Crank Stuff

Cranks, A and A+

Despite having different part numbers the A and A+ cranks are the same and are interchangeable. The only difference is that all A+ cranks have 'gutters' at the side of the journals to reduce crack propagation.

Cranks, differences between 998 and 1275

"The 1275 crank is bigger than all 998 regardless of A or A+. Then the rest of the dropgears wont fit the casing because the diameter of the bearings is bigger.

I believe though I haven't tried it, that the rest of the 998 A+ would fit the 1275 A+ gear. you need to use either all A or all A+ due to the different angle of the gear cut. No, it's the crank tail that is different between the different displacement engines. Won't argue the A/A+ differences, but all the Ss and the 1300(1275) have the step in the crank, and use a different gear than the smaller engines.

Taper is the same, but primary gear step is different.

The only flywheel with a smaller opening was the coil spring version used on early MkI 850s (thin tail crank as per DV). All others fit across the range."

Crank, cross-drilling

The Mini crank is not cross drilled as standard, leading to potential problems at high revs as the lack of restriction in the oil flowing out from the centre mains to the ends. This leads to centrifugal main bearing oil starvation.

Crossdrilling of the big end oil ways and the blanking off of the existing drillings puts more of an obstacle in the way of the oil's travel so for extended high rpm without mains failure cross-drilled cranks are the thing. Normally the oil has a line of sight trip out from the main bearing oilways out to the big end oil way outlet drilling.

Turbo cranks are not cross-drilled either. The reason that people say they are stronger is because very early Turbo cranks were manufactured in EN40B, but they changed it back to EN16 (cheaper) in around '86.

To check the difference so you don't get conned? Easy, just run a file over the nose of the crank. If it cuts, its EN16. If its EN40B, it won't leave a mark!

Crank dampers

The crank damper is the 'heavy' crank pulley fitted to all 1275 engines and some 998 engines.

This is basically an iron ring on a rubber inner ring that has it's mass and rubber spring constants matched to the 1st resonant frequency of the 1275 crank. It cancels out the resonances and prolongs crank life.

This may be because the crank has cyl 2&3 big ends on the opposite side to cyls 1&4 and at high revs this 'longitudinal' mass imbalance coupled with the spring constant of the iron (yes, it will have one) causes the crank to vibrate slightly. This cannot be solved by crank balancing, but can be solved by 'damping' out these harmonic vibrations.

The 1275 crank has 2 resonant harmonics within the 'usable' frequency range at 5950 and 6250rpm. If you don't use the damper, the crank may fatigue and break.

998 engines don't need a damper as the crank weighes less and therefore the 1st harmonic is beyond the usable range at 12k rpm or something. That is why the 998 engines have a 'light' pulley, and the 1275's a 'heavy' one.

Putting the heavy pulley (the damper) on a 998 won't do any harm, except adding to the rotational inertia, but putting a 998 pulley on a 1275 is asking for trouble.

Crank case ventilation

"On a worn engine combustion gas gets past the rings. This builds up pressure in the sump, reduces power and causes oil leaks. So, we exit the gas via the crankcase breathers. This is on the tappet cover on a 998 and timing/transfer covers on 1275. Flash people, like me, also vent the rocker cover. To comply with car build regs the designers just fed the gas back into the carb to be re-burnt.

Easy. So what's the problem?

Well, if you've ever looked inside the throat of a carb, you'll see that the vent pipe joins the carb just behind (on the atmosphere side of) the throttle butterfly valve.
Oh no, those clever designers didn't put the vent on the engine side so that the gas is sucked in on max vacuum (decelleration), they thought it would be clever to stick it before the butterfly so that the gas gets sucked in when the vacuum just before the throttle valve is greatest...

When is this?

Well, it's when the throttle is fully open, but the dashpot piston is just starting to rise. Or to put it in more general terms......it's when you floor it! All that horrible unburnable gas pouring into your carb just at the moment you want full power? MAD!

Disconnect it! Don't listen to those BL guys! They have designed granny's economical runabout...not a street racer!

So, if you know what's good for you, you'll block the pipe on the carb ONLY. Vent the crank case pipe to air, or to a catch tank or something. Obviously the down side is that the pressure inside the crank is no longer 'negative' and you may get a slight increase in oil leakage. If you've got loads of gas coming out of the vents then your engine is dying and the rings are worn."