Bonneville America Forums Forums Technical Forums Performance Mods Stroker cranks for Hinckley twins!!!

I emailed famed motorcycle crankshaft company, Falicon, to ask them about stroker cranks for the new Triumph tiwns. Here's their prompt reply:

"We do not manufacture a billet crankshaft for this model.
We do modify stock OEM crankshafts to the stroke of your choice."

Woo hoo! No more complaining about "small" engines! One of their mod jobs done to your crank, coupled with the big bore kit of your choice, and you, too, can thump with the best of them. And no further need for those danged engine-killing high rpms. You'll have plenty of power right off idle.

Now, a note about what Falicon said about modifying your crank. There are two ways to increase the stroke of a stock crank. One is to offset grind the existing journal to a smaller rod bearing size and use a custom (or other brand) con rod, which has a smaller rod bearing. This has been done for years with 350 Chevy cranks where the journal is cut down from 2.10" diameter, to the old "small journal" size of 2.00" Just use an early sized Chevy V-8 rod. Also, another make rod can be used, but usually the wrist pin has to either be bored out or sleeved down to the proper wrist pin diameter. The limitation with this method is you can't gain very much stroke increase.

The other way to stroke a stock crank is to weld up the outside of the rod journal and then regrind it to the stock size in an offset manor. The weld is applied to just the bottom of the journal, the area farther away from the centerline of the crank.

Welded stroker cranks have been around "forever" and it is a viable way to increase the stroke. One may not want to use such a crank in a 12,000 rpm racing engine, but for the street or even bracket racing motor, they're just fine. This mod is an offshoot of the standard welding repair job, done on a stock crank, to a burned/trashed/seized journal. It's always done on a forged crank, though a rare cast crank can be saved this way, too.

Many times a repaired journal is first undergound and then chromed back to standard size, making the journal even that much tougher. Hi perf cranks get chromed on all journals, along with oil holes chamfered or even enlarged.

Next consideration is what to do about the piston going farther up the bore with that longer stroke. Usually a custom piston is used, where the pin hole has been moved higher - half the distance of the stroke increase. The distance from the middle of the pin bore to the "deck" of the piston, or main flat part of the piston, is called compression height. If you stroke your Hinckley twin from 68mm to 78mm, then you would need to have a piston with the pin moved upwards 5mm to compensate for the change.

Sometimes this pin movement is not always possible (the stock pin location being as high as it can practically go), so one mod that has been developed for bike engines is a barrel spacer, usually machined out of aluminum plate, and installed under the barrels to raise them. A combination of spacer and wrist pin change could be done, too.

If one were to do a stroker kit in combo with a Wiseco big bore kit, Wiseco could move the pin for you as a custom operation when they made up your kit.

Yet another way to compensate for the longer stroke would be to get a shorter con rod. Since custom rods are commerically available for Triumph twins, you could special order some slightly shorter ones, say from Carillo. Heck, an accomplished machinist could even make his own rods out of aluminum bar stock!

Now all ya need is the money.......

How much would this cost and how much power do you think it would make??

Good questions. I didn't ask them about cost, but I'm sure it varies according to stroke increase.

As for power increase, man, that's a tough one. A really wild guess by me would be somewhere around 10%, but, that's from idle up to max revs.

The main idea is to do the stroker crank in conjuntion with a big bore kit, maybe bigger carbs, and possibly bigger camshafts.

The big deal is the torque increase, which should mean that one can run the biggest counter shaft sprocket and still pull Mama and loaded saddle bags up a Colorado mountain.

MATH CLASS

Going to the Wiseco big bore kit takes one's engine up to 904 cc's, as we all know.

Putting a 78mm stroker crank in such an engine (92mm x 78mm) would give one a 1037cc engine. (92 x 92 x 78 x 1.5708 = 1037.0295)

This is assuming one can put that long a stroke in the Hinckley twin. If you could only go to 75mm stroke, you would get a 997cc result.

There are several factors to consider when stroking a Triumph twin, besides those listed in the original post:

1) Are the rods going to clear the crankcase as they go around?
2) Will the con rods miss the bottom edge of the cylinders?
(less a problem with an increase in bore size)
3) If a barrel spacer is needed, will the cam chain have enough play for the resulting higher cam location?
4) What formula do you use to balance this whole thing when you're done? (It will need to be balanced before assembly!)

Once someone figures it all out, it should be fairly easy, if not expensive, to stroke your America.

Shame on me for not listing Falicon's web site. They make some really neat products - hardcore stuff!

http://www.faliconcranks.com/

Man, wish SCAT made motorcycle cranks, a pro-comp forged crank (only a mere $1000) along with Carrillo rods would be the shiznits. I was never a fan of the offset-grinding concept, you then have to find or fabricate oversized bearings, and now that journal is thinner :-( I really wish Jojje was on this forum, he's disassembled the bottom end of his engine, and he could probably give insight on how much clearancing the inside of the block would need for a longer stroke. The big bore guys would probably have good insight as to whether the Piston Skirts (stock and aftermarket) would need any clearancing. But oh the potential! Good Find PapaDean!

guess i was a day late and a dollar short with my previous !! Apologies to all

I wonder if they'd give somebody a discount if they took their bike to them as a test bed?

