Hot crank

[GT Guy]

Stripped my RB20 motor to sort a drive side oil seal and found the crank like this. There are no obvious signs of abnormal wear but I'm thinking that whatever caused the heat may well have ultimately caused the seal to fail. The crank has been in there for 3 years and was ok visually earlier in the year. The crank was built locally using a blasted pin that was loctited in place. Anyone have any thoughts regarding what might have caused the heat so locally? I will be getting it checked and rebuilt if necessary. Cheers

[drunkmunkey6969]

Hard to see from the pic alone, but it looks like the webs have spread on the pin.


Sent from my iPhone using Tapatalk

[Rich_T]

Looks like "Rod walk" to me. You see it much more on Indian webs because of the way they make the cranks (truing is fundamentally different to normal process). In basic terms your crank might be slightly out of true which means that the crank pin is no longer perpendicular to the piston travel. This means that halfway through the power and compression stroke the crank pin is not parallel to the crank axis or perpendicular to the piston travel. consequently the big end then favours a thrust to one crank web.

With the rod big end thrusting off one crank web there is inevitably frictional heating to that web only and this is what you are seeing.

I'd suggest clocking the assembly first to see if it is true or not. I think you can re-building with new rod, pin and big end bearing. There doesn't seem to be much mass off-set in those webs, does the engine shake a bit?

[Rich_T]

Yep, looking again I'd say your crank is definitely out of true. As Dan points out above the pin would appear to have moved but also you can clearly see the cheeks of the drive side web have been rubbing on the casing (circa 5-9 O'clock in your picture). This is a very strong indication the crank has moved because the drive side is constrained by the drive bearing. I'd check the width of the crank is 40.5mm too.

[GT Guy]

Thanks Rich. I was hoping you'd see this! Do you think I'd be better changing the crank altogether? I had been thinking about one of yours. Do you do a 116 rod?
To answer your question I do believe there is more vibration now but I ride this everyday so these things can go unnoticed.

[Rich_T]

I don't do a 116 rod I'm afraid. I think it will be OK to rebuild though. What is the piston and bore like?

[GT Guy]

I'm thinking about a 62mm replacement for this crank. Take the opportunity to try something new. I assume that will require a packer plate and some machining to the crankcase but is there anything else I should know?

[shocky]

I can strongly recommend Harry barlow at proporting for cranks rebuild truing or new
He often trues other dealers cranks for them

[Scooterdude]

Might be worth contacting Harry Barlow at Leicester regarding your crank, he'll build you anything you want + he tig welds the pin in so no future problems.
He'll probably be able to rebuild yours and again tig weld the pin.

[Lambretta Sash]

1st id get the crank on a set of blocks and check if its twisted, also is the little end coloured up ?
If crank is within tolerance on run out id suspect crank to bore alignment high i have found in a few casings and the worst was an Italian one btw