BUILDING   A  NORTON   CRANK    STROKER   TRIUMPH   in  650-750 UNIT & 650 P-UNIT CASES

This article concerns Triumph 1950 to 1962 pre-unit and all unit model 650-750 twins.

Both are reasonably easy to modify although with the pre-unit era there are the early 1950 to 1956 models that have a small timing side main bearing an RMS8 and are not suitable. Chances are though you may be able to get a later big bearing case to match quite well to the early number side you may want to keep. This I have done a number of times, generally, if the cams spin freely the crank will too but a small amount of decking may be required for an even cylinder base.

With unit cases up to the 1972 1/2 model, which is actually the start of 5 speed use, both mains are the MRJ type basically an 1 1/8”, usually a ball on the T/Side and a roller on the drive.  In 1972.5 the timing side went to the metric 306 as used in most Norton twins ,this can save a bit of machining. My preferred method is to bore the case main bearing bores to suit the metric Norton bearing and I always use a high capacity ball, ie. a 10 or 11 ball bearing then fit the roller to the drive side, usually NJ306 . The hi/cap ball gives end float control as it will be .003 interference fit in the case and firmed up with the pinion nut on the crank. The drive side on the Norton crank is a taper unless you opted to have a Triumph spline machined on to the Norton crank, then it too would be held by the sprocket nut but if not then is a tight drift fit to the taper axle. The hi- cap ball mentioned is std in all 5 speed model Triumph twins and did have a bit of a reputation for splitting the cage pins and collecting the balls together, I have witnessed a number of these collapses. I fit hi-caps to every engine I do, Norton or Triumph, they have a load rating almost as good as the roller, and no real end float fiddling is required once set up. Fitting the larger metric is in preference to grinding the main shafts back to the Triumph sizes of 1 1/8th but if you go this way, care should be taken to radius the axle to cheek interface. Keeping the stock Norton axle sizes in my mind is to keep a stronger setup. Triumph used a ball and roller setup just like Norton until the ‘80’s TSS. Norton used this ball and  roller through the Dommi years up until the ill fated 1972 Combat Norton double roller debacle then Norton changed to a barrel roller design on both mains. Trouble is it does not take long and excess end float becomes apparent with a loud knock or whack as sharp throttle is used and the crank slaps to the drive side. Ball and roller has none of this.

When machining the case for the metric bearings I prefer to set up under a mill using the joint face to find level with shims , if required, then centre the bearing hole and bore concentric .003 under ,rather than set up on a face plate of a lathe. I have done 2 dozen plus strokers and all have been fine, done under the mill, the one and only pair of cases set up on a lathe did not work and was out just enough to cause the crank to screw from right to left when spun then lock up. Junk cases.

Now whilst playing with cases and before fitting any bearings I like to take a flap wheel in an angle grinder and clean up any rough casting on each side where the crank cheeks spin. If your aim is to go with a stock street ride then a stock cast iron 360 degree Norton crank from any alternator 650-750-850, {they are all 89mm stroke and 1.750” B/E journal) will work although an E start ‘75 is not the best with extra long drive axle. The cast iron central flywheel is fine for general use and the occasional run through the gears, as you would , best to have it magnafluxed or x-rayed for cracks when in for a journal grind, before beginning any machine work on the crank.

The whole purpose of fitting a Norton crank to a Triumph is to increase the torque, it might only be an extra 80cc but it does, also keeping some weight on the crank helps. It’s all about the torque, being able to raise the gearing depending on the engine size achieved to be able to cruise at 60-80mph with 3-4000rpm on the tach and have good roll on in top from 2500rpm. As far as rev ceilings go, a stock Norton is basically limited to 5800-6000. The design has a poor rod to stroke ratio. Two guys now make a 6.3 rod and piston conversion for Nortons to increase this ratio from a poor 1.65-1 by way of a 3.56” stroke and 5.8” rod. The Triumph 650 has 3 ¼” stroke and 6.5” rod. Way better, when the preferred ratio area is 1.9 to 2.1 -1. The better this ratio the better the position of the rod small end in the bore when the big end is at 90degrees. At first I used to have  6.3 rods made by Herb Becker in Canada when otherwise not available so I could use the long rod big bore kits that are available for the 650 twins. Then MAP made me some  and the same 6.3 rod used in the Norton long rod conversion is used for the Triumph stroker. This 6.3 rod combo gives a much safer rod ratio, then basically lifts the Norton rpm ceiling.  I never use T140 10 bolt cylinders as they are ½” shorter and effectively then have a poor rod ratio when stroking. With the long barrel and its 76 mm piston using the 650 Tri 11/16” pin, which is the same as Norton you can probably see how it makes sense. The 89mm Norton stroke and the long rod big bore 76mm gives 810cc, same as the old Dunstall big bore for Nortons. There are still the odd Sonny Routt cylinders around, he was probably the first bloke  to make big bores for 650 Triumphs. Anywhere from 76mm to 79.5mm. A 79.5mm bore with a Norton 89mm stroke will give 883cc. I have 4 of these blocks I picked up over the years and therefore have 4 bikes in the 850+ range. Some of my personal collection photos on this site are among these completed bikes. So again T140 pistons cannot be used as they have a different pin to the deck and also use a .750” pin. The torque is basically why I do it and have built and sold only a half dozen strokers over the years but have 16 in the shed, some riders and some are still coming together, of which at least 7 are offset. Not all are shown on this site

