Author Topic: 750 main bearing clearance, a compendium of information. Re-run. (Read 14436 times)

Don R · « **on:** May 08, 2017, 01:15:14 PM »

My bearing saddles are all B and it appears the crank is BBBBB also as best as I can tell. I can't find color on any bearings but plastigauge says .001" across all main bearings. I don't measure any runout on the crank with my dial indicator but as the mains get to torque the motor tightens up. Is .001 enough clearance? I can find the max. clearance easily enough but not the minimum.

kmb69 · « **Reply #1 on:** May 08, 2017, 01:26:20 PM »

According to the Service Manual:
.0008-.0018 in
.02-.046 mm

Meant to attach a .jpg

MessnerMoto · « **Reply #2 on:** May 08, 2017, 02:13:55 PM »

When you add sealant between cases clearance will increase for ~0.01mm(depending on sealant) . One thing nobody pay attention when doing all the clearance measurements...

Captain · « **Reply #3 on:** May 08, 2017, 08:08:35 PM »

For any sort of life and particularly for racing .0018" is the figure. Smaller than this is less than desirable but also on the other hand, do not exceed. These engines are dry sumped and have very good oil delivery as the basic design is sound and the recommended .0018" oil clearance allows for greater heat transfer capability through increased oil flow.

Captain

scottly · « **Reply #4 on:** May 08, 2017, 08:42:55 PM »

1+ to what Captain says. Also, the main bores in the block may not be 100% true, which may be the cause of the tightening up when the case halves are torqued.

Don R · « **Reply #5 on:** May 08, 2017, 09:11:41 PM »

I agree, the mains may be off. This case has had the snot run out of it. It's in better shape than I remembered though, the clearance grinding went through a spot or two where the aluminum rods swing. It was welded up but probably gas welded from the appearance.
It plastigauged to .001 straight across and it was tight when sealed with Honda bond. I'll stop here and mic things to be sure what bearings to buy. Sure would have been nice to see any color, possibly a tint of yellow but not enough to be sure. I can torque to 120 inch lbs. and it rotates fine, any tighter and it binds. I'll get main studs also. 1124cc to the best of my measuring abilities.

Bill/BentON Racing · « **Reply #6 on:** May 09, 2017, 05:56:57 PM »

+1 on clearance and studs.

MRieck · « **Reply #7 on:** May 09, 2017, 07:26:38 PM »

There are lots of cases out there.

scottly · « **Reply #8 on:** May 09, 2017, 07:37:11 PM »

Don, do any of the bearing shells show more wear than the others? You might try assembling the case halves with only the outside and center shells installed and see if the crank turns freely. If it does, add the other shell pairs, one at a time until you find the tight journal.

MRieck · « **Reply #9 on:** May 09, 2017, 07:59:31 PM »

Quote from: scottly on May 09, 2017, 07:37:11 PM

Don, do any of the bearing shells show more wear than the others? You might try assembling the case halves with only the outside and center shells installed and see if the crank turns freely. If it does, add the other shell pairs, one at a time until you find the tight journal.

But even if he finds the tight saddles/or journal what can he do about it. Yellows?

scottly · « **Reply #10 on:** May 09, 2017, 08:17:27 PM »

Let's cross that bridge when we come to it, Mike.

Bill/BentON Racing · « **Reply #11 on:** May 09, 2017, 08:21:06 PM »

I have cases......

NobleHops · « **Reply #12 on:** May 10, 2017, 08:59:27 AM »

Quote from: scottly on May 09, 2017, 07:37:11 PM

Don, do any of the bearing shells show more wear than the others? You might try assembling the case halves with only the outside and center shells installed and see if the crank turns freely. If it does, add the other shell pairs, one at a time until you find the tight journal.

Fantastic tech

Don R · « **Reply #13 on:** May 10, 2017, 10:10:52 PM »

It turns freely with all mains torqued to 180 inch lbs except the two closest to the dyno only torqued to 120. When I tighten the outer two left side bearings above 120 it begins to drag. The PO spun the flywheel end off a crank once, not sure if these cases were involved. For sure not the crank.
I took apart an old block that was also all BBBBB no color on any bearings there either. I was hoping for something to compare to. I called my local 750 guru, he said just buy green bearings, that's all I ever use. According to the case and crank codes, these should be green. I may roll in two bearings from the other set just to check. I still have plastigauge left.

I was hoping not to take the top end apart again but I'm getting used to the idea that I may need to. Or buy a micrometer set.

scottly · « **Reply #14 on:** May 10, 2017, 10:28:02 PM »

A B crank with a B main calls for a green bearing, but, due the the range in B sizes will result in a clearance from .025-.046mm. The yellow bearings are the "loosest" next to the green, and have as much as .008mm, or .003" more clearance, if I did the math right.(It's late

) You might try yellows in the two left side bearings?

