Author Topic: CB750 Main bearing replacement Question (Read 19776 times)

seebee750 · « **on:** December 08, 2007, 09:31:52 am »

Hi,
I am ready to replace the 5 main bearing shells on my 1976 CB750 K6.
In the Honda repair manual it shows numbers stamped on the crankshaft counterweight by the center sproket. My crank has no numbers or letters stamped any where.
Also the bearing shells have no color coding.
The top half of the engine case does have B B B B B stamped on it. Is this the bearing that I need?
Honda part # 13316 300 013
thanks

754 · « **Reply #1 on:** December 08, 2007, 10:14:38 am »

All the cranks should have numbers or letters, they are put on with an engraving pencil, not stamped so if crank is very dirty they may be hard to see.

Regardless of crank code, crank should be measured as well if it is worn.. may fall out out the tolerance code marked on it.

seebee750 · « **Reply #2 on:** December 08, 2007, 10:25:31 am »

I have cleaned the crank and looked it over 3 different times with a light and magnifying glasss. No numbers on it anywhere.
The crank looks in good shape and I will measure it. I just want to be sure that the BBBBB stamped on the upper transmission case is what I can use to pick the original bearing size.

754 · « **Reply #3 on:** December 08, 2007, 11:20:06 am »

Look on the flat portion beside the area of the crank with the cam and primary sprockets, move it around in the light to read it. If someone polished the sides of the crank then it may be missing.

seebee750 · « **Reply #4 on:** December 08, 2007, 11:31:22 am »

I looked over and over again nothing stamped or etched there on the crankshaft.

If the engine case is stamped B B B B B then the crank would not need to be.Is this correct?
The Honda repair manual shows both methods

MRieck · « **Reply #5 on:** December 08, 2007, 11:32:00 am »

Quote from: 754 on December 08, 2007, 11:20:06 am

Look on the flat portion beside the area of the crank with the cam and primary sprockets, move it around in the light to read it. If someone polished the sides of the crank then it may be missing.

Like you said earlier I'd just measure (the crank journels) OR take it to someone who can. If you buy a LCD digital 1"-2" mic it is almost a no brainer. Yamaha doesn't give actual measurements (bull#$%* IMO)....only codes. You have to measure the case bores if you want a real #. You should be able to see the color codes on the sides of the bearings anyway. If it ran OK just replace them with the same color.

seebee750 · « **Reply #6 on:** December 08, 2007, 11:48:57 am »

There are no color codes on the bearings either. They may have been replaced with aftermarket bearings as they are stamped A- std on the underside.
I am not the first to have this engine apart

bryanj · « **Reply #7 on:** December 08, 2007, 12:31:34 pm »

Nobody makes aftermarket shells, they will have a small bit of colour on the edge BUT it does discolour with heat and age.
Similarly unless the crank has been lightened and/or polished the marks will be there just very faint---they are difficult to see on a new crank.
The letters relate ONLY to the bore in the crankcase and you need the pin code to use the table to pick shells.
Finaly if the shells aint worn they dont need changing and if they are the crank will be worn as well and need replacing as they CANNOT be re-ground

scondon · « **Reply #8 on:** December 08, 2007, 03:46:26 pm »

As bryanj said, the BBBBB stamped on your cases refer to bore sizes in the crankcase, the "missing" letters on your crank would refer to the journal sizes on the crank. The table in the Honda manual shows which bearing to use for each combination of case/ journal letters( Example: "B" case/"A" crank= Green bearing).

This doesn't account for wear on used parts so even if you get the correct bearing by using the table it might not be the size you actually need. Start by plastigauging your current bearings to see if they even need replacing.

Same goes for the rods, each has a # stamped on it that refers to the big end bore. These cross reference to the #'s engraved on the crank(next to the missing letters) for selecting rod bearings. The letters stamped on each rod refer to weight and should be the same letter(weight) on all four rods.

Honda did not engrave codes on cranks from the last year, or so, of SOHC production. I haven't found any markings on three F3 cranks. I think, however, that they were still marking them in '76. They are really hard to spot, as others have said.

HondaMan · « **Reply #9 on:** December 08, 2007, 09:32:22 pm »

The CB750 manual has both crank journal and case hole sizes. If you can measure your journals, I can translate them back to metric and look up the right shells from the table. The "B" size is OK to use for the shell's outer, unless you think the cases have been align-bored (rare, but sometimes done in RR bikes). PM me with those journal sizes: try to mic to .0001" size, as this is the accuracy chart supplies (it's actually in .0004", or .01mm, increments).

