1961 99 Dominator 600cc Performance Engine Build Advice

Hi all, I have a 61 99 600cc Domi I am about to rebuild the engine on and was looking for advice on performance upgrades.
I recognize these aren't the best base to hot up however would like to do what I can during this build.
Engine isn't apart yet so i'm unsure on bore size etc however I will be sticking with STD bore and re-sleeving if required as I understand these barrels are weak and unobtainium.

Compression: I believe std comp ratio is 7.4:1? are higher comp pistons available? and what would be the limit I should push for on 98 Octane Pump Fuel?
Rods: Seems these are a weak point even at standard performance? or is it only the early Domis? what aftermarket rods are reccomended / available?
Head: Any Advice / Guides on porting?, larger valves?
Balancing: Will look to have rotating assembly balanced, advice on Balance factor?
Oil Pump: Recommendation on aftermarket pumps?
Oil Flow Mods: Any advice on modifications to oil flow / pressure?
Carb: Currently running an Amal Concentric 930 and would prefer to stay single carb

Cheers from Aus

Had an early slimline 99 back in the sixties. Fitted it with a high comp pistons. Don't remember CR. Made it considerably faster than a friends late wideline 99.
Had it at standard balance factor. Didn't have an issue with reboring.
Kept the standard single Monobloc setup as I wasn't good at tuning twin carbs back then. Did some slight inlet polishing and carefully matched the flanges. Had long straight pipes until I got tired of often had to outperform the cops. Then fitted a pair of Dunstall mufflers with no noticeable loss of power.
Did nothing to the oil system except fitting a piece of rubber tube on the copper pipe for top feed. Pipe did break and oiled the rear tyre.
Spent a lot of effort on making it lighter. Alloy rims, engine plates, mudguards. Clipons etc.
Top speed approx. 115 Mph clocked at a 30 miles straight highway. If memory don't fail me.

The orriginal head is already a high comp one and fitted with valves the same as a 650SS , it will already have a "Daytona" cam with timing not much less than the 650 SS cam ,so its not really a cooking engine. Higher comp pistons Nomad ones will give a pretty high ratio that will provide a bit more stress than the old rods need. Some aftermarket pistons are not reliable and too heavy. The 99 barrel is not especialy strong and a resleve can leave it even weaker as it often breaks thro the casting to the outside. I would just get it well set up as std. My 99 std often leaves 650 ss's struggling in its wake , so its not slow if you are a rider . Orriginal 650 SS pistons will fit and may be lighter than other options . Thunder /or Corrillo rods may be a good investment but may need some work to fit some pistons . Compression will be around 8.25 , 7.4 was the earlier head/piston combo. Andover does not have any new 88/99 rods . I dont think any new rods have been made for the 88/99( by AMC or Norton )for about 60 years , After 63 I think they just used up the stock and took back stuff from the dealers stocks to build the last 88 SS bikes. I recently came accross a pair of new old stock rods ( absolutely hens teeth) ,but they were so poorly tolleranced they would have to be re-sized !, Probably factory rejects sent to the USA to complete an order but scrap really.

Cheers Bob, Nomad pistons hey.... Will look into that but yes sounding like its just going to be a meticulous standard rebuild due to parts and weaknesses.

Appreciate that info

With your head Nomad pistons will probably come out to a higher ratio than pump fuel will tolerate . But not sure about this !!.

is 70% Balance factor still what to shoot for?

Egad .

The " Domiracer " & " Daytona " twins are probly where youd look for the Top Line tips & tricks , if its a ' street / Compition ' S/C trip your on ?

Uprateing the oiling'd be a no brainer . Crack test crank .

A Domi 99 SS would likely be a better ' ride to work ' set up . Manual Advance with extra available , and you can smoke the rear seeing off those new kawasakis
expertly riden . There was a corner after . mustve been around 67 . Ex D. R. uncle arrives elated at sucess . Had a 54 SS & 58 inter , self maintained . imaculate .

Beelow . This might not be entirely sensable .



