Crank bearings 250GS -81

[dirtbike]

Hi all!
I'm trashing the M25 crank bearings and go for the plain 6305 ball bearings. A friend of mine that races a 250 -81 says that they hold up just as good if not better. The only problem is that one has to heat it up during both mounting the halfes as well as splitting it!

Does anybody have good opinions in this?
See the project here:
http://www.mxbikes.com/forum/view.php?site=mxbikes&bn=mxbikes_ktm1&key=1067983004&first=1067983004&last=1065276392[?]

[john durrill]

Dirtbike,
 the stock 3 piece bearings will work better, last longer and give you more HP.
  They are called self aligning bearings. if the 6305 would work as well the Factory would have used them for sure. the 3 piece bearings are a lot more expensive and would not be used if not really needed.
 It will be very hard to get the 6305's off if you need to reshim the crank. dont know how you would do that with out ruining the bearings.
  Honest, you really need the orignal 3 piece bearings  to have the motor work was well as it did new.
John D.

[dirtbike]

The factory do use them and so does most marquees nowadays! All newer KTM's have plain ballbearings(or ball/rollar combi). The M30's on the 350-495 seem to last quite long, but the M25?!?!. Well, let's put it this way. I haven't dismantled an engine yet finding them in proper working order. A friend of mine that races a 250MX -81 in vintage racing built 5 engines last year. The M25 was junk in all and he is bone hard in his view, he trashes the shoulder bearing in favour of the ballbearing even if it's good, as he did with his 420 recently.

I haven't compared as of yet but I'm a bit dissapointed of the M25's appearance in the last two engines I splitted recently.

[john durrill]

I guess i should have given a time frame and which factory we were talking about . chuckle chuckle.
 Both Sachs and KTM in the 72 to 79 100 to 400 motors used the 3 piece bearings. it may have to do with case tolerances  or the ability  not to be able manufacture to a certain tollerance over and over again. Ktm and Sachs would have oppted for the one piece bearings in a heart beat to save $ on every motor made if the one piece would have done the job. it would be very difficult to shim the main bearings for proper side clearance on the crank with a one piece.
 If everything does not line up exactly right the one piece will bind and drag.
 if the inner race of the one piece is an interferance fit how would one measure side clearance? nothing will give. the outer race is fit tight to the engine cases and the inner is fit tight tight the crank.
  that would means no end play. if every thing is not perfect in the alignment , crank and cases, then you have a bind in the bearing.
 I KNOW I have not learned everything about engine building yet.
 * GRIN *
I believe the day i quit learning i will have passed on.
 so help me and the other folks out here. $10 bearing as opposed to a 55 0r $65 bearing is a no brainer if it will work just as well.
 John D.
 
 
 
 
 

[Larry Perkins]

This is is one of those debates that comes up again and again here and generally goes on and on each time.  Here is the bottom line without 20 experts with a gillion years experience having to say it for the umpteenth time:

The self aligning bearings when properly installed and shimmed are the way to go.  The normal style bearing WILL NOT WORK LONG TERM WITHOUT CAUSING GRIEF TO THE CRANK OR The CASE OR BOTH.

The regular ones DO NOT FUNCTION AS WELL, LAST AS LONG, NOR WAS THE ENGINE DESIGNED FOR THEM.

There is one reason and one reason alone that people try to make them work and find reasons they should work.  THEY ARE CHEAPER!  So were Yamahas so if you want that type of bearing a Japanese bike might be in order.

Now you can listen to your buddy or the advice of long term experience.  Good Luck to you either way.

Hope this is not too harsh-JUST THE TRUTH.

[firstturn]

Larry does know what is is talking about and stated it well.  I have a engine that has a single bearing arrangement and is for show and slow ridding only no RACING.  That was the way the engine came to me.

Ron Carbaugh

[dirtbike]

No Larry, it's not to harsh but does indeed requires you to explain how they did fail, after how long and the appearant reason for doing so!

