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Engine bearings Crank and Big End

Edward Buck

CB750 Enthusiast
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Great Malvern, Worcestershire
Hi all, I am in the process of rebuilding two CB750's a K1 and a K4, I split the cases yesterday on the K4 having previously removed the head and barrels. The main bearings look like they have some marks See photo below of worst one, and I'm going to change them having gone this far - the big ends have very little movement in them although there is some. I have no idea how many miles the engine has done. Whats the advice - bite the bullet and change main and big end shells?
Also as the shells are matched to the bearing/big end by the numbers on the case and crank, what do you do if you take it apart and the crank is scratched/worn? Can you buy over size shells or do you get the crank reground? I'll be taking off the big ends when it warms up bit here.
Fianl question are the big end bolts stretch bolts or can you reuse them?
Thanks in advance
Ned
IMG_9487.jpeg
 
no grinding on the crank. Pretty much all plain bearing motorcycle engines use select fit bearings. There are code stamps on the case and the crank used to select the bearings. The bearings are color coded. Best thing you can do is get a good shop manual...I recommend factory honda.

If that is the worst bearing honestly I would use them. Some bearing sizes are getting harder to find and getting somewhat expensive. We used to use good old bearings when needed just a little more extra clearance than what a new bearing had.
 
no grinding on the crank. Pretty much all plain bearing motorcycle engines use select fit bearings. There are code stamps on the case and the crank used to select the bearings. The bearings are color coded. Best thing you can do is get a good shop manual...I recommend factory honda.

If that is the worst bearing honestly I would use them. Some bearing sizes are getting harder to find and getting somewhat expensive. We used to use good old bearings when needed just a little more extra clearance than what a new bearing had.
Dirt digger thanks. I have a Haynes manual (which is not much use) and have watched 'Hackaweek' on Youtube at least three times. Im retired so this is just a hobby for me. I thought the bearings looked OK but as its split seemed like the right thing to do to buy new - now I'm not sure - main bearing shells are about £13 each shell so no, not cheap for 10 of them!.
Are the big end bolts stretch bolts do you know? Or can I reuse them.
Thanks
Ned
 
Dont take much information from hackaweek, have seen wrong information and laught and stopped watching the cb750 series.

The connecting rod bolts should be replaced, many use the f2 rod bolts as they are a bit stronger. I have reused the stock bolts but it is not recommended as they do have a tendency to stretch.

There are many things I always replace even on stock engines.....heavy duty cylinder studs, heavy duty cam chain, primary chain, cam chain tensioner, primary chain tensioner, undercut transmission, rebuilt oil pump.
 
Dont take much information from hackaweek, have seen wrong information and laught and stopped watching the cb750 series.

The connecting rod bolts should be replaced, many use the f2 rod bolts as they are a bit stronger. I have reused the stock bolts but it is not recommended as they do have a tendency to stretch.

There are many things I always replace even on stock engines.....heavy duty cylinder studs, heavy duty cam chain, primary chain, cam chain tensioner, primary chain tensioner, undercut transmission, rebuilt oil pump.
OK thanks for the heads up. I have only done one cb750 so far - a K1 which was a learning curve for sure. I didn't do the crank on that as it ran and sounded lovely so just stripped the top end and replaced valves etc. The rest of the bike has been gone over of course. I'm more used to cars but my wife said enough!! So bikes it is!
 
no grinding on the crank. Pretty much all plain bearing motorcycle engines use select fit bearings. There are code stamps on the case and the crank used to select the bearings. The bearings are color coded. Best thing you can do is get a good shop manual...I recommend factory honda.

If that is the worst bearing honestly I would use them. Some bearing sizes are getting harder to find and getting somewhat expensive. We used to use good old bearings when needed just a little more extra clearance than what a new bearing had.
Dirtdigger, thanks for the replies. Whats your view on rising the corned bolts and nuts - they are very expensive - the nuts are £10 each! I have read conflicting advice about reusing them - I don't know if they stretch or not - the torque setting seems very small to stretch a bolt? I did a Landrover Discovery head and that had stretch bolts - you could actually feel them give - tightening was a question of angles of degree not torque.
I took the crank to a engine shop I have used many times over the years - he said although the shells are not shot he would get new ones - its racking up the cost but equally no point doing all this for it to break!! So what's the view on the bolts and the nuts?
Many thanks
Ned
 
Bolts are a weak point but I have reused them on stock rebuilds with stock rpms. Yes the bolts stretch. Rod bolts are also arguably the most important bolt in an engine when you look at the job they have to do. I dont use stock connecting rods anymore anyway in any of my builds as the aftermarket connecting rods are so much stronger.

