CRANKSHAFT/ Billet T40 (T120) 89mm "Stroker" 76 Offset EN40B

Welcome to the M.A.P. Cycle world of a "Hot-Dog" stroker crank for your Triumph 650/750 Big twins.  You wanting increased displacement along with more low-end grunt? Well, here it is, we've upped the torque anti with our 89mm EN40B Offset Billet "Stroker" crank. The torque of a Norton with the simplicity and plethora of aftermarket performance parts of a Triumph. A perfect match. Having always been known for their excessive vibration, due to both pistons rising, stopping then falling, the longer stroke made matters worse (example a Norton Atlas). Now by offsetting the journals, vibration returns to an acceptable level all the while enjoying the dramatic increase in torque of your "New" engine.  Coupled with our Billet alloy cylinders you have from 810, 834 up to a whopping 884cc (cylinder dependent).
The added torque and displacement are a wonderful addition, but especially in a side car application where the add torque is a wanted necessity!

Designed to drop into virtually any 650/750 Triumph twin with only the need of a set of M.A.P. "stroker" rods (stock rods, by this time, should be replaced regardless to avoid shattered cases and a destroyed engine caused by a broken rod).

Fits:

• All Triumph 650-750cc Twins built after June 1971 (GE27209) though 1983 with metric timing side main bearing. Can be adapted to earlier 650cc twins by purchasing a "conversion" bearing (metric ID with inch dimension OD) available at M.A.P. Cycle  (see below)

Specifications:

• 89mm stroke


• 1-3/4" journal (use with any replacement OEM Norton 750/850 rod shells (B2101LC))


• Crank Journal Offset (76 degrees)


• Long snout for better alternator rotor support


• Approximately 19.75 pounds


• Dynamically balance with a pair of 604gram bob weights creating roughly a 49-51% balance factor dependent of remaining components

Features:

• EN40B Billet Material - EN40B steel


• Rated 123,000-144,000 psi 


• The strongest alloy used for cranks especially for a British bike engine


• "Tear-drop" journal oil holes for better big-end oiling 


• Heat treated


• "Ion Nitrite" treatment for longest life (similar to British Tuftriding)


• Ionized nitrogen is vacuum deposited onto the crank surface in an oven, penetrating .010 to .012 inch into the metal surface and changing the microstructure of the steel, surface hardness is doubled from 30 to 60 on the Rockwell scale, while fatigue life is increased by 25 percent.


• Shot peened to reduce internal stresses


• Generous Crank "Fillets" for strength and rigidity


• Out-bounded Counterweight to lessen the possibility of crank flex

Note:

• Requires MAP7063 (4340 ultra light steel "Stroker" rods)


• The 76-degree "Stroker" crankshaft can be used on pre-1971 models, but it requires the use of a "conversion" timing side main bearing (70-3835/MI (ball bearing) or 60-7362/MI (latest factory located roller) or the crank journal can easily be turned to fit imperial unit (inch) bearings.


• This must be used in conjunction with a 76 degree electronic ignition, and 76 degree camshafts (stocked at M.A.P. Cycle) 

About crankshaft bob weight:

• Bob weight is the amount of weight added to each journal before it is spun also known as dynamically balanced. It is calculated in the following manner:

Bob weight = (complete piston assembly + small-end rod weight) X (balance factor) + (big-end rod weight + bearing shells)