Bent crank?

[IndyIan]

Hi Guys,
I think I've got a bit of saw (372)trouble...  Last night after I was cleaning it up I noticed that I had a tight spot on the chain when I spun it around by hand.  Did some research online and found that it could be a bent crank or an egg shaped sprocket.  Since the crank is not turning it has to be the sprocket!  YAY! they're cheap!

Then I put on the compression release, pulled the chain out of the groove at the middle of the bar and turned over the motor...  I could feel the chain tighten and loosen significantly with each revolution...  not good...  this can only be the crank since the sprocket isn't turning...  

So am I in for a new crank?  I was planning to take the saw in this week anyways as it has starting revving to high for my liking unloaded.  Related maybe?  Still cuts fine and still "four strokes" a bit but when compared to the sound bite on madsen's saw tuning page it revs alot higher.  Could be different saws I guess but I don't want to risk it.  

Any advice?  I have been quite careful not to over tighten the chain when hot but I have got pinched a few times and have seen the chain pulled pretty far off the bar while I was freeing the saw...  Tried not to pry it out but I guess I did... Rookie mistake :(

Anyone have an idea what the allowable run out is for the crank?  Can I measure it with the clutch on?  I'll have a look tonight pulling it over with the bar off and see it I can see it moving.

My buddy has a 371 so I'll go over to his place tonight and see how it compares.  Maybe I'm making a big deal over nothing.
Thanks for any help,
Ian

[Kevin]

I have to ask ... what did you lift the chain with while the engine was running?
What does the bearing look like under the clutch?

[Weekend_Sawyer]

Could it be that your chain is streached unevenly?
This happens with motorcycle chains, they seem to tighten up and loosen as you spin the tire slowly.

It also seems that if you took the spark plug out and spun the engine while holding an edge near the sprocket you could see if the crank is bent.

[Tillaway]

A little knowlege is dangerous so heres my 2 cents.

It is more than likely a cutting attachment problem.  Chain drivers, sprocket, bearing under clutch hub, (as Kevin stated) Bar tip damaged or something like that.

A bent crank will destroy the crank bearings, crank seal and eventually "wallow" out the case itself.  When the seal goes the saw will  sieze up taking out the ring piston and sometimes scoring the cylinder.  If you have been having problems with the saw siezing up then it may be a bent crank.  A crank bent enough to be noticed in how the chain rides on the bar would have siezed up long before this was noticable I would think.

Basically, I really doubt you have a bent crank.

[IndyIan]

Thanks for the help guys,
Kevin, I didn't hold onto the chain with the saw running!  With the saw off I gently pulled it over while pulling on the chain like when you check tension.  The chain doesn't rotate but I could feel it get definitely tighter and looser as the engine turns over...   I think this test removes all the drive line variables(bar tip, chain stretch, oval rim sprocket) except for the clutch bearing since the chain doesn't move.

I did the same test last night with my Dad's walmart saw and the chain didn't change tension at all.  

I haven't taken off the clutch yet to check the bearing but I guess it could cause the varying tension in both tests if it got warped.  I do grease it every 8 or so hours of use.  

Also I bought a new plug and did a few cuts to see how the mixture is doing.  


This is with the high as rich as it would go.  The low seems fine, revs up quick and idles fine.  I guess with the plug not in the centre of the cylinder the side hear the wall burns less efficently?  
It's never seized but I do think this is too lean, part of the plug is snow white, so it could be a clogged fuel filter or as Tillway mentioned the crank seal...  The old plug has alot of carbon on it from the breakin period and when I was first learning to use the saw so I think something has changed very recently.  I checked the air filter for holes and found none so I don't think the carb is plugged.  The saw has about 40 hours on it.  

Maybe I should take it to the shop as it's still on warranty?  I think I'm beyond my mechanical skills anyways.
Thanks again,
Ian

[TJACK]

The high reving could cause the crank bearing to go out. This is not that uncommom if you are cutting smaller wood or  doing a lot of limbing.  Call the technical service manager at Husky if you need information.  

TJACK

[Mark M]

How about the chain brake? Could it be causing the problem?

If the crank is bent they can frequently be straightened.

[IndyIan]

TJACK,
I was doing a lot of limbing but to be honest I don't use full throttle that often, only when the saw will be cutting for more than a second or in a 2" or up branch.  

