E: Engine Big End Replacement

[Cyborg]

On some cranks we have pulled apart just to inspect, and found the clearances too tight, so we hone the rod liners out until my machinist is happy.

I just pulled apart a Norton crank. It didn't have any play, but felt tight in some spots and wouldn't rotate as freely as it should. It was sitting for decades before I got it, so tried flushing it out, but that didn't help. Once split, it took some real encouragement to get the rod to slide off the bearings. Turns out, the bearing is new and has never run which is a good thing. The outer race was obviously pressed into the rod and not honed at all. I'm thinking whoever assembled it must have warmed the rod prior to sliding it over the rollers. It was that tight.

 

[Chris Launders]

Oh you lucky person, finding a brand new big end not some rusted up wreck.

 

[Cyborg]

Oh you lucky person, finding a brand new big end not some rusted up wreck.

I thought I was lucky too until I peeked into the disgusting looking hole in the flywheel that the new pin came out of.

 

[Cyborg]

Anyone know what would be the optimum interference fit between the crankpin and the flywheel?
PR's book only mentions shafts (at .0039) for a Vincent, but no mention of the pin. Tuning for Speed says (no specific make mentioned) "a light drive fit, about .001".

 

[mercurycrest]

Try for .004". You'll need a good press to do it.

 

[litnman]

Lanolin will reduce galling during the pressing.

 

[roy the mechanic]

The big end pin on my twin was .006, took advice, ground it to .003 went together nicely.

 

[passenger0_0]

The big end pin on my twin was .006, took advice, ground it to .003 went together nicely.

This figure sounds better than 0.004" . As I recall the original flywheels are made from EN8 (read soft as butter) so pins with higher levels of interference will simply cause plastic (permanent) deformation of the flywheel bores and negatively impact flywheel alignment. Pulling the big-end pins nuts tight has more affect on flywheel rigidity due to the shoulder on the big-end pin pulling up against the flywheel face.

 

[david bowen]

They were pressed together on a hand press at the Vincent Factory. big end nuts tightened by a box spanner with a 3 foot shaft,

 

[Cyborg]

Pulling the big-end pins nuts tight has more affect on flywheel rigidity due to the shoulder on the big-end pin pulling up against the flywheel face.

That jives with the statement in Tuning for Speed. His statement (in the book) “a light drive fit about .001” seems low to me. It mentions the shoulders of the pin providing support. I won’t be using this (Norton) crank for a while. I pulled it apart to check the stiff bearing, because there is an Inter big end assembly on EBay for a decent price. Turns out my bearing is perfect except for requiring more clearance. The flywheels have gone back under the bench for now. The pin holes will require some weld and machining, so if anyone has advise on that, fire away.... although I need to get back to the Comet. Timing chest bits have arrived. Commando gearbox is finished and belt drive is on the way.