M25 crank shaft bearings

[Keith Meatyard]

All bearings of the same size but different manufacturer will be the same size dimensionally, they all meet the same tolerances.  If the bearings are loose (not interference when the case has cooled down), then the bore in the case is worn and will affect reliability.  In the ball bearing world, one of the fits, either the outer race or the inner race is a slip fit while the other is an interference fit.  In the case of these motorcycles, the shafts have the slip fit in the inner race to allow assembly.  The outer race can be inserted into the case bore when the case is heated because of the greater thermal expansion coefficient of the magnesium verses the steel race.

Keith Meatyard

[Andreas Piepke]

Any experience with products like Loctite 640 retaining compound? These are advertised to address problems like this.

Andreas

[Wesley Plunkett]

Andreas,

It appears from your photos you have a decent set of cases. So how loose are the bearings in the cases? Were the old bearings bad, did they spin in the bore and hog it out? Is there alot of clearance, or only a few thousandths? You may be able to loctite them in and be fine. Loctite 271 high strength (red) should work if it's not too big a gap. I have not used Loctite 640 but in my experience their products work as advertised.

This is of course your call as a mechanic.

I have used SKF ball bearings from a local industrial supply, they are  good bearings but as Kieth said would not vary in size from FAG or any other brand.    

 

[Andreas Piepke]

The original bearings were loose too. The engine was not in running condition when I got it so I only know what I can see now.
There is a slip fit between case and outer bearing race, the bearings will simply fall out when the case is turned over. I don't want to operate the engine like this because this can only become worse if they start spinning in the case. The gap is indeed very small, I can't even get the 0.1 mm feeler gauge in between case and bearing. I Loctite is probably the way to go. The thing that worries me: will I be able to get these bearings out if I ever need to? That's why I am asking whether there is some experience out there on how to fix this problem. This sure must be a common issue with old engines.

Thanks for your input!

Andreas

[Daniel P. McEntee]

My experience with materials like Loctite is that it can be heated with a torch to a point it will release when needed. Carefully heat the bosses to a point where things start smoking and the Loctite will crystallize and release the part. On machine parts with a loose bearing fit, I sometimes lightly dimple the seating surfaces with a center punch. This raises some metal and gives something for the outer race to push against, and also use Loctite to secure the bearing. Do a thorough job on the rebuild and unless you really thrash the engine by racing it hard, you may never have to worry about getting back in there again.
  Good luck and have fun,
  Dan McEntee

[Andreas Piepke]

I used the center punch method to secure the transmission bearings:

http://s1341.photobucket.com/user/andreas_g_piepke/media/Penton/100_3550_zpsd963d5cc.jpg.html?sort=3&o=1

http://s1341.photobucket.com/user/andreas_g_piepke/media/Penton/100_3551_zps5ef1da3f.jpg.html?sort=3&o=0

The bearings are tightly seated now. Let's hope they stay that way once the case warms up.

Thanks again for the input!

Andreas

[pastmxa]

6/13/2018

Hi, guys, rebuilding my 74 1/2 250 and I called Barlett in AL.  They found plenty of these bearings at a supplier of theirs called Consolidated Bearings.  The brand they are sending are FAG and have a plastic cage.  Just an update FYI in case anyone is looking for these bearings.

Rgds

[skiracer]

Where are the bearings made?

quote:Originally posted by pastmxa

6/13/2018

Hi, guys, rebuilding my 74 1/2 250 and I called Barlett in AL.  They found plenty of these bearings at a supplier of theirs called Consolidated Bearings.  The brand they are sending are FAG and have a plastic cage.  Just an update FYI in case anyone is looking for these bearings.

Rgds,

Richard

1976 MC 5 Original Owner
1976 Penton 175 XC

[pastmxa]

received the bearings, they look ok, FAG brand

[Andreas Piepke]

I am avoiding bearings with plastic cage in two stroke engines. I am worried that the contact with the gasoline weakens the plastic over time. For the rebuild I did I got FAG bearings with brass cage.

Cheers

Andreas

'73 Penton Hare Scrambler
'73 Norton Commando
'77 Maico 440 GS

[JP Morgen]

I didn't read all the postings, but one could switch to a two piece roller bearing on the mag side and use a standard ball bearing on the drive side, engine would still come apart easily and you wouldn't have to mess with shims!! Send checks to....lol!

[skiracer]

smart....

quote:Originally posted by JP Morgen

I didn't read all the postings, but one could switch to a two piece roller bearing on the mag side and use a standard ball bearing on the drive side, engine would still come apart easily and you wouldn't have to mess with shims!! Send checks to....lol!

1976 MC 5 Original Owner
1976 Penton 175 XC

[Andreas Piepke]

You have this combination of roller and ball bearings in Maico engines. This allows for the different thermal expansion of the engine case and crank shaft without the need for shimming. I just rebuilt a 1977 Maico. However, these roller bearings are hard to find and pricey too.

Andreas

'73 Penton Hare Scrambler
'73 Norton Commando
'77 Maico 440 GS

[JP Morgen]

https://www.ebay.com/itm/SNR-30305-25-X-62-X-17-TAPER-ROLLER-BEARING-30305-SNR-30-305/113149662474?hash=item1a583ed50a%3Ag%3AEBYAAOSwQJxa0lj7&_sacat=0&_nkw=25+x+62+roller+bearing&_from=R40&rt=nc&_trksid=m570.l1313