Would you like to build the smoothest Triumph twin possible? Then, you'll need to start with the 76-degree M.A.P. Crankshaft.
Triumph twins have always been known for creating a lot of vibration. That's because both pistons rise and fall at the same time on the standard 360-degree crankshaft. Over the years there have been multiple attempts to smooth these vibrations. Some attempted using a 180-degree crank . It is smoother than the 360, but it suffers from rotational vibration due to increased rocking couple. This is best described as plane imbalances on the Y or Z axis of the crank (basically twisting at the ends). Some crankshafts have counterweights to offset these vibrations, but these add to the weight of the engine and have limits as to how much they can help.
Legendary motorcycle and automotive racing engineer, Phil Irving believed the 76-degree crankshaft offered the best solution. According to his design theory, the 76-degree offset crankshaft allows one piston to be at top dead center while the other piston is just reaching it's maximum velocity. The 76-degree crank may not offer perfect harmonics, but when you consider the confines of the narrow Triumph crankcase, this configuration offers the smoothest possible motion of the crank and eliminates much of the vibration inherent in the stock 360-degree crankshaft of the 650 and 750cc Triumph twins.
It has been stated that a 90-degree offset crankshaft has better harmonics. This may be true, but the added offset increases the rocking couple. This was understood by Irving. Our tests confirm he was correct. Harmonics is secondary to reducing rocking couple because of the Triumph's narrow crankcase mounting width. Rocking couple creates vibrations which are harsher to the rider than those originated from harmonics. We believe overall rider comfort is paramount and we belive Phil would agree.
- 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 (see below)
- 82mm stroke
- 1.6235-1.6240" journal uses standard Triumph OEM journal diameter and accepts OEM shells (rod bearings)
- Approximately 21 pounds
- 4340 American-sourced steel which is one of the most popular alloys used in performance crankshaft manufacturing
- Rated 140,000-145,000 psi tensile strength
- Triple heat treated
- "Ion Nitrite" treatment for longest life (similar to British Tuftriding)
- Shot peened to reduce internal stresses
- Generous Crank "Fillets" for maximum strength and rigidity
- Out-bounded Counterweight to lessen crank flex
- May be used in conjunction with any stock type Triumph connecting rod utilizing B2026M big-end shells (bearings)
- Use of some rods and piston combinations may require re-balancing based on precise bob-weight specifications (which is out of our control). Some very heavy rod and pistons combinations could require the use of mallory metal where precise balancing is required
- Direct replacement for 1971 and later T120 (650cc) and T140 (750cc) models
- The 76-degree crankshaft can be retrofitted to pre-1971 models by using a specially modified bearing with a metric ID and an Imperial (inch) OD. We stock these bearings for this purpose. For a ball bearing request part number 70-3835/MI. For a located roller bearing request part number 60-7362/MI. The timing side journal can be re-ground to fit imperial (inch) sized stock OEM bearing (70-1591) if desired
- Cranks are dynamically balanced at 50-percent using a bob weight of 585 grams per journal
- This must be used in conjunction with a 76 degree electronic ignition (we recommend and stock Tri-Spark) and 76 degree offset specific camshafts
- A high-quality additional oil filter such as our MAP6500 or MAP6510 is highly recommended
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 weight) X (balance factor) + (big-end rod weight + bearing shells)
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