So the crank will need the diameter turned down to around 6” similar to a stock Triumph crank. This can differ if you intend to use some performance cams as Triumph cams can sometimes be quite close to the flywheel, so another .040” maybe. I like to narrow the flywheel to the same as the Triumph as this also lessens the weight a little but don’t try to remove too much, this may inhibit a dynamic  balance. A cast iron flywheel is fine for general road use but if you intend to go racing then a steel flywheel is the only option. Believe it as I have seen 4 engines destroyed at race tracks when the cast iron gives up the fight and it pretty well takes the rest of your engine with it. The steel flywheel can be made from a piece of nominal 5/8” thick flat plate bored in the centre for a dowel or another stud, then turned to 6” or so? after cleaning up faces for .625 finish. Then drill for the cheek bolts, at this point decide if you wish to stay with the 5/16 bolts or go to the late 850 3/8” bolts. Some banana shaped weights will need to be bolted /welded preferably both ,to the bottom of the new steel plate as counterweights. Similar process if you want to go with an offset motor but the weights will be a bit longer ¾-1” and protrude forward of the cheek each side. Offset can be whatever you want but 90 and 45 are problematic as  1 cheek bolt hole lines up at 47 and then again at 99degrees.  On paper 90ish deg  @ 50% balance is best. So I have done a 47, 50, 70, 76, 90 and 99 so far, this route is more time consuming and costly as special cams will be required. See pic (no 1) of a 47deg crank with MAP 6.3 rods for a Triumph dirt dragger with cams at 23.5 separation for a quick fire twingle. Two beats in the same 360 rotation, No1 fires at 360 TDC and No 2 only 47 deg later,  instead of only one per 360 rotation. Normally to get a “V” twin beat the cams would be 23.5 from their normal 360 opposite position.( 47-313) a 90deg( 90-270) a 70 deg( 70-290)? Pic (No 2 ) is a Nourish short stroke 360deg Norton NOS. No 3 is 76deg

Anyway, .010” can be cleaned off the outer cheek face that is close to the case face I mentioned cleaning up but leave the bearing shoulder alone and radius the interface, no sharp edges, they are places to crack. Then remove some more material , about 1/8th from the angle clearance at the cheek lower. This also helps clear the case in particular with a pre-unit which has an oil return pipe in the timing side case , some finessing will be required here. In most cases I cut out a shim of about .010-.015 thick to put behind the T/Side main in the case before heating and dropping in the Hi Cap ball, this may not be required in 1972.5 metric models because the 306 is shallower than the MRJ series. When this cools check to see if crank spins clear it may also eventually need another .010-.015 shim but this time betwixt ball and crank. This is where having cleaned up the main journal a smidge to a push fit on the main, helps. While you are at it heat the D/Side and drop in the 306 roller and fit the inner to the crank. At this point you can close it up and check spin and end float etc. The crank does not have to be dead centre of the case as eyeballed through the case join. There is a fair bit of lateral clearance on both the rod to crank and piston to rod.  If this comes up good then fit the rods to the crank temporarily then  also fit the piston to the rod ,put the crank and rods in one side only and drop the cylinder on (no rings) hold with one base nut . This procedure allows one to see how close the big end of the rod is to the cases when slowly turning over . Material will surely need to be removed around  the case bolt castings, with a dremel, a bit at a time. Pull apart , tape over the bearing then take a small amount. Repeat, repeat etc. When happy, go to the other side check again , sometimes material will need removing around the cam bosses and sometimes one has to remove material, (make a slot) for the rod to clear the cylinder spigot, just like a Norton but mainly on the smaller bores. I like to use the timing wheels from the  67 to 72 they are ½” wide  unlike early unit and pre-unit, they are narrower at 3/8”. I find the narrow gear wears and causes lots of backlash noise. With unit or pre-unit I go for the wider set. I sometimes use T140 ramp cams, 2 x inlet and with the heavy wide gears that the 750 used ,in a pre-unit , same ½” wide gears but not the gutted lighter style used from 67-72 which you may prefer if doing a hotrod. In a unit the T140 inlet and a ½ race exhaust (LF Harris) cam, can be a good combo. The Triumph pinion gear is the same diameter as the Norton so no work required except to narrow the key to fit the crank. So

