Crank binding
Turbo Mini Forums

This is a new one to me.....

I'm building a race 1293. I've had the block line bored with steel caps and a freshly cround crank. All sounds well, but there's a problem.
After torquing up the mains the crank spins freely for a while. If I stop it and leave it, it sticks. I need a lever to get it spinning again, but as long as I keep it rotating, it spins freely. I thought it was the lube used, but I've now removed the crank, and fitted it againg with regular motor oil. Same thing happens.

Has anyone else ever experienced this? The main clearance is 2thou as it should.

i had this when i first built the engine for the Don, it was the centre main that had distorted,

is the crank straight ?

any marks on the shells,

carl

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE

Are you building it in an ice cave?

1/4 Mile 14.3secs 96Mph Terminal 10psi of boost.


Fibreglass Parts? - http://www.tdkracing.co.uk/
Split Rims? - http://www.force-racing.co.uk/

I have had similar when installing one of Carl's 1" CMSs the wrong way up in a moment of brain fade.
I doubt you have a CMS but it does add weight to Carl's theory above.

I was being serious by the way? Is it really cold at the moment?

1/4 Mile 14.3secs 96Mph Terminal 10psi of boost.


Fibreglass Parts? - http://www.tdkracing.co.uk/
Split Rims? - http://www.force-racing.co.uk/

I have come across this a few times and its either down to the center main strap, cap dowels, or the crank run out (bent).

Try and find the bearing that is causing the nip. With all caps correctly fitted, remove one and see if the nip disappears. Refit the cap and remove another, and so on. I once had one where it the crank would spin easily with two caps, no matter which two, but nips with three. It ended up being a bent crank, and we are only talking 3 thou across all journals! but then if you have 3 thou run out and 3 thou bearing clearance, something will tend to bind at some point.

Check the run out by removing one main bearing cap at a time with the other two still correctly torqued up, and place a DTI gauge on the journal away from the oil feed hole. Rotate the crank. Measure any run out on each journal.

The center straps are a pain in the arse and can be hit an miss. I personally think they still need a line bore, or at the very least a hone contrary to what most actually do. In my opinion, for the effort, you might as well fit a four bolt center. That said, you already have had the line bore done so this might not be the issue. also, if you do have the cap AND the strap, machined flat and then line bored, at least mark the strap for its correct orientation since if the strap fitted upside down or back to front can some times distort the cap enough for the crank to nip.

I have aso had problems with the crank nipping up as a result of worn locating dowels, allowing the cap to move a little when it is torqued down. new dowels > problem solved


Lastly, I asked by main man about this, and he said as long as it turns easily and continues to turn without nipping with a finger and thumb once it is moving, it will be fine. The reason it 'nips' is because the oil film is pushed aside when stationary at the high spot. Once its moving, oil is drawn in and will continue to do so as long as there is oil there. It is still a bit worrying though when you have to use a lever to get it moving, and rightly should be easy to spin from stationary.

Regarding the mains dowels, the swiftune billet ones are very good, pricy at 15 quid, but worth it IMO as they seem a bit snugger than std ones.

i have had simlar problems the dowells are the culprit

It could be that who ever line borewd it did not have the mains bolts torqued up fully, thus boring a circular hole when the cap is not fully clamped , then when you apply the full 60 lbs it nips down,

in the inatance i mentioned above, i could put 55lbs into the bolts and it spun freely, but 60 lbs would have it stick as you describe V.

carl

WWW.FORCE-RACING.CO.UK PLEASE CLICK HERE