their Nortons a bit more go in the engine department. Quite a few 650 owners, found out to their delight, that the same barrel and piston kit could be bolted onto their engines with only the small amount of engineering to the barrel flange (easy with an alloy barrel) andcrankcasesas mentionioned above. I guess they used a 750cc head to save more bodging.

Attached somewhereis a photo showing a 750cylinder head gasket sitting on top of a 99 version. OK the 99 gasket is for a spiggotted barrel but as you can see, all the front holes are in alignment and the rear holes are not too far away from each other.

The first Atlas 750 engines made at Bracebridge Street for the 1962 bikes had the exact same crankcase castings as the 500-650cc Dominators, but they were machined differently. Bored out for the larger cylinder, cylinder base bolts moved outwards for clearance as in Phil Hannam's photograph, and the camshaft breather had to be moved along with the oil drain from the cylinder head, all to clear that big-bore.

I have seen a number of these 1962 Norton Atlas bikes in the USA over the last few decades, some of them still in service, and there is no evidence of any problems with the crankcases not being strong enough for practical use.

A 1961 Dominator 99 might have the "big base" cases that would accept the 89mm stroke crankshaft. I know the first 650 engine was numbered 93601, but I do not know if all bikes from that number onward received the 650-style cases or if there were some of the old-style crankcases used up on the smaller Dominators for a while afterwards.

Most of the Dominators shipped over here to the USA for the 1961 Model year were the 650cc Manxman bikes, and I have never ran across any 1961 Model 88 or 99 bikes that I could examine for information. Maybe someone like Phil with a lot of 99 experience could tell what the earliest serial# he has seen for a 650 case on a Model 99?

Apparently I have the second 750 Atlas engine here, with engine shop #2 stamped on it, serial # 1017xx, and it was fun comparing the cases side-by-side with those of other Bracebride Street Dominators of smaller displacement to see what they did back then. It was a strange thing for me to end up with as for quite a while I have had the third 650cc Norton built which also has an engine shop #2!

Previously wrote:

The first Atlas 750 engines made at Bracebridge Street for the 1962 bikes had the exact same crankcase castings as the 500-650cc Dominators, but they were machined differently. Bored out for the larger cylinder, cylinder base bolts moved outwards for clearance as in Phil Hannam's photograph, and the camshaft breather had to be moved along with the oil drain from the cylinder head, all to clear that big-bore.

I have seen a number of these 1962 Norton Atlas bikes in the USA over the last few decades, some of them still in service, and there is no evidence of any problems with the crankcases not being strong enough for practical use.

A 1961 Dominator 99 might have the "big base" cases that would accept the 89mm stroke crankshaft. I know the first 650 engine was numbered 93601, but I do not know if all bikes from that number onward received the 650-style cases or if there were some of the old-style crankcases used up on the smaller Dominators for a while afterwards.

Most of the Dominators shipped over here to the USA for the 1961 Model year were the 650cc Manxman bikes, and I have never ran across any 1961 Model 88 or 99 bikes that I could examine for information. Maybe someone like Phil with a lot of 99 experience could tell what the earliest serial# he has seen for a 650 case on a Model 99?

Apparently I have the second 750 Atlas engine here, with engine shop #2 stamped on it, serial # 1017xx, and it was fun comparing the cases side-by-side with those of other Bracebride Street Dominators of smaller displacement to see what they did back then. It was a strange thing for me to end up with as for quite a while I have had the third 650cc Norton built which also has an engine shop #2!

Hello Ben yes that right the First bike 18-93601 was pulled from the line and it as workshop number 7 . So Number 18-93602 was workshop stamped number 1 . And as for Dominator 99 Crankcases these where from the 650 Manxman with the hump at the back so a 750 top end and crank should fit OK , I was thinking of the Earlier (99) at First so Now I have Corrected my self . Your AJD

I don't think anyone who walks on two legs got the idea that a 750 Atlas top end will bolt onto the top of ANY 99 or 650 cases without major welding and re-machining being done to them.....but as usual AJD gave it a go......