As Hogslayer started off with ' 99 ' engines , reading up on that , ' search ' . wouldnt be amiss . You might not want to run Nitro on the street , but fuel injection wouldnt be amiss .

The only 99 I've ridden was a standard 1957 model. It's a pretty bike, but very slow. Maybe 20 HP or so at the wheel. My BSA A10 feels sporty after riding the 99.
So I think some hotrodding would be a good idea!

Did the late 99 really get the Hele 650ss type down draught head?
I recall reading that even the last 99 ss did not get that head, however the 88ss did.

Glen

Not seeing the downdraught here

Sorry, read it wrong. Norton Bob said " valves the same as 650ss", not head the same.

CHARGER said:

is 70% Balance factor still what to shoot for?

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Balance factor depends on the revs you intend to use. If you are going to road race, you might go as high as 78 %. If I had your bike, I would concentrate on torque and reliability, rather than more power. If you want to have fun with it, buy a 5 or 6 speed close ratio gearbox. With that, you can ride it better around the twisties. From memory, the Domi 99 only has one carburetor, so it was never intended to be high performance. So just love it and ride it. Many old bikes are too valuable to shag by hotting them up. Particularly Nortons.

I think we all need to remember, that when we own an old motorcycle, we are only custodians until the next guy gets it. I would love to go down into the main street in our town and see somebody with a genuine 60s Norton twin. I have seen about one Commando in about ten years. But what I did see one day, was an immaculate Norton Rotary. I am probably silly, but I have a real psychological need to know that Tasmania is still there, and not full of bullshit.

I hear you acotrel, it is a very original bike so I am only doing a mechanical restoration with no over polishing etc, just want to make the motor the best it can be while I have it apart. I prefer seeing a survivor / original bike over an over restored one any day.
Cheers

PS will be lapping tassy on a Survivor 1975 XS650B in March next year

Hi had a 1958 99 years ago, un molested, ran against the Triumphs and BSA 650s of the time on the road with lots of handling advantage, it ran well against them, only trouble I had was it had sat for a few years prior to my purchase in 1968, and the rings had siezed into the pistons, and as the rings wore out it lost compression and eventually stopped.
If the rest of the bike is good, I would just rebuild to standard rather than try and hot it up.
I still have the original fuel tank, plus gearbox/clutch that I have used in various racing adventures.

Keep it standard, build it and if not happy then update it with later parts but remember these bikes are becoming rarer as the years go on.
Burgs

Slotting in a stock 650 motor would be the way to go, or you could piecemeal a 650 motor one bit at a time onto/into the 600. The 650 specific bits were all beefed up to handle the performance increases the 600s couldn't, hence the reason why the 99ss was only moderately hopped up.

Easier said than done, I agree.

Put a different lump in it, go for a good complete Atlas engine. Parts are more readily available and builders/machinist have more experience with the later engines. A single carb 600 is a nice ride but anemic.

BALLS .

A Triumph Crank ( preferably a 67 od ) would be the same stroke , and good for 8.000 + r.p.m.

As the Short Stroke 750 is good for 8.000 r.p.m. , we would assume the valve gear also , would be
able to be set up for 8.000 also .

This just leaves the rods & pistons to get awkward over . The earlier ( 1961 ish ) 650 Triumph rods
were LIGHTER , thus less inertia , thus quicker . And seem to hold up fine - to high r.p.m. .

Therefore , suitable pistons , to match wrist pin hight & deck , would sort it . For a fster Lap of Tassie !

If you went the ' S S ' dual carb route .

You are right in that airflow in/ exh gas out with minimal restriction is important for power, as is in cylinder air motion(swirl) influenced by the throat/valve seat detail. However the bottom end is equally so. The cam profiles dictate torque curve shape and hence power, and the engine speed equally so as power = torque x speed.

This is where a '99' has an issue. 50 years ago the alloy rods were new/nearly new. Now they are 50 years older. The rod shape was always a compromise and nowadays as Robert says it is a bit of 'russian roulette' as to how good the rods are. They always suffer greatest stress as you close the throttle from high rpm (mininal gas to compress), and this is where and why original rods will let go near the small end.

