Topic: Notes about the plain bearing bottom ends.

[HondaMan]

This isn't so much about selecting the bearing sizes (that's been beaten to death here at the forums) as it is about the details surrounding their clearances and construction.

When Honda made the cases on these Fours, they finish-machined the clamshell halves and torqued them together without any sealant in the interface. The bottom of the engine was pegged in place using the 2 little "drain plug" holes on either side of the bottom cases (on the 750), which are now plugged with small drain plugs (that leak from hard O-rings after 40 years...). Then they clamped the cases down with approximately 2 kg/m of force and align-bored the 5 holes for the bearing seats. This situation creates the "original round hole".

As the cases cure from years of heat, they shrink a little from front to back. The crankshaft pushes downward on the bearing seats in operation: between these two factors the lower half gets ever so slightly egg-shaped, big end down (about 0.0002"). In most engines that have had good oil and haven't spent time on a racetrack (i.e., extended high-RPM loads), the lower bearing is the one that wears. Honda knew this from the beginning, so like the points that first came out (moveable part of the points only), they sold the bearing shells in halves. In Japan, their mechanics were trained to replace just the lower half if the clearance was getting big, to save $$ on maintenance and give Hondas a good overall reputation for low maintenance costs.

When you are Plastigaging your bearings, if you have the time, gage both top and bottom of the bearings and you will see the inequality of the wear. If the wear is still within spec (i.e., less than 0.0032" worst case) you may find that just the bottom bearing needs new. I have seen this many times. If your crankshaft is in good condition and not worn in the journal at this site, replacing just the bottom half with the same color bearing as the top half can provide a good "budget rebuild" for you. However, if you are planning on spending a lot of time near that redline, replace both halves so the bearing crush zones will align perfectly upon case torquing: this will ensure even oil distribution to the top half of the bearing.

If your journal is worn (evenly) you can gain like-new smoothness by stepping down one size in clearance from the original bearing. Each step is (in English) 0.0004" smaller, and this action requires that you replace both halves at once. Break your engine in for at least 1000 miles below 5500 RPM if you install (a) tighter one(s): it will be like brand-new at that site and needs some time to polish the bearing and crankshaft interface.

If the journal is not worn evenly, and removing the taper will still net the proper clearance for the BLACK bearing shells (smallest), then you can save the crank and still get like-new performance. If you have to leave a little taper in the journal but can get at least 75% of it flat and even (and aren't going racing), it will still be a very long-lived engine, because a 5 main bearing bottom end under a 45 cubic inch engine makes it like a tractor bottom end, able to leap tall buildings in a single bound. 

When Honda originally assembled these engines, the case contact areas around the bearings did not receive sealant. This allows the area directly around the crankshaft to pull inward slightly more than the rest of the case, which prestresses the whole assembly to stiffen it more around the crankshaft and helps mitigate the extra clearance from the upcoming case-cure cycle (which won't happen to you this time, BTW). So, when you assemble the cases, always start with the big 8mm bolts first, fully torqued. Then install the center 6mm bolts next, torquing them from the center ones first, to about half the final value (50 inch-lbs) and then to their final value (90-100 inch-lbs): 3 steps is better. I usually torque the ones across the front of the cases last: this seems to keep them dry there longer over the miles. Be sure the engine gets at least 24 hours of drying time before putting oil in it, or it will develop leaks from washing away the edges of the uncured sealant on the insides (which is why so many racing engines leak: 30 minutes between application and startup!). 

[MRieck]

Mark....I honestly have to ask you....will 3 or 4 microns really make a difference in regard to bearings? First....who can count on bearing thickness consistency? Plus....who can measure .0001 accurately on the is board? Also....I've seen leaks from no sealant on the towers. People get to hung up on this stuff....we have to make it easier. I find bearings all over the place in regard to thickness and in all honesty a black bearing or green??.....the engine will run just great loose. A loose bearing is a happy one as oil flow over the surface increases and the bearing runs cooler . Keep in mind.....you cannot even get black or yellow bearings for the 500/550 anymore. Like I said...... I think folks belabor this bearing thing to much.  Put greens in and forget about it. I was told by a Yamaha race person ( the real deal) many. many years ago to go brown (their green) and forget about it. No offense but guys get all up tight about this stuff. My thoughts.

