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What is the overall weight of a heavy 650 crank? I have 2 cranks here, one with a full circle flywheel, and one with the top half shaved down. Both weigh 10kg or 22lbs. I would like to put a heavy one in my Flattrack motor im about to start on.

Hi PHE, I've not put the 2 on a scale. There is a 3rd also??

The 1968 shop manual states the light flywheel is 2-1/2# lighter section B42. The illustration B44 showing oil tube (sludge trap) is a light flywheel.

Visually the light flywheel is very different looking. Illustration B45 in same manual shows light flywheel on balancer.

If you are looking at light flywheel the flywheel is cut away to a narrow part near the big ends.

The heavy flywheel is solid (the same thickness) all the way around with no narrow section. The edge of the wide section is tapered.

To me it looks like the pork chops (counter weights) of the crank itself is same on all.

I've personally held & seen the light & heavy. To me lifting heavy feels heavier.

But.... not so simple as there is a middle one. The flywheel is flat, & cut narrower near big ends, but only slightly narrower. (light one is quite narrow in comparison) I've never seen/held one of these in person. I don't know what it may weigh. Maybe it's same as the tapered one?? My hunch is it's the same as the metal is thicker on rim, but not as wide. So may be the same weight??

Here's ebay of light one. I've held these.
https://www.ebay.com/itm/Narrow-Crankshaft-Connecting-Rod-1968-Triumph-650-Bonneville-T120R-TR6R-T1107/202921520680?hash=item2f3f10ae28:g:-7gAAOSwAeBeTxqd

Here's ebay of heavy one I've held. Feels heavier to me.
https://www.ebay.com/itm/Triumph-Crankshaft-Piston-Rods-650cc-T120-TR6-1971/323940103246?hash=item4b6c55b44e:g:6RgAAOSwekxdmojN

Here's ebay of the wider flat one that I've never personally held.
https://www.ebay.com/itm/Triumph-Crankshaft-Piston-Rods-650cc-T120-TR6-1972-122/223930199913?hash=item342347af69:g:VgIAAOSwi-BeWIrL

Of the 3 which 2 do you have?

Riding a 68 or 69 Bonnie compared to a later Bonnie or Tiger, truthfully I can't tell much if any difference. I actually paid attention to this a while back. If I didn't know I couldn't tell you which it has. On the race track I expect an experienced ride could feel a decided difference.
Don

The later wider flatter one was introduced 71ish, I believe is it the same weight as the heavy flywheel but is machined across the full width on the OD to try to minimise vibration from side to side inbalances. A similar solution to the fully machined clutch basket, by machining all over the cast surfaces you reduce variability in mass.

1970 crank in my 650 racer....look close you can see the flywheel mass is slightly reduced near the journals


71 crank used in a 750 conversion...You can see the flywheel difference near the journals...




One of the cranks used in my double engine racer...I was told they were 1969 vintage, noitce the two notches for TDC and full advance..

Thanks for the input guys; I went to see my mate at Thunder Engineering (who makes the rods and crankcases) He had one of the ultra thin width flywheels (69) And yes it is very different to my 2, which are the heavy ones, even tho they look very different they are the same weight, Thanks for your homework Don, but i do not get a crank when i open your leads, just a standard Ebay page for some reason.

Late heavy metric crank 21.8# on my digital bathroom scale which seems to be pretty accurate when weighing barbell wts. for calibration and for comparison my new MAP billet crank 23.8# on same scale. This MAP crank is his 76* offset so I'll be interested in how the offset and 2# heavier feels and winds up, this combo could possibly make for a good flat track 750 engine. Mark

I forgot to add that if you should find a nice late metric crank and want to run it in early inch crank cases then you can either machine the timing side main (30mm) down to 11/8" for early bearings or better, leave the larger 30mm timing main journal and order one of MAPs conversion bearings with inch OD and metric ID that they make for this purpose...I recently bought one, the 3 pc roller made by NSK (Japanese) but I believe they also have it in the metric ball bearing.

Would be interesting to get some feedback on the 76 crank.

And that is why I almost didn't add that bit about the offset MAP...I knew you'd be interested but also realized I wouldn't have it done in a timely manner so was just teasing you, sorry. However, when I do get it running I'll post my opinions, just to tease you a bit more, it's going in a C&J track frame but for the street. Mark

Hi Mark, Even though offset crank is heavier, it doesn't have a flywheel on perimeter, so the effective "flywheel momentum" may not be all that much different.

