Bearing Istallation

[Terry Keeley]

Terry, do you have a dimensioned drawing for a cmb 67 case? I like that fixture it would make the job easier.

I could go buy a new case but to me this kind of stuff makes the hobby more interesting and fun.

Brent

I got 44mm (1.732") between the vertical flats under the lugs, I'd make the case fit tight in the fixture there to help align it lengthwise to the spindle.

The holes are 52mm wide (2.047") x 31mm long (1.220"), check these to be sure, it's from an old drawing I dug up.

You have a bore gage? Gonna make up some plug gages?

 

[Will slagel]

B

 

[Terry Keeley]

Terry, is that a pcd tipped boring bar? PCD cuts aluminum like butter.

It's just a WCOW carbide insert with a TiAIN coating, I got that little boring bar a few years ago and it's been great, C06-24W405 on pg. 53 if interested.

The case bushings actually have to be done from the back so the 4" long solid carbide bar really helps.

I did get a Sumitomo PCD insert (holy crap $!) recommended to me by Neil Lickford, I'll use to make some RSA 431 pistons this off season.

 

[Daniel's Racing]

CMB's bearing to case fit on most sizes is between about 5-7 tenths, the tight side of M5-M6:

https://www.skf.com/ca/en/products/...ring-interfaces/tolerances-and-resultant-fits

If you were somehow able to press in a bearing with a 2.5 thou interference fit it wouldn't even turn as the high end of C3 internal clearance for that size is 1.1 thou.

How are you going to do the job? How are you going to measure the bore?

I was thinking that would be really tight but it actually came from SKF specs on a random 16100 2Z 28mm bearing for interference press fit in a housing. Maybe it was a rubber housing. Lol.

Didn't compare to a chart for that size before posting. Probably done by Artificial Intelligence.

Have you noticed that about everything coming from Artificial Intelligence you read that is technical has so many mistakes and often exact opposite of what should be most times. Completely worthless reading 1/2 the stuff on the internet as of late.

 

[Will slagel]

Ops

 

[Daniel's Racing]

Will,
Thanks man. No worries I understand it was just a mistake. I will edit it now so as to be more clear.

 

[tnrcboatracer]

It's just a WCOW carbide insert with a TiAIN coating, I got that little boring bar a few years ago and it's been great, C06-24W405 on pg. 53 if interested.

Thanks for the info Terry. Very interesting tooling. I like the micro inserts with coolant thru.

IIRC, the 10mm 4 flute PCD reamers we used at work were north of $1k ea. But we could resharpen every 10k holes up to 10x. So 100k parts/reamer. This was a dedicated transfer lime spitting out a completely machined v6 cyl head every 36 sec.

 

[Tom Foley]

Thanks Al, to add to the info here's how I do it but I now also put the case in the oven at 250F. You can use a torch but you havta be careful how much heat you use and the most important thing is to let the case cool naturally.



edit: crank sizes (from CMB at least) have gotten better so no need to loctite the rear bearing anymore.

Question for you , do you think that our bearings inner races were spinning or rather skidding on the cranks causing the loose fit ? I have noticed on my VAC 91 cranks in particular that even after installing new bearings you can turn the crank and not see the bearing turn . If you put a little axial load on it with your finger as it turns then it will turn . I always think of the rotation of the engine under load keeping the bearing turning and not skidding . Just a thought .

 

[Al Hobbs]

Question for you , do you think that our bearings inner races were spinning or rather skidding on the cranks causing the loose fit ? I have noticed on my VAC 91 cranks in particular that even after installing new bearings you can turn the crank and not see the bearing turn . If you put a little axial load on it with your finger as it turns then it will turn . I always think of the rotation of the engine under load keeping the bearing turning and not skidding . Just a thought .

Jim Allen liked the inner race to be a tight fit on the crank.

 

[longballlumber]

Jim Allen liked the inner race to be a tight fit on the crank.

Nice to see you back Al!

 

[Terry Keeley]

Question for you , do you think that our bearings inner races were spinning or rather skidding on the cranks causing the loose fit ? I have noticed on my VAC 91 cranks in particular that even after installing new bearings you can turn the crank and not see the bearing turn . If you put a little axial load on it with your finger as it turns then it will turn . I always think of the rotation of the engine under load keeping the bearing turning and not skidding . Just a thought .

CMB has tightened up their tolerances on the crank journals recently, they used to be about 1/2 thou sloppy (our VAC 91's), they're now about 1 tenth under size (perfect), instead of the bearings dropping on you now have to push them on with a "light press fit". The loose fit wasn't a problem for the front bearing as the inner race is clamped between the crank step and the flywheel collet but the big bearing's inner race could spin on the crank and overheat, expand and cause bearing failure. I've had inner races turn blue and if not caught in time it Brinells or the balls start to chip. Ceramic balls help but recently I've discovered another issue.