[MCRider]

Thx Mark, that's pretty much what I was taught in my very brief stint as Honda mechanic. But I never had any of the back story.

And why put sealant where it can do no good?

Then once realizing how bullet proof the bottom ends are, what Mike says rings true too. In the drag race shop where I worked for a bit, it was always throw in a set of greenies and go. Cases and cranks were being swapped out all the time, codes were ground off, and there wasn't time to decode or even to plastigauge.

Something else will break or wear out long before the bottom end bearings, racer or streetbike.

At least I think so.

[HondaMan]

Mike & MC: You're both right, no doubt of that. 
I just get a lot of e-mail on the topic at home, so I posted this so I don't have to re-type it every time: I can just "point" to it, now.

I get around 30-40 e-mails per week from this forum.

On the "fine tuning" side, though: if the owner is after smoothness, that comes from the exacting bearing selection that Honda went to all this trouble to get. I've found that keeping the bearings in the 0.0008" to 0.0010" clearance range delivers great smoothness, while opening them up to 0.0012" to 0.0018" improves top-end power and the freedom for the engine to rev easily. Above 0.0020" clearance, the vibration starts to feel like a Kawaski Four, and images in the rearview mirrors get real blurry. Above 0.0028" or so, the oil rings get overwhelmed from the excessive oil being released from the bearings, and the bike starts using and oozing oil, especially out the pipes. Honda was always about this "tunablity" in the 1960s, which carried into the CB750 design.

Other than outright damage, I have never seen an engine with Honda's stated limit of 0.0032" clearance. Even mine has only reached 0.0022" after 138,000+ miles. And, it feels like a Kawasaki, time for new bearings (sitting in my drawer, waiting for this next rebuild). 

I always remember Sochiro's famous statement that the Honda bean counters have long strived against: "If you are going to sell a motorcycle, you must always supply the parts for it forever." I think the Big Four and it's effect on Honda overall has probably earned it some of this respect, compared to the other models, over the years.

My goal now is to find other vendors who are willing to supply the parts that Honda is trying to quit, like pipes, rocker shafts (I'm making those myself, now), cam towers, etc. Rather like David Silver Spares, I hope to become something along those lines over the next 2 years or so. 

[MRieck]

Mike & MC: You're both right, no doubt of that. 
I just get a lot of e-mail on the topic at home, so I posted this so I don't have to re-type it every time: I can just "point" to it, now.

I get around 30-40 e-mails per week from this forum.

On the "fine tuning" side, though: if the owner is after smoothness, that comes from the exacting bearing selection that Honda went to all this trouble to get. I've found that keeping the bearings in the 0.0008" to 0.0010" clearance range delivers great smoothness, while opening them up to 0.0012" to 0.0018" improves top-end power and the freedom for the engine to rev easily. Above 0.0020" clearance, the vibration starts to feel like a Kawaski Four, and images in the rearview mirrors get real blurry. Above 0.0028" or so, the oil rings get overwhelmed from the excessive oil being released from the bearings, and the bike starts using and oozing oil, especially out the pipes. Honda was always about this "tunablity" in the 1960s, which carried into the CB750 design.

Other than outright damage, I have never seen an engine with Honda's stated limit of 0.0032" clearance. Even mine has only reached 0.0022" after 138,000+ miles. And, it feels like a Kawasaki, time for new bearings (sitting in my drawer, waiting for this next rebuild). 

I always remember Sochiro's famous statement that the Honda bean counters have long strived against: "If you are going to sell a motorcycle, you must always supply the parts for it forever." I think the Big Four and it's effect on Honda overall has probably earned it some of this respect, compared to the other models, over the years.

My goal now is to find other vendors who are willing to supply the parts that Honda is trying to quit, like pipes, rocker shafts (I'm making those myself, now), cam towers, etc. Rather like David Silver Spares, I hope to become something along those lines over the next 2 years or so. 

Mark....Honda is the greatest manufacturer of motorcycles if...for nothing else...their commitment to their product. They have set the standard for any company in regard to supplying replacement parts. Their parts system was the envy of all bike companies.
 I agree with the bold print. You'd have to suck something bad to damage a 750 crank. I personally think they can withstand something in the area of 225 to 250 HP. That is based on length and bearing area/support.
  Mike