  1. Machine overall diameter

  2. Machine flywheel width

  3. Machine extra angle relief on edge of web

  4. Machine .010-.015 of web face

  5. Machine material off timing side axle length to mimic Triumph oil quill

  6. Machine and grind a left hand thread Norton oil pump nut to fit in late pinion gear

  7. Machine bore case halves under mill for metric bearings

This whole process is a lot easier now days with the availability of MAP chrome/moloy steel 6.3”  conrods. They have a much smaller big end knuckle compared to the previous Alum unit or for that matter, trying to use Norton rods as I did on my first ever attempt in the late 80’s. That was a preunit with a 10 stud short barrel and bored rod small ends to accommodate .750” pins of  T140 pistons . It ran good but not for long as I had removed so much material to get things to clear, the cases cracked. I learnt so much that first time. The old school of hard knocks. My fave at the moment is my 828cc 1970 bonnie lookalike, luv that grunty thing. It runs 2 x 34mm Amal Mk 2 carbs and has great roll on above 2500 in top and 3000 in top is 60mph ( 100kph). Smaller carbs may make the roll on torque a smidge better. It is a modified cast iron crank. This bike is  essentially a Trident frame and balanced dynamically to 74%. 50-80mph is smooth as , a real treat. The reversed head streetfighter on this site is very quick and fun, so one does have to temper the wrist a bit, it is only a cast crank. This thing runs 2 x 38mm Mikunis and will idle when warmed up. The 3 stoker motors  that I have actually had dynoed gave  60hp+ on the ground , so to speak . A stock 850 Norton is factory quoted at 60hp at the crank. These bikes have over size valves , are either 810 or 828cc and ran 34mm carbs. No head flowing and cams equivalent to T140 inlet with 1/2mm more lift. A Norton 850 is actually 828 on a std 77mm bore.

Now for the HARD part, or easy if you can throw money at this sort of project. As with the above, a lot of the work can be done for you by suppliers such as MAP in the US, Lyte Drives in Melbourne, Tighe cams in Brisbane, Newman cams in the UK, Nourish Cranks in the UK and so on.  As you will have picked up on by now, is the fact that we now have a taper crank drive and a taper mainshaft input for a typical Truimph 4 or 5 speed gearbox. You could get a spline machined in place of the taper of the Norton crank and I did this to 4 cranks many years ago, simplifies things . Otherwise this double taper is a problem, how does one get a chain alignment with two tapers? Easy, fit a specially made  belt. 36-68 teeth with an 800mm x 8mm x 25-30mm wide HTD belt, on a pre-unit the belt length will be at least 880mm long. There is an economy belt drive for Triumph unit and pre-unit twins offered that has a steel front pulley . This can fairly easy be converted to a taper with some machining of an Atlas type single row chain sprocket. Removing the front pulley centre, turning down the sprocket and fitting into the pulley with bolts, hey presto. Still not cheap but cheaper than a full custom drive. ( pic 2) I have done this a few times and the latest job was on my upcoming 883cc 1962 Duplex frame  Tourer. Whilst at it in order to have a crank seal I make a spacer to fit in the case and take a Norton crank seal. The other option is one I have also done a few times and is to choose either a duplex or triplex chain drive , take the spline out of the Triumph front sprocket and the teeth off the Norton sprocket, I had done by water jet a few times and then machine clean  leaving an interference press fit, not too tight. On and off and on and off again or at least move a bit back and forth until correct alignment is achieved with the clutch chain wheel. The beauty of this with the taper is one can turn the front sprocket and check run out  with a dial gauge. The  clutch pack will need to be mounted complete on the mainshaft, less chain of course and with a straight edge, a 12” rule, fiddle about for half a day till you are happy. Gently take remove and tack in 2 places check again then give it 4 good 1” long welds.  Good to go. Fit a new quality chain, this chain conversion is all about the unit models only. Now if you are doing this to a pre-unit then by all means fit a chain , same method but with  single row chain 428 Norton and Triumph cush drive era sprocket. Try the Norton taper sprocket first up for alignment , never know it may just work  without machining and welding. I usually go with belt drives on pre-units for the lack of oil required , (although primary oil can be a good rear chain oiler via the near useless slinger disc) and set and forget low maintenance schedules.  Along with electronic ignition, decent carb and proper oil filtration it makes classic riding more fun.

Good luck,   Brian

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