When your running in very cold temps I don't think the bearings are loose enough for the "new" fits CMB are giving us. I've recently been running my SAW boat at temps in the low 30's and had a bearing Brinell. Even thou I fit them to the high side of C3 (4-10 tenths for 10-18mm bore) the cold would squeeze the case down and there wouldn't be enough room. For fun I put a case in the freezer overnight and the rear bearing pocket went from 0.0006" to 0.0018" undersize, it shrank over a thou! I remember Jim Allen was a big fan of C4, I don't know what his press fits were but I can see why. The rear bearings on our little motors are kinda unique in that both races havta be secured, guys have tried other ways over the years (clamping nuts, grinding the inner race right into the crank etc.) but for us I think just extra clearance might be needed. I'm pretty sure David Spitzer ran into this also at the Cinci record trial, it was low 30's each morning and he smoked a good set of bearings in his CMB 80.

Is this what's happening with the new K21 as well? Either the bearings don't have enough internal clearance for the case/crank fits, or the crank is too short between the journals compared to the case?

ps: I'm thinking about having the cranks on my VAC91's flash chromed, a couple tenths per side would be just about right...

 

[John Beardslee]

CMB has tightened up their tolerances on the crank journals recently, they used to be about 1/2 thou sloppy (our VAC 91's), they're now about 1 tenth under size (perfect), instead of the bearings dropping on you now have to push them on with a "light press fit". The loose fit wasn't a problem for the front bearing as the inner race is clamped between the crank step and the flywheel collet but the big bearing's inner race could spin on the crank and overheat, expand and cause bearing failure. I've had inner races turn blue and if not caught in time it Brinells or the balls start to chip. Ceramic balls help but recently I've discovered another issue.

When your running in very cold temps I don't think the bearings are loose enough for the "new" fits CMB are giving us. I've recently been running my SAW boat at temps in the low 30's and had a bearing Brinell. Even thou I fit them to the high side of C3 (4-10 tenths for 10-18mm bore) the cold would squeeze the case down and there wouldn't be enough room. For fun I put a case in the freezer overnight and the rear bearing pocket went from 0.0006" to 0.0018" undersize, it shrank over a thou! I remember Jim Allen was a big fan of C4, I don't know what his press fits were but I can see why. The rear bearings on our little motors are kinda unique in that both races havta be secured, guys have tried other ways over the years (clamping nuts, grinding the inner race right into the crank etc.) but for us I think just extra clearance might be needed. I'm pretty sure David Spitzer ran into this also at the Cinci record trial, it was low 30's each morning and he smoked a good set of bearings in his CMB 80.

Is this what's happening with the new K21 as well? Either the bearings don't have enough internal clearance for the case/crank fits, or the crank is too short between the journals compared to the case?

ps: I'm thinking about having the cranks on my VAC91's flash chromed, a couple tenths per side would be just about right...

View attachment 332588

Or, you can also use a light line of green loctite or bearing mount loctite on the crank journals when assemble. I use a tooth pick and put a very small line across the journal. This used to be a common practice on many bearings in my rc heli’s.

 

[RaceMechaniX]

I believe Jim also favored steel bearing carriers for his motors to match the thermal expansion of the case with the bearing more closely. You can run tighter tolerance bearings in cases that have a similar CTE.

 

[Terry Keeley]

Or, you can also use a light line of green loctite or bearing mount loctite on the crank journals when assemble. I use a tooth pick and put a very small line across the journal. This used to be a common practice on many bearings in my rc heli’s.

I tried that for a while but in the end it never worked for me...

 

[John Beardslee]

I tried that for a while but in the end it never worked for me...

What were the symptoms? It still spun?
A flash of hard chrome should do it, isvery controllable. Depending on how much you need to apply will be the determining factor. It’s the edges that could be the issue. It can build up extra in that area.
All my students tools that they built in class were all flashed with 50 millionths of hard chrome. A great rust prevention plus the fact that Modern Hard Chrome was close by and the work was done for free was a nice bonus for my program.

 

[Terry Keeley]

What were the symptoms? It still spun?
A flash of hard chrome should do it, isvery controllable. Depending on how much you need to apply will be the determining factor. It’s the edges that could be the issue. It can build up extra in that area.
All my students tools that they built in class were all flashed with 50 millionths of hard chrome. A great rust prevention plus the fact that Modern Hard Chrome was close by and the work was done for free was a nice bonus for my program.

Yup, still spun. It would only take 1-2 tenths per side to make it right, hell I might just end up doing it at home...

 

[John Beardslee]

Yup, still spun. It would only take 1-2 tenths per side to make it right, hell I might just end up doing it at home...

I had a large lead tank set up for years at home when I was doing a lot of gun parts. I got rid of that and went to a glass battery jar, powered with a battery charger. All cheap but I have to believe the hard chrome plating solution is pretty pricey nowadays. You can also buff hard chrome to look bright just like decorative chrome.

 

[Terry Keeley]

I had a large lead tank set up for years at home when I was doing a lot of gun parts. I got rid of that and went to a glass battery jar, powered with a battery charger. All cheap but I have to believe the hard chrome plating solution is pretty pricey nowadays. You can also buff hard chrome to look bright just like decorative chrome.

I still have the chemicals (chromium trioxide flakes, sulfuric acid), and a power supply, it wouldn't take much to make an anode from a lead pipe and set up a tank with a small Pyrex jar.

I would probably put on too much then polish it down the the right size with some 3M diamond strips I have.

Hmmm....