It's hard to apologize for seeing something that suggests the opposite! I honestly don't know but I do have my thoughts which is
as follows:

The narrow axial play tolerance it is said to be a problem entirelly caused by the shoulder bearing itself. You need to shim it so the shoulders on each side keeps the balls firmly within the 0.05mm limit. The problem as I see it is that one should lock the ballbearing with a plate. There is no plate of course and that could be a cause of concern. I believe that the KTM factory used the shoulder bearings as an inheritage from the sachs era. Well, there we don't have straight cut gearwheels on the primary drive. Therefore a pretty high axial load. There is no such thing on my 250 and the 250's from the early 90's doesn't have shoulder bearings either.

I rarely find an 20 yrs design so good that it cannot be improved! The magnesium case is crap for example. The engine mounts isn't up to the job and so on... There is this support ballbearing too. Why doesn´t these problems wreck that one instantly?
The M25's i have found is old stock and manufactured in machines that cannot be compared to newer computer controlled machines with far better tolerances. The bearings themselfes are made out of metal produced in newer plants with a complete different control over the entire production line all the way from the mine, I think.

[john durrill]

Dirtbike,
       the reason for shimming the bearings is to allow for differences in engine cases , rod kits, and crank width, when the crank is pressed together. not the bearing design. The bearings used are not inferior. get new run m25's if you are worried about that.
       The extra bearing on the drive side of the crank is not an interferance fit. But a  looser fit than the inner races of the m25's or 30's or whatever depening on which engine you have. so the crank will move and align itself the right distance as soon as the engine is turned over. i dont shim the mains with that bearing in the cases but install it after shimming is done. If you dont shim the crank right
 ( Ie , when the shims dont work out even so you have the same amout on both sides of the crank ) you run the very real risk of haveing the mag flywheel contact the backing plate/ stator with the engine running. more than one stator has been damaged that way. always add the extra shim on the clutch , drive side of the crank.
 running a higher oil to fuel mix would help if the m25's are failing at an abnormal rate. or they could be failing due to hooking  down shifts to often or constant over reving. thats given the air intake system is clean and free from leaks to the outside.
 spectro or maxmuim at 32 to 1 would be my choice if i were MX ing or riding CC. that mix give a significant increase to engine life at high rpm's
       Magniezium is an expensive weight saver. it will fatigue with age and fail sooner than aluminum. i perfer aluminum because it last longer in years, but it weighs more.
 on a new set of mag cases if the mounts are kept torqued they last a very long time. Magnezium dosent like water either ,another reason i like aluminum better. but its still a trade off. do we want it as light as possible or can we do with out and handle a bit more weight.
       the reason Sachs didnt use straight cuts is cost. they built one gear set that worked on all their 100 - 125 cc engines from about 67 to 75. straight cuts are noiser and  not necessary for a lot of the bikes the engines were bought for. that design won a lot of MX and medals in the Six-Days over many years
     everything we get from any manufactuer is a trade of with cost and what the customer will settle for.
    i can tell you one of the ways the one piece failed . it spins the bearing in the case or on the crank journal. both are very expensive to repair. with a small and finite number of cranks and engine cases availble for replacement it gets even harder. If this happened 1 out 5 times  its not worth the risk to me. the reason for this is one post back chuckle chuckle.
   a KTM or Sachs engine built right will last a long time in competion. they did when i sold them in the 70's and will last every bit as long today with the right replacement parts.
   John D.
     
 
     
     
 
     
     

[tofriedel]

It is interesting to read the pros & cons of the 3 piece main bearing VS the 1 piece.  All of this is or will be moot shortly as the 3 piece bearing is no longer manufactured.  All you can find today is old stock.

Also interesting is that other Euro bike manufacturere did not use the 3 piece bearing, Maico & Husky come to mind.

But, the pros & cons will not matter in the near future.  The 3 piece does make splitting the cases and reassembly an easy task.  

