Main Bearing Oil Clearance with new bearings


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Hamza
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Main Bearing Oil Clearance with new bearings

Post by Hamza »

Hi guys,

So i think I might have jumped the gun a bit...Just before I put the crank cases together, I realized I did not check the main bearing oil clearances. Upon initial engine break down, I had measured the oil clearances and found them to be 0.076 mm (Plastigauge)..I decided to replace them and used the tables to get new bearings. My selection was as follows:
Bearing Selection.jpeg

The problem I feel is that some bearings seem to be at 0.076 mm still...or just slightly under save 2 which are about 0.051 mm..The manual says that factory spec was 0.02 mm to 0.06 mm. Also as a short cut I torqued on the main crank shaft bolts......not the others....

Also as can be seen the plastigauge was not really in line axially with the journals..maybe as I was fitting the top half, the movement caused them to move out of alignment...can this cause for an inaccurate reading...

I wanted to ask the gurus what to do now? Shall I continue assembly of the engine?

btw, the con rod oil clearances came in sweet...they all came in about 0.038 mm..

Photos below...



Journal # 7 (Left most being 1)
P_20211108_183502.jpg
Journal # 6
P_20211108_183523.jpg
Journal # 5
P_20211108_183555.jpg
Journal # 4
P_20211108_183623.jpg
Journal # 3
P_20211108_183626.jpg
Journal # 2
P_20211108_183639.jpg
Journal # 1

The Plastigauge fell out during case installation!!

P_20211108_180906.jpg
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Last edited by Hamza on Mon Nov 08, 2021 11:27 am, edited 1 time in total.

Hamza
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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

Also wanted to mention that I the old bearings that came out of the engine do NOT have the oil recess path...the new ones I got from David Silver Spares UK have the oil recess....are these even the right ones for the CBX engine? When I got them, I thought maybe the new design superseded the original on the part number..

Original Bearing Type:
Original Bearing.jpg
New Bearing currently installed:
New Bearing.jpg
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bobcat
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Re: Main Bearing Oil Clearance with new bearings

Post by bobcat »

Hi Hamza !

The bearing shown in the top picture is a Rod bearing.
The bottom picture is a Main bearing.
On all the motors I've built, race or street I always aim for .038mm (.0015")
for rod or main bearing clearances uniformly. Consider that the clearances loosen
slightly on start up as the finish of the bearing is "burnished" by the crank. (new bearings).
I always have the journals polished before fitting new bearings.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

Thanks Bob,

Yeah thats the spec I was aiming and hoping for to be honest...so I read on the forum last night that one of the pair of bearings can be changed to a bigger size to reduce the oil clearance. So for example, if my crank et al has been bolted down with fresh bearings in the upper case, I can get a slightly thicker bearing for the lower case and close down the oil clearance. If this is true then I can save the effort of dismantling everything and save some money and get bearings for the lower case only...

Is that something that can be recommended and work at all?

Also does it matter that I inserted only the crankshaft bolts and torqued those down rather than the entire case?
Last edited by Hamza on Mon Nov 08, 2021 10:37 pm, edited 1 time in total.

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

bobcat wrote:
Mon Nov 08, 2021 1:01 pm
Hi Hamza !

The bearing shown in the top picture is a Rod bearing.
The bottom picture is a Main bearing.
On all the motors I've built, race or street I always aim for .038mm (.0015")
for rod or main bearing clearances uniformly. Consider that the clearances loosen
slightly on start up as the finish of the bearing is "burnished" by the crank. (new bearings).
I always have the journals polished before fitting new bearings.
Also it would seem then that someone over the course of the bikes history had put in conrod bearings into the main bearings!!!!!!!!!!!!!!!!!!!

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

This is the post I was referring to:

viewtopic.php?f=25&t=1399

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Re: Main Bearing Oil Clearance with new bearings

Post by shiskowd »

Hamza - my suggestion would be to have your crank journals polished per Bob's suggestion and then measure the journal diameters with a good micrometer and/or measure the oil clearance again with plastigauge. Page 12-12 on the '79 service manual shows the corresponding bearings for a measured journal diameter. You likely need to go up a bearing size or two (A-Black or AA-Blue) if the crank has worn given some of its service life had the wrong bearings installed. Each bearing size up closes the clearance ~ 0.008mm so you'll need both bearing shells to close the clearance to spec.

I'm surprised your con rod bearings measured within spec. The reason the crank bearings have that groove in the middle is to pass pressurized oil from the crank journal through the crankshaft oil passage to the rod journal. Without that groove (using the rod bearing shell) the amount of oil would have been limited to the thin film between the crank the the bearing. Surprised the engine ran for any length of time that way.

The manual suggests installing/torqueing all the case bolts for the plastigauge measurements. I've always done it that way but not sure that will make a difference over just the crank bearing bolts. As for the plastigauge position on the crank journal, best if it can lay across the entire width of the journal to see if there is uneven wear but from the pictures you should have a good reading.
2010 KTM 690 Enduro Rally, 1979 CBX 'A' & 'B', 1975 Moto Guzzi 850T
Long Sleeping '79 CBX Restoration > viewtopic.php?f=102&t=11699

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

shiskowd wrote:
Mon Nov 08, 2021 11:24 pm
Hamza - my suggestion would be to have your crank journals polished per Bob's suggestion and then measure the journal diameters with a good micrometer and/or measure the oil clearance again with plastigauge. Page 12-12 on the '79 service manual shows the corresponding bearings for a measured journal diameter. You likely need to go up a bearing size or two (A-Black or AA-Blue) if the crank has worn given some of its service life had the wrong bearings installed. Each bearing size up closes the clearance ~ 0.008mm so you'll need both bearing shells to close the clearance to spec.