That's an interesting question.

I read in David Gavin's Triumph restoration book Triumph went back to heavy flywheel mid production 1969 to reduce vibration. I don't know I could feel the difference in vibration either. I find it most puzzling how some bikes are very decidedly smoother than others of same year & model. That has puzzled me for many years now.

Will be most interesting to see how your bike feels. I'd be willing to trade bikes to compare. I'll ride over to your house. Let me know when.
Don

Dick Harris has ridden 10's of thousands of miles on a 76 degree 70 650 Triumph.I saw the bike and Dick started it.Yes sounded like small Ducati..I reved the engine and iit didn' t walk across the parking lot while on the centerstand,lol. I suggest you PM Dick Harris.......

Don, yes this crank is also smaller diameter than stock so heavier is ok by me. I believe that a heavier, offset billet, dynamically balanced crank with weight matched parts is the best I can do. My rod weighing jig isn't done yet but just for comparison purposes, I weighed what I thought were a set of matching stock rods and found them to be 40 grams apart whereas the MAP steel rods are within 0.1 grams of each other and match the total wt. of a brand new T140 alloy rod...of course until my jig is done, I won't know the most important end to end difference. Most gracious of you to offer to trade bikes, I've actually never ridden an OIF Triumph. I got a ride on the back of an OIF BSA 650 back in '72 while deployed in Germany and I must say I was impressed by how that bike pulled through the gears.

Hillbilly, thanks for that suggestion, I've read everything Dick has written in Vintage Bike, a real inspiration that man. His crank was 2# lighter I believe and he said as a result of that and the offset, he had to set his idle up a bit. He may have also said the next time he'd make it heavier but I'll have to check on that. Interesting on the center stand revving test.

Sorry about the hi jack OP...my take is go with the heavy crank as I believe most flat trackers went that route back in the day. Mark

Its amazing where these conversations can lead to. But all interesting to note. At least i now no i have 2 heavy cranks, The 76 sounds ideal for the road, but i do not think there will be a big benefit for my purpose's. There are so many ponderables when building a track bike, made more of a head scratcher because of the small loose tracks we run on, Handling and traction are the most important to get right in my mind, I have decided to go with a single carb head, As i seem to remember John Healey saying a Tr6 with a 750 conversion and flatslide carb was one of the best british bikes he had ridden, Would like to no what cams where in that bike, as that is my next puzzle to sort out.

PEH, one of the nicest off road Triumphs I rode was a mentors Rickman Metisse TR6 with 750 Chantland kit, 34mm VM Mikuni and manifold (JRC nowadays?), H&C desert cams...no matter what rpm, just open the throttle and go with nice flat torque curve. I couldn't tell you which crank he used but I don't think you can go wrong with the hvy one.
There are quite a few mid range cams out there, my desert sled runs stock '70 T120 cams with small port TR6 head and single 32mm AMAL Concentric...pulls good.

Handling wise the '65 or earlier steering head angle (steeper) would help with handling esp. on tighter, shorter tracks...mine has later frame and was ok in desert but not as good on TT tracks. I tried another 1" longer swing arm (from desert racer) but that slowed my handling down even more on the TT's so removed after one test.

Mine is in a late Trackmaster frame Mark, so the handle should be good. I have a JRC 32mm in stock and a Tr6 head that needs refurbing. a lot of folks like the T140 inlet/3134 combo, but im thinking a pair of 3134's (as yours should be i think) should be enough for my needs on the short tracks, Do's yours rev out ok? as thats what i will need to pull out of the corners hard. Is yours 650 or big bore 750?

Mine is 10.5-1 cr, 650 with over bore but can't remember how much, I like the T140 inlet and 3134 exhaust idea with the inlet moved to 100* lobe center as has been recommended on this forum by the likes of the late Pete R. Inlet timing around 45* open 69* close. My stock 3134's rev out fine with my open 1 1/2" ID open desert side pipes and still ran good with my later 2 into 1 siamesed 1 1/2" primaries into 1 3/4" secondary capped off with supertrapp muffler with a whole lot of discs. It did not rev out good with the smaller diameter stock TR6 side pipes and didn't seem to pull any better down low so I never tried them again. Mine with stripped down stock frame and steel tank is going to be much heavier than your Trackmaster. I need to pull my engine down some day as the last outing years ago ended with some bad noises from the top end. This time I will install my Chantland 750 with perhaps some H&C 7054 desert cams (mild) I have laying around, not sure about compression but Triumphs like it if you've got the good fuel. I have a 34mm Mikuni flat slide that will go on also but I will straighten out manifold as best as possible. All I want out of it is a good midrange fire road bomber...a dual sport if I can get it plated. Mark

Sounds like a great project in the offing. didn't no about the Chantland barrel. My friend at Thunder Engineering is looking to make some thing similar. Looks like the numbers of the 3134 is similar to the H&C cams. Usefull info on the pipes. thanks Mark.