I still would reuse the mains. Like said I have reused slighly worn bearings to get correct and even clearances, these cases have a tendency for the main bearing bores to get out of alignment, the reason cycle x now has a service that does slight honing to correct this. An engine machine shop is always going to tell you to replace bearings...they dont want to be held liable if something fails because they told you to reuse a part. If you have the extra money to spend then sure go ahead and replace the bearings....definitely not going to hurt anything, keep in mind you have to get the correct color code.
 
Bolts are a weak point but I have reused them on stock rebuilds with stock rpms. Yes the bolts stretch. Rod bolts are also arguably the most important bolt in an engine when you look at the job they have to do. I dont use stock connecting rods anymore anyway in any of my builds as the aftermarket connecting rods are so much stronger.

I still would reuse the mains. Like said I have reused slighly worn bearings to get correct and even clearances, these cases have a tendency for the main bearing bores to get out of alignment, the reason cycle x now has a service that does slight honing to correct this. An engine machine shop is always going to tell you to replace bearings...they dont want to be held liable if something fails because they told you to reuse a part. If you have the extra money to spend then sure go ahead and replace the bearings....definitely not going to hurt anything, keep in mind you have to get the correct color code.
Dirtdigger hi,
well where to start - first I had the casing vapour blasted which included glass beads. I thought I had cleaned it out completely - but - on putting the head on I found glass beads in one of the threaded holes. Then I found some more - so stripped the engine completely again as I can't be sure of it being clear of glass beads.
Took the case apart and removed the crank - then the main bearing shells and found that one side of each shell has scratch marks on it - it has only been rotated by hand a few times just to check it was all in line etc. Used assembly lube on everything.
See photo of one of the shells - do you think I need to 'plasti- gauge' the bearings as they surely shouldn't have these marks after only being turned by hand. Of course it could be the glass beads?? What do you reckon?
Thanks
Ned
IMG_9910.jpeg
 
those glass beads get everywhere and could be hard to shift out of oil ways , i have known engines wrecked on the first few miles. you need to get soap/water and then petrol and compressed air everywhere, get cotton buds in the holes and get it all out
 
Agreed you need to do a good wash, honestly a dish washer is one of the best ways for an initial clean after glass beading. Take out the oil gallery plugs so you can get a good wash through the gallery. I also use a gun cleaning kit with the brushes to get in all the galleries including the crankshaft galleries. You would be suprised at how much junk collects in the crankshaft galleries up against the plugs. If you have a crank with thread in plugs you can take them out. If it has press in balls for plugs many places take them out and then thread them for plugs so they can be sure the galleys get clean.

If you look at the new bearings they have the same marks as the old bearings which indicates the case holes are little out of shape. Not much you can do other then let cycle x lap the cases. If the clearances are good run them. I am assuming the bearings with the marks are on the side that had the weight of the crankshaft while turning? Even with assembly lube the crank will still very slightly touch the bearings even through the lube. Always check bearing clearances to make sure it is good...never assume.
 
Agreed you need to do a good wash, honestly a dish washer is one of the best ways for an initial clean after glass beading. Take out the oil gallery plugs so you can get a good wash through the gallery. I also use a gun cleaning kit with the brushes to get in all the galleries including the crankshaft galleries. You would be suprised at how much junk collects in the crankshaft galleries up against the plugs. If you have a crank with thread in plugs you can take them out. If it has press in balls for plugs many places take them out and then thread them for plugs so they can be sure the galleys get clean.

If you look at the new bearings they have the same marks as the old bearings which indicates the case holes are little out of shape. Not much you can do other then let cycle x lap the cases. If the clearances are good run them. I am assuming the bearings with the marks are on the side that had the weight of the crankshaft while turning? Even with assembly lube the crank will still very slightly touch the bearings even through the lube. Always check bearing clearances to make sure it is good...never assume.
Dirtdigger thanks for the reply. Yes the weight of the crankshaft was on the side which is showing the marks.
When you say case holes are a little out of shape what do you mean? Also ‘lap the cases’ I know what lapping the valves is but don’t know what lapping the case means. Can you explain please?
I’m going to use some plastigauge to check the clearances which I didn’t do. I just assumed using the colour coded shells would be enough. Especially as the engine generally didn’t show much wear.
Thanks
Ned
 
The crankshaft bores in the case can be out of round or out of alignment. Cycle x has a bar that they use to get them closer without changing the bore diameter enough to affect bearings.
 
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