Mark M,
I think the chain brake is fine, it works and the clutch spins freely when its not engaged.  I had the outside of the clutch off last night and the bearing seems good.  I took out the plug and had my girlfriend pull the saw over and put a block of wood just under the end of the crank and I'm 99% sure its bent, the saw moves a bit on the antivibe mounts.

So, I'm thinking I should bit bite the bullet and take it in and see what the mechanic thinks.  How long approximately should it take for them to tear down a saw and check the crank?

Thanks,
Ian

[Mark M]

Hi Indy

I wouldn't think it would take more than a couple hours or so to take it apart. Hope they can straighten it for you.

Good Luck

Mark

[Minnesota_boy]

A decent shop would put a dial indicator on the end of the crank and tell you immediately if it was bent.  Might take them a week to tear it down and fix it though. >:(

[J Beyer]

What does it cost to replace the crankshaft on average for parts and labor?  kind of curious if it ever happens to the 066.

JB

[Tillaway]

I have seen this done quite a bit, even helped from time to time.  The cranks on the older huskies were pretty stout and believe it or not you could have done the entire thing yourself on the older saws.  They thinned the cases of the newer saws to save weight so now you need special tools the pop the case apart.  A good tech can do a complete rebuild in a couple hours or so providing they don't have to deal with stripped out threads and snapped allen screws.  Tear downs take about 1/2 hour... maybe.  There is really not much to them.  

The first thing is to determine if you really need to pull the crank and as I stated before I really don't think that is the problem but you never know.  An easy way (trick) to determine if the crank seals are good is to fire up the saw without the bar, chain and cover off.  Spray some starting fluid or carb cleaner near the seals not anywhere near the carb or air intake.  You will know if the seal is bad. ;)

J Beyer
If you need to replace the crank its time for a new saw.  The cases usually go bad before the crank.  Crank bearing though, do go bad and are a normally inspected and usually replaced as part of a rebuild.  Stihl parts are usually a little cheaper than huskie parts.  Huskies are thought to be a little easier to work on so it saves on labor there.

[Dennis]

A bottom end rebuild is not as easy as some make it seem.  With all the correct tools and know-how it will take about an hour to tear down.  The rebuilding takes longer...and requires more special tools.  I have rebuilt many Stihls and Huskies.

as far as cranks going bad...it is quite common on the 372 for the big end (rod) bearing to go. And once again...I have replaced many.  Rarely does a case go bad unless a tree drops on it.  Side bearings are always replaced with a bottom end rebuild and are included every time you buy a crankshaft.

I can do a Husky rebuiltd for about 100 bucks less than a Stihl rebuild any day of the week.

Seals are tricky to test, it is next to impossible to spray them with anything while the saw is running with the flywheel, oiler and clutch in the way.  The best test is a pressure/vacuum test.

I can honestly say that I have never seen a bent crankshaft...I have seen broken ones..and "turned" cranks...the only way I can see a crank bending  is manufacture defect or if something very large runs the saw over.

The dial indicator, as Minn. said....will tell you in a heartbeat.  
but that wont tell you if the bearing is gone out of the saw..and that is causing the movement.

[Dennis]

With all the evidence...the white plug...the high revving...the crank movement...I would bet on the side bearing and/or seal.  Probably flywheel side, and definitely warranty work.

[IndyIan]

Thanks for help everyone,
The saw is off to the shop Monday.  I let everyone know what the verdict is when I find out.  
Thanks again,
Ian

[isawlogs]

Have you tried another chain? I've pinched a few times and the links on the chain itself was causing for the stiffness changed the chain and kept going ...

[IndyIan]

Well, I got my saw back.  The carb was out of adjustment, they think what happened was that I didn't have enough hours on it when I got it set up after the break in period.  Also I think I had some not so fresh mix in the saw when they set it up the first time, they probably had to lean it out just to get it to rev.  Not real bright on my part.  Now the saw doesn't rev so high and 4 strokes nicely. 8)

On the crank issue I didn't insist on them checking the run out, the service guy just wrote on the work order to do a check over.  So they did a leak down test and found no problems and the bearings are tight so I'm going to just run it.
Today if I have a minute I'll try and do some rough calculations how much the crank is out based on the difference in chain droop.  That will be a good test of my grade 12 trigonometry! ;D