How delicate is that new crank you are trying to "ease" into your case?

Just wanted to start some general discussion as I have done some "research" on this and ended up still not sure of the answer


Q: wonder why that bike is so rough running, perhaps worse  at idle?

A: Crank out of whack due to improper "brute force" installation?


One forum poster said handle it like glass, tapping it in with a hammer is a no no.  One poster said you would be surprised at who is doing the "no no".  So sure I had to carefully tap the case around the crank to get it on.  HAd a bad nights sleep when I learned it was possible to untrue a crank if you "stared" at it too long.  So I conducted and experiment.  I have an old crank or two lying around so I measured the distance across the web (opposite to where the connecting rod pin is), and hit the shaft with a plastic hammer several times.  Web distance did not change, so I hit harder, then changed to blows on the old bearing that was installed on the shaft.  These were moderate to hard hammer blows no change, so I generally beat on it moderatly for a while and still no change (0.01 mm or 0.0004 inch)   I concluded that perhaps these units are not that delicate?  Any comments


And yes, run out with a dial indicator is more important, but I don't have one, and this was an extreme test.  And of course it is very prudent to use heat and cold properly and get everything set in place as smoothly as possible, but at times this does not happen, partucularly for the unpracticed as heat and cold fitting must be done pretty fast.  So how much force do you think a crank can take during installation?


Thanks guys, just wanted to get some discussion going!

i think you want to learn the engine building business or at least proper assembly procedures. cool

Absolutely.  Tricks to the trade, and do's and don't based on real world experience.  Not all assemblies go smoothly for a wide range of reasons

When rebuilding a rm125, a local shop told me to put a pipe over one end of the crank that has some rags on the bottom and beat it in lol


I decided to borrow a friends crank puller....

they are middle of the road, they can take a few smacks but how hard is up for debate. I only have to hit a crank toget it out, and when i do that it is to push it out so it makes no difference.  When putting a crank in i freeze the crank and heat the cases. Literally drops right in with zero effort required.

Freeze the crank, heat the cases. 

No more than 15lbs pressure should be used 

Cranks have to be pretty tough. It usually takes a very meaty press to push one together. It'd probably take some good beating by a 2-3 pound sledge to seriously get one out of whack.


Where you have to be careful with cranks are the ball bearings. Ball bearings cannot take much axial load (as in the same plane as the shaft going through it), and beating on the crank can damage the bearing race/balls.

Great point about the bearings.  They to often take a hell of a pull as the crank is "sliding" in or out, that's got to be significant force. 


I been thinking and correct me If I am wrong, the Pin that holds the con rod must be is press fit and incredibly tight.  Its thick and it's mounted through the entire section of the crank webs.  At any rate to deform the crank by hitting one of the shafts, would require the pin to either bend, or the webs bend.  Not really possible is it?   PErhaps if the cranks counter weights could be easily twisted about the pin that hold the con rod. then the crank would really  become out of balance. If so could a twising motion be imparted by  blows on the end of the shaft?  I'm guessing a bit here but it would be difficult.  So then cranks that go out a balance are they typically twisted about the conrod pin, rather than being. bent slightly inward or outward?

Edited by bikedad1

Hey Kranie, Like the number 15 lbs.  It does put it into perspective.  You know youve done it right if it really does just ease in!  Had a bearing stick in an rm250 case. IT was discoloured and looked pretty ugly.  I figured I was in for a stressful extraction.   Had to hit it pretty hard and I hated doing it!

Ive built bike/quad motors for myself and locals for about 8 years now. I always install cranks with the freeze/heat procedure. Ive set back and drank brew while watching friends beat them in with a deadblow. Ive lost cranks in my own stuff and so have they. So i think its all in preferance. I will say ive always had the smoothest running motors after a full rebuild. My motors are always barely felt in the handle bars upon first startup. however that slowly starts to change after 30 or so hours of run time. My dads hobbie is tractors. He has built tractor motors as far back as my memory goes. He always stands his spare crankshafts up on a wood jig that he built just for that reason. I remember asking him once "Why do you stand all your cranks, and cams up instead of sealing them up and laying them in the shelf?" His immediate answer was "You lay a crank down and its gonna warp" Logical theory, I wonder just how much a crank would warp laying down, if at all??

My Jury really is out on this too!  I think there is a lot of rituals out there.  Its really hard to know for sure.  I like your experience.  Beer beating, vs cold and heating.  No longterm conclusions based on what you've seen.  Stranding a crank up?   The "warpage" would then be about the big end conrod pin.  Standing it up would put more force on it (IMO).  Perhaps laying it in a bed of feathers with the pin facing down and turing it every three days will balance it perfectly  a for a good beat in LOL!  I'm still thikling they don't bend easy and it must be a twist about the pin that causes problems.  I saw one post that recommends eyeballing the webs looking straight on "through the shaft" and if if they don't line up you take a plastic hammer and tap them till they are "prefect".  Thier was even I diagram.  Looked very official.  Still not sure it its for real.  I just did the crank on an RMZ and ya i did use a plastic hammer to bring it "home".  Gently.  Runs smooth as silk so far.  Also did and RM last week and used the case bolts to carefully bring it home.  I think however next time I will take the zen appraoch and try the 15 lb rule Kranie mentioned.  Heat and cold sounds sensible.  Had a friend say really!! do you think the shops have a freezer in he back....good question.  how many do?

I use a heat gun on the cases. Ive gotten away from the freezing of the crank just because of the damn condensation on the crank and the thing may as well be under water.


But the crank is the first thing I drop in, I heat that bearing saddle up with a heat gun until it practically smokes. Drop the crank into the right side case, the bearing seats all the way, then drink the beer and wait for it to cool.


On a CRF, you shouldnt have to break out the plastic mallet. The bearing saddles are damn near perfect slide fit most of the time. RM's have been a different experience for me. I had an RM 250 that I swore was going together wrong it was so friggin tight on the main shaft left side bearing saddle. Ran perfect, but had me sweatin, tell ya that.


I cringe when it takes force to assemble these things.

The Rm250 I worked on Everything was tight!  Needed a puller just to get the kickstarter off the shaft and it took a gob load of force.  THat was the start of the adventure!   THe RMZ was pretty smooth, but I was to do it all over agian, based on what I have learned here I would use heat and beer!  Beer is a very important tool in my garage, ones always open.  Problem is I get so focused usually I can only manage one in about 3-hrs.  Love the heat till hot, then drink cold beer till cool!  Seriously though I've done two cranks so far in my life and plan on many more.  This advice is absolutley fantastic, and I feel I am gaining a much better understanding!

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