I'm surprised your con rod bearings measured within spec. The reason the crank bearings have that groove in the middle is to pass pressurized oil from the crank journal through the crankshaft oil passage to the rod journal. Without that groove (using the rod bearing shell) the amount of oil would have been limited to the thin film between the crank the the bearing. Surprised the engine ran for any length of time that way.

The manual suggests installing/torqueing all the case bolts for the plastigauge measurements. I've always done it that way but not sure that will make a difference over just the crank bearing bolts. As for the plastigauge position on the crank journal, best if it can lay across the entire width of the journal to see if there is uneven wear but from the pictures you should have a good reading.
Thanks for the guidance...I was wondering if I should torque it to the maximum limit...I torqued the bolts to 20 ft.lb instead of the 23 ft.lb....reckon that will help in getting the clearance down abit?

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

I tried searching for the AA bearings. could not find them..would you have the part number?

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Re: Main Bearing Oil Clearance with new bearings

Post by shiskowd »

I can't find the AA bearing in any parts or price list either. Before you measure the oil clearance again ensure the lands on the cases are perfectly clean/dry as well as the surfaces that the bearing shells rest in. Your crank may be worn to the point you won't be able to get the clearance to spec. As long as your under the service limit (0.08mm) and you're not racing the bike you should be OK IMO.
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Long Sleeping '79 CBX Restoration > viewtopic.php?f=102&t=11699

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Re: Main Bearing Oil Clearance with new bearings

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Last edited by EMS on Thu Nov 11, 2021 5:29 pm, edited 1 time in total.
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Re: Main Bearing Oil Clearance with new bearings

Post by bobcat »

Hamza,
I would never replace just half of a bearing with a different size to get a tighter clearance.
You might get a tighter clearance but but uneven actual oil clearance around the journal/pin.
Even with used bearings, replacing only half (1 shell) is not a good idea. I ALWAYS replace
bearings as a set using the same size (color code) top and bottom.
As far as torqueing the case bolts to measure clearances, I've tried it both ways, all bolts
or just the main bolts, and saw no difference. Tightening to just 20 ft/lbs ok too.

Others may disagree but ..just my 2 cents worth.
Bob
82 CBX, 81 CBX, CB1100F, 79/82 CB900F (avatar)

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

Hi guys,

Thanks for the advise to measure the crank!! I am so glad I did open it back up....I discovered a mistake I had made....

I had counted the Left side of the Inverted Top case as 1..so the case was inverted for assembly so the left was actually the right!!!!!! upside down...so what I thought was no. 1 journal was actually no. 7...this is not a mistake I made on the lower case as it was the right side up...

Journal 1 was supposed to get green bearings and journal 7 the brown ones.

Due to this mistake, I had 1 green and 1 brown in Journal 1 and Journal 7!!!!!

Glad I found this when I did!!!

So I measured the crank journals and took 2 readings both 90 degrees to each other with a calibrated and verified micrometer. The reading that says final pass, I manually fixed the micrometer to that adjustment and felt my way all around to come up with an equal feel across all journals. I have made a table for better decision making.
The boxes highlighted yellow are what are in there now. With everything out I will fix the Journal 1 and 7 and plastigauge and see where I land. Otherwise then I am assuming I will have to go with all black bearings..correct?

Thanks guys!!

Oil Clearances.JPG
So except Journals 1 and 7 which I had mucked up Journal 3,4 and 6 are similar to my plastigauge results....more to follow..........
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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

Might be realistic to factor in the largest journals i.e. 35.98 and then try and aim for a clearance and match the smaller journals to that..so in this case based on the readings above;

Journal 1, 4 and 7 can make do with brown bearings and Journals 2, 3, 5 and 6 can be black?

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Re: Main Bearing Oil Clearance with new bearings

Post by Hamza »

Hi guys,

So just an update..I got the crank back from polishing the journals..look super shiny now...I measured the journals and some of them are down by about 0.005 to 0.01 mm................

All black bearings in the crank case...and photos below..most are still 0.076 mm and one is above I think......

I dont know at this time how possible a crank replacement is sitting here in Pakistan....are there any other over sized bearings other than the stock OEM for these situations? Also I have been reading a bit on oil clearances, what if I use a 20w-60 or 20w-50...this bike is going to be a weekend cruiser and not a racer....would really love to hear some options...I also got in touch with the guy I bought this bike from and he reckons about 15 years ago he had the crank machined...he did not touch the con rod pins on the crank so no wonder they are fine....
P_20211125_123439.jpg
P_20211125_123512.jpg
P_20211125_123531.jpg
P_20211125_123544.jpg
P_20211125_123557.jpg
P_20211125_123621.jpg
I would really like some help here to figure out what my options are other than a replacement crankshaft.... :sad-roulette: :sad-roulette:
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