Tony

[sachsmx]

M25 bearings are still available from many bearing mfgrs. I have at least 2 current catalogs that list them. The M30s are not easy to find, however. Its not so important as to which bearing is better, but which bearing the engine was designed to use. Speaking as an automotive engineer, the intent of the engineers who originally designed the engine was to use the 3 piece bearing, and all of the components are built in such a way as to make it very difficult to properly use another type of bearing.
If I were to use a conventional ball bearing I would have the crank journal ground down ever so slightly so that you have a slip fit between the crank and the bearing ID at room temperature.

[dirtbike]

Here is the continuation of the story.
http://www.mxbikes.com/forum/view.php?site=mxbikes&bn=mxbikes_ktm1&key=1068465460&first=1068488855&last=1067267636

[Dr.2Stroke]

You want know my experience,as i can say im a real expert on aircooled two stroke engines? The best bearings which were constructed in these kind of engines were the shoulder bearings!!!!!
 i sold a 125 from 83 which had new bearings in it(the L 25,which are smaller than the M 25),and this engine holded over 20 000km on the road.
then im driving a 250 from 81 on the road,and always full throttle,and no play on crank.
 and all the 350 420 and 495 engines i have taken apart: never seen a real damaged shoulder bearing in it!!!!!
do you really want to know why they gonna break? because of the guy which has installed it,did not measure the axial play and did not adjust it as he should have!if the mechanist is not capable to mountr these bearings,the damage is programmed. it is a very difficult thing to install and balance these bearings!i have always 2  hours just to measure it out in one engine!
second reason is wetness. they really dont like only one drop of water  or invisible water vapour.if a engine standed over years in a barn,it has collected some fine vapour in the engine,and so the bearing has very little corrosion in it,which is absolutly TOXIC for the bearing!
and the third reason is: use high quality 2 stroke oil!!!
i use valvoline two stroke competition oil SAE 50,which i guess is coming from pennsylvania,and this oil i just magic for your engine!
just the notes of one who has tested everything in matters KTM engines.......

[dirtbike]

It might well be so that the M25 is the best bearing. On the other hand, one is entitled to think that a mechanic devise that sensitive to mounting, moisture or whatever can't be a particular sound design.
Unsound designs have a habit of dissapearing over time.

I'm really not into declaring the M shoulder bearings as bad in any way. What I'm into is weather the 6305 works. I know a few more today then yesterday using them and can put it this way. It obviously works just fine. There are risks as with the shoulder bearings but I haven't heard of one bearing coming loose or failing. Only a whole lot of what could happen. To draw my attention would require a real world situation containing: to whom, when, after how long, what happened.

Dr.2Stroke.. Ask our friend Tommy about this!!

[LynnCamp]

Went to look at your crank bearings on mxbikes.com.  Can I add a woman's and an antique collector's advice and perspective to all of the great advice you are getting?

I would shoot you if you put that stuff on my pine (which is very soft) dining table!!!!!!:(

[BrianTaylor]

Sachsmx I would like to know what manufactures  you show still making these bearings because I checked with  FAG Bearings and SKF Bearings today and these "Magneto Type" bearings have been obsolete for over 15 years. Yes there are probably surplus houses that may have some but as as factory fresh inventory I dont think they are there.The "magneto type was used mostly in the electric motor industry but they have all gone to the "Conrad Type " years ago  thus no requests for the production of this type and not enough after market usage to produce.My $.02 CDN worth
Brian Taylor Canadian Bearings Windsor

quote:Originally posted by sachsmx

M25 bearings are still available from many bearing mfgrs. I have at least 2 current catalogs that list them. The M30s are not easy to find, however. Its not so important as to which bearing is better, but which bearing the engine was designed to use. Speaking as an automotive engineer, the intent of the engineers who originally designed the engine was to use the 3 piece bearing, and all of the components are built in such a way as to make it very difficult to properly use another type of bearing.
If I were to use a conventional ball bearing I would have the crank journal ground down ever so slightly so that you have a slip fit between the crank and the bearing ID at room temperature.

Brian Taylor