Jojje, the Turbo Swede, has had Carillo rods done for his bike. He has this nice pic comparing the differences between 3 different connecting rods for his Bonnevilles. I'm sure Carillo could come up with something for this stroker.

1. Use the longest rods you possibly can. The rod length/stroke ratio is critical. Short rods mean exessive side thrust on the crankshaft, converting torque to bearing load. This is hte reason 318 and 340 Mopar engines can easily produce more power than 327 and 350 Chevies.

2. Don't forget that increasing the displacement, even if the pistons are still in the same position at TDC, will increase the compression ratio. so, you may need to use higher octane fuel.

3. You can get a ballpark figure for your power increase, but keep in mid that this is an educated guess. Power will vary as a function of displacement +/- a scaling factor.
Let's say you are going to destroke your engine in order to race in a particular class, maybe 650. For engines of this size, the scaling factor will probably be around 5% to 8% and it adds as you go smaller, subtracts as you go larger. So, rounding off stock numbers to make things easier, dividing 60 HP by 800, multiply by 650, THEN add 5 to 8 percent of the difference. Increasing the displacement, do the same thing but subtract 5 to 8 percent of the difference. So, going from a stock 800 to 1100, 60/800 * 1100 gives 82.5 hp. .05 * 22.5 gives 1.125 and .08 * 22.5 = 1.8, so you can expect 80.7 to 81.357 hp.
The reason for the scaling factor is that a bigger bore means flinging heavier pistons up and down in the bore and an increase in friction area with a bigger bore, or flinging the same weight around faster and farther with a longer stroke, which eats up a bit of power.

4. I tested the theory that there is a difference in torque increase when displacement is increased by boring or stroking. Much to my surprize, given the same combustion pressure and calculated at 4 different crank angles, the same displacement gives the same instantanious torque no matter what the stroke and bore are. The real differences are caused by tuning the engine to make the most of differences in the operating limits caused by varying piston speed and inertial forces.
So, the tradeoffs are that a very long stroke will limit the maximum speed of the engine because of inertial forces caused by the crank geometry and piston speed. (think of the crank as a lever, which is what it is. The longer it is, the more force a given weight will generate) A very short stroke and large bore are limited by the maximum flow rate through the intake and exhaust systems and the inertia of the air.

Another factor that I know exists, but don't have any figures for, is that there is a limit to how quickly the air passing through an engine can be heated and expand. I am quite sure that there is a point at which a piston will be pulled down the cylinder faster by the crank than the burning mixture can expand and push the piston. So, if you built an engine with a 15 inch stroke out of very strong materials, you couldn't run it at half million rpm's unless you find a different fuel/gas misture that heats and expands faster than gasoline and air.

I think a compromise with rod length would be more in order, running a long rod will reduce side wall thrust, but will also slow down the piston speed vice a shorter rod. The slower piston speed can have an effect of making the torque curve more "peaky" and not quite as flat. And yeah, the longer the rod, the more barrel shims you'd have to use to maintain the proper deck height, so your compression wouldn't run away with you. Anyways, a really cool discussion, glad this came up :-) Regards to All

Is not Bill Gately's Bonnie Flat Tracker a Falicon stroked motor?? Yes! I know so - at 989cc 105 HP.

In Bill Gately's race motor, rod length ratio would be of some consideration, but let us not forget that we're talking of street bikes here.

While shorter rods do increase cyl. wall loading by the piston, at lower rpms it is not a problem. Standard rod length in a small blk. Chevy is 5.7 inches (144.78mm for you Imperially challenged folks), but, when Chevrolet designed the 400 small block, they kept the piston compression height the same as the 350 and shortened the rod to 5.56 inches (141.224mm).

For a street motor, this short rod is just fine. Now, when hot rodders want to turn more rpms, such as in a 400 Chevy bracket engine, they usually go with the 5.7" rod and custom pistons. I worked for a place (Performance Chevy Products) which had its own dirt circle track "late model" with a 434" small block. It had a 4 inch stroke(!) and used a custom 6 inch long rod. The compression height was 1 (one) inch even!! That put the center of the pin one in down from the deck. In fact, the engine had Wiseco pistons with just a compression ring and an oil ring, no second ring. And, there was a special machined aluminum pin button with had grooves in it for ring support over the end of the pin.

Obviously there's no need for this in a street engine, just as there's no need for a longer rod, or even stock length rod, in a late model Bonneville engine.

Now, as for those 318/340/360 Mopig, er, Mopar engines making more power than the 327/350 Chevies, I can say as a Mopar fan that it just ain't true (usually). The limiting horsepower factor for the Mopars is the cyl. head. I know, I ported a couple sets (both 318 and 360 iron) for my son's Duster motors. I've also ported way too many Chevy heads over the years......no comparison - Chevy (sadly) wins. The W-5 Mopar heads are very nice, though.