Crank Pin Interference

[Northracing]

I had a wide-case 250 crank pressed apart, 30mm pin. Both pins pushed out with no more than five tons on the hydraulic press, I thought they would be tighter. Crank had been rebuilt before because it has an aftermarket rod.

Everything will be carefully measured, but can anyone let me know what an acceptable range of interference fit for a crank pin is?

Thank you.

[themoudie]

Aye Northracing,

I'd suggest that you await some sage words from Nigel Lacey and or Eldert.

I can only tell you that when my 1974 450 MKIII widecase was taken apart it required 28 tons of pressure to press apart the crank. When Nigel serviced my 1975 Mototrans built 350 MKIII widecase engine's crankshaft, I have no idea what pressure he used to separate the crank!

I cannot remember, and don't have it to hand, if the Clymer Ducati 160 - 450 service manual through 1974, has the information that you are looking for. When I get the chance tomorrow, I will have a looksee and post data here, even if others express their practical experience/actual pressures and interference dimensions, used regularly on competition engines.

Good health, BillR

[bendik]

My crankshaft turned in the bearings after I changed the bearings. Got the following advice from Nigel ; "If the crank is worn below 29.95 then it definitely needs repair or replacement. Even 29.96 is marginal, but if both sides are that sort of size, it was not good for racing"

[LaceyDucati]

Hi,
Generally speaking between 3 to 4 thou (0.1 to 0.075mm) interference on a 30mm pin is good. Absolute minimum would be 0.05mm, but even that is a bit marginal. I would expect the bore in the flywheels to be between 29.90 to 29.935mm if the hole is in good condition.

Narrowcase 27mm pin cranks are generally about 2.5 to 3 thou interference and 32mm widecase cranks are between 4 to 6 thou!

5 tons is not a lot for splitting a crank, are the holes "broached" in anyway. Some original pins are undersize maybe 29.99mm, I generally specify my pins at 30.01mm.

Hope that gives you a guideline.

Regards Nigel

[Northracing]

Thank you Nigel, that is the info I was looking for. The pin that came out measures 29.98-29.99, I have not measured the flywheels yet.

One more question, if the holes in the flywheel are worn large, can they be bored to take a 32mm pin? Would the oil holes line up? Not sure I need to do this, I have a spare crank but interested to know if it's viable.

Thank you.

[LaceyDucati]

If the holes are good but just stretched in theory you could just use an oversize pin. You could hone the holes to rectify any minor imperfections and also fit an oversize pin. However one off oversize pins can be expensive. Any major damage could be bored/ground out to another size, but finding a point of reference and doing this accurately enough to end up with an acceptably true crank assembly is no easy feet of engineering. Minor errors will amplify to produce an out of true crank, this is not a job to be undertaken lightly. Any wear on the shafts or run out of centres would need addressing before any work on the holes was started. In my opinion it is seldom an economic repair unless you are a precision engineer with good machinery and plenty of time. This is work that needs to be done in a few microns or a tenth of a thou over the length of the hole and the same on either wheel.

Regards Nigel