The Chantland is a sleeved barrel with same shape fins as head, very distinctive look that I like but these barrels are no longer made. The MAP 6061-T6 alloy billet barrels are nikasil plated and also the delta shape and can be had from 76mm bore up to 79.50 mm (825cc claimed) and 7.5-1 cr to 12-1 cr (alcohol). I just purchased MAPs long rod 77.5mm kit (774cc) with 9.0-1 pistons and I must say it's the nicest, most complete kit I've seen. His latest machining has made the fins appear narrower at outer edge as compared to the original kits and looks better with the Triumph head. Pistons are designed for zero deck height and tight squish so you can actually set them up for real squish, they also can be had with T140 crown and 650 pin to use with a 10 bolt head on long rod barrel. This is what I did for best of both worlds.

In another post awhile back, I asked about offset rocker buttons for more lift using my standard 3134 cams. I also read about and had forum member confirm, that the Norman Hyde half race cams are 3134 profile but with the lift of the T140 Spitfre profile .322" cam lift I think. The megacycle mildest cam has decent cam lift .348" and specs that favor good low to midrange and I would think they would work good with open pipes and a TR6 head for tight loose tracks. MAP offers UK made version of this cam and others at a better price point. My stock cammed engine ran shortened TT 1 3/4" pipes on the TT track and although felt a little soft on bottom but pulled really well. Only shortened because they came off my street bike which had been shortened for 12" 'shorty' mufflers...hey, little money but much desire...I still used 2nd gear for starts to try and eliminate wheel spin and early shift.

Ive been looking at the cams and still undecided, The desert cams appear to be very similar spec to the T140 inlet cam, which has a lift of .350" i believe, just a bit of adjustment needed on the triumph cam timing to get it closer, the map cams are made here so i will try to find out who. was that 1 3/4" od or id on the pipes?

Standard 1 3/4"OD TT pipes...I measured T140 inlet cams at .322" cam lift so yes approximately .350"+ valve lift. MAP cams are Neuman made I believe. In the past it's been recommended on this forum to set T140 inlet at 45* open/ 69* close (@0.040" lift) for LC of 102* for improved midrange. Stock is 40/74 with LC of 107* supposedly but many are being found from factory with LC of 99* and loss of ability to breathe on top. Some of the forum members have been documenting this disparity between factory specs and actual timing found on unmolested T140/TR7's.

I'm not sure I'd pick the T140 inlet for a loose dirt scrambles type bike, I'd probably go for something with less duration but the nice thing about Triumphs with separate intakes and exhausts is that you can play with timing and try different settings to try and tailor a desirable power band.

Yes, i have read some of the disparity on T140 factory specs, I am pretty tempted to go with a pair of 3134's. I had a suspicion they where made by Newman, he makes them for Harris and Wassel/emgo,

All I know on this is the '66 Bonnies came with light flywheels, accelerated very quick but vibrated like Hell.

Two riding buddies had '66 Bonnies and couldn't keep the bolts tight on them
This is one problem I didn't have with my '67.

My '66 didn't' seem to vibrate any worse than buddies '67 T120 or '68 TR6, also really couldn't tell if it accelerated any better because of the crank alone but then mine had 750 cc and cams so did anyway. I think luck of the draw on balance of parts, diligence of assembly etc. plays as big a part as the actual wt. of crank, however, the factory did go back to the heavy crank for a reason. So I agree with you Irish Swede but just not as a blanket condemnation of all '66 light crank bikes. I wish there had been an EI available back in the '60's for a back to back comparison because I think that may have been as big a difference as the cranks. Mark

Can't go wrong with 'less cam' but you can with 'to much' cam, I would still try to find the Hyde 3134 half race with the .350" valve lift vs the Harris 3134 half race with less lift since the option is there. Intake and exhaust can make or break any cam selection, not just the cams specs. And again compression makes a big difference if you have the fuel or head/piston crown that allows it.