Some of the cranks had the numbers stamped on the alternator end, on the side of the crank weight. Not clearly, either, but there.

seebee750 · « **Reply #10 on:** December 09, 2007, 04:05:35 am »

Thank you to all,
I will take some measurements and study the crankshaft one more time for markings.
The bearing shells are worn but the crank looks Ok. I will have to get some plastigauge and start there as suggested.

seebee750 · « **Reply #11 on:** December 09, 2007, 08:05:24 am »

OK guys, I now can see the marks on the crank shaft,
The letters are quite large and look like grease pencil marks.
I tried different types of light to finally see the marks. Looking with a magnifier and bright light was the wrong approach.
Now that know what I am looking for it seems that the marks on the crank are
(C) J AAAAC (P) L 4444

The engine case is BBBBB
Rod big ends are F2 F3 F3 F3

Now I should be able to figure out what bearing shells were originally installed since there is no paint on the crank bearing shells. They are marked underneath with a circular Logo and D60-A STD. not the original shells I presume.
Thanks for the patience.
This has to be the hardest part of restoring one of these bikes.
Attached is a photo of the crank where you can just see one of the letters A

Verbal · « **Reply #12 on:** December 09, 2007, 09:18:36 am »

Jerry Rxman Griffin aka MuthaF'er · « **Reply #13 on:** December 09, 2007, 09:38:41 am »

Seebee,

Good job and better persistence. Thanks for listening to those that know. You're one of us now ;)You really have to see them the first time to understand. No stampings, no fancy letters. Just some Japanese guy knocking out measurements by the thousands on a daily basis and scribbling on some very hard metal. You have to read these in a "Japanese writing accent". They sure don't look like someone with native English writing skills did them, do they?!

Crank = A-B ; A-B ; A-B ; A-B ; C-B
Rods = 4-2 ; 4-3 ; 4-3 ; 4-3

Go to the chart and match up these combinations now. 4 greens and 1 brown for the crank.

There is a real good chance those are the original shells. I've never seen afermarket shells myself. The D60-A STD markings you indicate are normal. There's a good chance the color on the bearing ends is long gone. How about posting pictures of some or all of the shells. Good chance they can be reused. If so you will Plastigauge them for wear and clearances first. Did you pair up the bearings for the correct journal when you removed them? If not it will be more difficult to match them up. I don't know that you can make any sense of the D60-A STD markings. If that were possible it sure would be nice to have a chart for this too. From determining you have 4 greens and 1 brown then if the markings on the back mean anything you should have 8 markings that are identical with 2 different but identical. Even if you do get new bearings at about $9 per half shell ($90 total for the crank) you're still going to need to Plastigauge them JUST LIKE YOU CAN DO WITH THESE ORIGINAL USED BEARINGS. Do not skip this step! Do not give in to temptation. This is not like women. This is precise. I repeat DO NOT SKIP THIS STEP! Go slow. Just do it! You're getting close so don't get in a hurry. This is real engine building. This step makes or breaks your whole investment.

754 · « **Reply #14 on:** December 09, 2007, 10:12:16 am »

Actually its not hard..

It just looks...and sounds hard.. but once you get your head around it it is quite easy and straightforward..

Just make sure your journals have not wore out the marked range.

seebee750 · « **Reply #15 on:** December 09, 2007, 11:51:29 am »

Yes I see the light,at last,I cannot believe how easy it is to see the markings when you know what to look for. I go back from time to time now just to look at the crankshaft and to see the letters jump right out at me. You need just the right light angle and the letters are large.
I am glad to be among with so many knowledgable, willing instructors.I am a retired millwright and an enthusiastic pupil I hope to get all our 5 CB750s on the road in no time.

I did not mix up the bearing locations.I will get some plastigauge tomorrow.I will measure the clearence as it is to get the feel for the use of plastigauge and bolt torque sequence.I will mark down the measurements as a reference point.
2 more questions,
what type of sealer do I need to seal the transmission cases back together?
I will need to get some crocus cloth to polish the crank journals right? Are there different grades of fineness?

I have 2 CB750 K0 engines to do and 2 CB750 K6 engines.
My K4 K8 and F3 engines are all running.

I will put my new found knowledge to good use.
I am putting in new bearings in all.
I just like the sound of 'new'.
I have put off the engine rebuilds for years but now have no excuse for not forging ahead.
For this K6 I have new pistons, rings, rocker arms,primary and cam chains, primary tensioner and cam chain tensioner,slider,sprockets,
Genuine Honda gaskets,
Seal kit,
New genuine Honda phillips head screws.
Polished covers and all. I have posted a photo of the K6 with its NOS tank side covers, emblems,seat,exhaust and front fender.I have thousands tied up in this bike.The first engine rebuild is for this bike.
My Honda dealer sends me a card at Christmas and calls me on my birthday. I have an charge account there and he sees me more than any other customer.

I bought a new 1973 CB750 in 1973 and a new 1977 CB750K K7 in 1977.
They are long gone but I have managed to take a trip back in time by just being around these bikes that I collected up.