With new (uprated) rods and pistons, together with a decent oil pump the 99 could become a reliable higher power bike, but the argument is really whether it is worth the cost to achieve that or not.

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The rods are marginal, so if you want to rev an old engine, buy some new Thunder Engineering rods. They are well made and just go in. (FWIW the rod bolts thread into the main rod, rather than having a nut below). I bought a pair for my standard 500 - a previous owner bent the big end eyes by fitting -0.010" shells onto a standard un-ground crank (he confessed when I asked him how the rods got bent), and they got binned

Another good mod is a Commando oil pump, they have wider gears, so you get more oil pressure and faster scavenging. A hot early motor will have 0-5psi at idle, and maybe 40ish with some revs,so more is very good. Even standard, they wear well if the oil is kept clean and adequately fresh - I detected under 1/4 thou of wear on the drive side main on an engine with 80 or 90k miles on it. Fit an oil filter. But the 6-start pump gears on an early engine just cause trouble. You have to re-engineer the whole system.

The early inlet tract is restrictive, but don't go bonkers opening it out. The 650ss was ported 1 1/8", but worked better with the standard 1 1/16" carburetors. 70% of that area makes 1", and the standard inlet tract in the head is 15/16". The inlet valves are 1.4", big already, and there's no point in fitting 1.5" inlets intended for an Atlas. Those pots are 50% bigger. Similarly, the hot cams you mention were developed for racing 750 engines. A 500 will not make good use of them, and the high, rapid lift will just cause wear and drag. Anything beyond the '60s Dominator SS cam (the Commando SS cam is different and unsuitable) is a waste of time, and just wears things out and spoils low-down power.
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by anna_jeannette_dixon on Mon, 09/12/2019

good advice​

Hello Now here some Good advice From 50 years of experience of Norton twins Now you have a 62-year-old machine with limited parts So you must follow some rules these machines do not like to be overstressed in the engine department So do not overdo the mods now things you can do are machine out the cam tunnel to take an S2 Cam profile and RGM pushrods they make in stainless and flat type cam followers and gas follow the head with atlas valves mean a valve seat recut to fit then Bronze guilds with seals and terry springs and shims under the valve springs as for the bore Do not bore go over 40 oversize the next its to go back to standard as these barrels will not take big bores as they tend to weaken the barrel, next you need to check and large ends and main bearings and not to forget the OIL PUMP and the pressure release valve in the timing cover and do check and clean all oilways and rocker spindles and there wear and tear, and do get all the reciprocating parts in Balance, as for compression ratios goes 9:1 or ( 9:45:1 is about fine with lead subitues in the fuel, carburettors you need a good pair of amal 376/288 and 376/289 chopped mono bloc with a twin pull cables exhaust silencers a pair of Dunstall replicers with baffels will be fine spark pugs champoin N7BYC or N4 igntion spakrex D102 now go have fun yours Anna J

Ere , now wots all that . ( liberated from ' elsewhere ' .)

The ' 88 ' racer ran ' 99 ' size jourals , which 99's have ( bigger diameter )

But a fresh crank ( Triumph - crack tested - first undersize ) would be stronger .
note the Notes above , re RODS . If the Triumph Lengh were right , Low Time
650 rods would hack harrasment farly superiorly .

The Dual Start twice speed laer oil pump set up , with the later PRESSURISED
Top end Feed , it would run cooler . With Adequate Oil Volume ( Big Tank ) -



Making it wider / deeper crossways , by say the amount of the tin thing , if
acceptable , would be a start . If one had two old oil tanks & a hacksaw ,
you could get it good . Hot Tanking the tank wont hurt , either .


This bloke here
" Son of Siredward "

Has a long build thread on a 88 speed record build , if SOMEONE ccan find it .

Aye , its a matter of Seeing it Dosnt Blow , removing obsticles obstructing output , then ' dialing in ' tune / improvements .
rather than a mish mash of uninterelated absurdities . and Let Er Rip .

https://www.motorcyclespecs.co.za/model/norton/norton_dominator_99 1956.htm ( the missing page )