Don R · « **Reply #15 on:** May 10, 2017, 10:30:44 PM »

Quote from: MRieck on May 09, 2017, 07:26:38 PM

There are lots of cases out there.

Mike you're reading my mind, I have most of a low mile F2 engine that could be flycut to allow the big sleeves and a backcut F2 trans that big Jay did for me a couple years ago. I could also use the K7 cush drive I have to line up the sprockets and get a little more tire clearance for the 16x5" M&H slick.
This bike belonged to a local street hero that taught me a little about 750's. In his honor I wanted to just clean it up and get it running on the assumption that he put it away in good shape, and recreate the big 4 express. At first it seemed like it was falling together like I was supposed to do it, then things slowed down. I really want to make it run this year even if it's just an easy pass down main street.

Don R · « **Reply #16 on:** May 10, 2017, 10:36:04 PM »

Quote from: scottly on May 10, 2017, 10:28:02 PM

A B crank with a B main calls for a green bearing, but, due the the range in B sizes will result in a clearance from .025-.046mm. The yellow bearings are the "loosest" next to the green, and have as much as .008mm, or .003" more clearance, if I did the math right.(It's late ) You might try yellows in the two left side bearings?

Not sure of the math but I am considering that also. Lots of B's around here. I can try different cranks and/or blocks.

scottly · « **Reply #17 on:** May 10, 2017, 10:37:14 PM »

If the crank has run-out, cases won't help. Check the run-out with the crank supported by only the outer two shells; if it runs true, try new cases.

Bill/BentON Racing · « **Reply #18 on:** May 11, 2017, 07:43:20 PM »

Yep, crank must be true...step #1.

Captain · « **Reply #19 on:** May 12, 2017, 01:53:04 AM »

Checking the crankshaft for straightness is very easy.......
Using either one (top or bottom) of the cases, remove all bearing shells except #1 & #5 and lay the crank in place. Fabricate a solid and flat steel plate and bolt to the cases so it doesn't move or flex. Then mount a mag stand and dial gauge and place probe against the centre journal. Turn the crank and read the dial.
This will tell you in seconds if it's bent or not and if it is by how much.
Captain

Don R · « **Reply #20 on:** May 15, 2017, 12:10:55 AM »

Agreed, I checked it in the middle, the problem is in the two bearings near the flywheel. Probably the crank is bent which will end this project for a while but it still may be incorrect bearings on those mains. I have no idea if there are rods for these pistons and a stock stroke crank. Thanks for all the good ideas, patience and not making me feel dumber than I already do.

I've been occupied with $1200 in repairs to the rear end in our race car. After 4 seasons the ring gear bolts backed out, 3 broke, all were loose and it wobbled the spool out of round. So, I bought a steel profile milled and lightened spool, bearings, pinion spacer, billet u joint caps, set up kit and new pro gears. The dial indicator told the story there. It now has ARP Aircraft style ring gear bolts and I still need to safety wire them.
It would probably not have made it down the track one more time. I just felt the need to check the rear end and BOOM, there it was looking at me. It performed well on the last pass last year but when we lost that round it was a good thing. Could have let loose off the starting line, revved the 567 to the moon and ended racing for me for quite some time.

Jim F · « **Reply #21 on:** May 16, 2017, 01:01:28 PM »

Quote from: scottly on May 10, 2017, 10:28:02 PM

A B crank with a B main calls for a green bearing, but, due the the range in B sizes will result in a clearance from .025-.046mm. The yellow bearings are the "loosest" next to the green, and have as much as .008mm, or .003" more clearance, if I did the math right.(It's late ) You might try yellows in the two left side bearings?

more like 0.00011812. Your getting into the micron range when you get three places past decimal in MM
toolmaker just leaked out
sorry
Jim

754 · « **Reply #22 on:** May 16, 2017, 02:09:48 PM »

When you are turning it with all but the tight bearing in, then check it with the indicator. As Scotty said, probably a looser bearing out there.
I have seen strange fixtures in the backrooms of machine shops with bottle jacks on them, when I asked about them, they were either crank or cam straighteners. I asked if they used them much, they said yeah quite a bit..

Don R · « **Reply #23 on:** May 16, 2017, 02:30:20 PM »

I got back into the shop today, put the dial indicator on the crank and don't find a wobble on the ends either. My plan is to check a full set of yellows.
I read somewhere Honda had people sitting at fixtures checking bearings and marking the colors. The ones that measured too tight were fine sanded on the back side to make them looser. That's why some bearings have different size shiny spots on their back side near the oil hole. Anyone ever hear of this?

754 · « **Reply #24 on:** May 16, 2017, 02:42:50 PM »

If it was so close, might work.when it is torqued will it turn with reasonable effort.?
If it were just hanging on an edge, you could scrape it. Do you have prussian blue? And a scraper..?
This will sound silly, late in the game, but did you check for debris or small norg or burr?

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Author Topic: 750 main bearing clearance, a compendium of information. Re-run. (Read 14436 times)