Photos for the worst crank main bearing and the BBBBB stamped on the upper half of the engine case

seebee750 · « **Reply #16 on:** December 09, 2007, 12:05:32 pm »

This is my K4 recently finished. I have given it to my son Paul,age 26. He helped me with the rebuild of the engine top end,so he now has the force. It has been passed to the next generation of CB750 lovers.
For 5 years I have hunted down parts for this bike. I spent thousands of dollars and hundreds of hours.New seat, front fender,,exhaust,tachometer and speedometer,tank,side panels,emblems, and on and on.
It has new everthing on it .It even has new smell and you can't get that just anywhere,and new smell is not cheap either..

seebee750 · « **Reply #17 on:** December 09, 2007, 12:11:23 pm »

This is my K0
I gave it to my oldest son Darrell, age 28
I have thousands of dollars tied up in this one too.I have every thing for it except the crank and rod bearings.
I hope to get Darrell to help with the engine rebuild on this one.Then he too will have the force.

dave400 · « **Reply #18 on:** December 09, 2007, 12:21:35 pm »

Nice work, those bikes are beatiful!

seebee750 · « **Reply #19 on:** December 09, 2007, 12:33:54 pm »

This is my K8,
I gave it to my youngest son,Jonathan 21 years old.
I am the third owner and now he is the fourth. He has the force too.
Not as much time tied up here yet.I will do a total restoration in a few years.
The top left and right mufflers are new.The sidecovers are new from Honda. I need the K decal stripe for both side panels.
The grab rail is new, I have new fork ears for it and will rebuild the engine someday with Jonathan.

seebee750 · « **Reply #20 on:** December 09, 2007, 12:40:06 pm »

This is My K7. Before and after photos
I put good used parts on it to restore it,

I drove it this past summer.
It has a K8 engine in it, K8 tank and one K8 sidecover.
I traded it last week for a nice 2 owner CB750 Super Sport F3

bwaller · « **Reply #21 on:** December 09, 2007, 12:41:15 pm »

You're quite a dad seebee, good man for sharing with your boys. Five 750's make for a heap of force too!

seebee750 · « **Reply #22 on:** December 09, 2007, 12:51:46 pm »

This the 1978 CB750 F3 Super Sport that I got in an even trade for my K7
I am the third owner.
I have a new seat on it since the photo and an original muffler coming from a fellow sohc4 member.
Also I have the original carbs and handlebars.
I am looking for a non faded speedometer.

dave400 · « **Reply #23 on:** December 09, 2007, 12:52:36 pm »

I’m available for adoption as long as you build me one of those nice bikes.

HondaMan · « **Reply #24 on:** December 09, 2007, 03:01:40 pm »

A little extra info for the winter months' thinkers:

On mine, as an example of "what to do when the crank wears down", I just bought the next size down from the original sizes. I had replaced the bearings several times before 1980 (due to roadrace injuries, mostly), but always with the same size as stock. Now, 120,000+ miles later, the crank's journals are worn to the point where they are nearly .0008" undersize from their original size, just from the miles. They are still round (this would not necessarily be true of the rod journals!), so I have gone "down" 1 step for this next upcoming rebuild. Honda's bearing came in those useful .0004" increments, and I could go 2 sizes down, but have learned from experience that an extra 1/2 thou crank clearance makes for a happier engine on a real hot day in city traffic. (For roadracing, I'd jump down 2 increments, though, to keep the crank accurate at hi RPM.)

Rod journals: they wear more on the "power stroke" side, i.e., the side that faces up the cylinder when the piston is near TDC. This is true because this is the side that sees the push from the piston on power stroke. The rod journal wears a little less on the "other side", from pushing up during the exhaust stroke, but the sum of the wear is a slight oval shape on the journal, about .0013" on my worst one last year. It's not practical to just undersize the rod bearings like the crank, because the "wide" part of the oval then becomes too tight. Rod journals usually need to be repolished down to round again in hi-mileage situations like mine.

All of this extra clearance in the crank wouldn't be a problem IF new oil pumps (or hi-flo Yoshimura pumps) were still available, because the top end of the engine would not starve for oil if a hi-flo pump could be plunked into place. But, as the bottom end's clearances (and resulting leakages) increase, the top end gets less flow. Going to a heavier oil base (20w-something instead of 10w-something) helps because the higher viscosity makes the worn pump move more oil flow again. There is a point where even this improvement is not enough (mine is there, now), and the top end does not get enough flow at low engine speeds (mine: 2500 RPM on a 100 degree day will run it pretty dry), and the overall pressure drops a bit, too. All this signals the need for new bearings.

I just bought some used oil pumps: I'm going to attempt some rebuilding to see how it goes. I sure wish I'd bought some of those Yosh hi-flo pumps back then.....

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Author Topic: CB750 Main bearing replacement Question (Read 19776 times)