E: Engine Big End Replacement

[Bill Thomas]

I have just had my crank rebuilt for the third time in around 80,000 miles.

The first two times it was done by a now deceased local 'expert'. The first time he did it he fitted an oversize crank pin which involved boring out the crank pin holes in the wheels, lightened the flywheels and 'pin bored' (his term not mine) both ends of the conrod. That big end lasted less than 5,000 miles before total failure caused, according to the same expert,by poor case hardening of the crank pin. The second job he did on the crank lasted around 20,000 miles.

The chap that fitted the latest crank pin reported the following issues with my crank assembly. 1. Each flywheel as wedge shaped - he assumes that when the wheels were 'lightened' it was not done in a mill or a surface grinder but by some other less accurate means resulting in the face of each wheel NOT being parallel. 2. The crank pin holes in the two flywheels were 0.007" different in their relationship to the main shafts - so each wheel was giving a different stroke. 3. The crankpin holes were not an interference fit, but rather an easy slip fit with just the pressure of the crank pin nuts holding things together. The result of this was that while the main shafts were running more or less true, the crankpin itself was at a slight angle relative to the main shafts and to the conrod - as a result the entire load in the big end was focused on one end of the big end bearing rollers with no load at all on the other.

It was touch and go but he managed to save the flywheels but it took over 10 hours of machine shop time to fix them - and that excludes the time he spent to assemble the new big end once the wheels had been repaired.

Having only just finished putting my Comet all back together with the only 'new' bits being main bearings and the rebuilt bottom end I can tell you that it is performing was way better that it ever has in the past.

So when it comes to rebuilding a crank assembly it not just the main shaft runout you should be checking. You need to check and double check all of the dimentions around the crank assembly - assume nothing!

I wish I had the space, skills and equipment to all this sort of precision engineering work myself so I would not need to send anything out to others.

I also saw a bloke who was putting an engine together for a Vintage bike run "Banbury ", he worked in the same place my crank was being done, And he held one flywheel in one hand and with the crankpin in it, And just slipped the other flywheel on, Clunk !!, They all had a good laugh, I could not wait to get back to the Smoke ie London. Cheers Bill.

 

[Chris Launders]

No Bill, I'm sorry they just come as they are although most are old stock.
I say "we" I've never had to hone any yet, but the guy who did has just retired so I suppose I will have to learn.
Chris.

 

[greg brillus]

The rod eye is always out of round, it is not very strong, that's why you hone it back to being round, press in the new liner and then hone to the desired size/clearance. Not indifferent to liners, muffs and pistons.

 

[Bill Thomas]

Greg my carillo rod is brand new, How much can you grind the inside of the outer track, Before it's knackered ?. Cheers Bill.

 

[greg brillus]

Hi there Bill, I'm guessing the Carrillo's are probably stronger all over verses a stock Vincent rod, But in any case you are only talking a few thou here and there, it only takes some careful measuring to find if it needs attention or not. If the rod eye is out, then pressing in a new liner will end up out of round as well. It might seem like a lot of extra trouble, but i suppose it might be the difference between it lasting a long or short time.

 

[Cyborg]

Read my post again. It was NOT the previous owner.

BTW with the most recent rebuild - the nuts on the crank pin have been tightened to a reported 200 ft lb

I was referring to how my crankshaft was disassembled.

 

[Cyborg]

I work part-time in a bike shop and we rebuild quite a few roller bearing cranks, most are ok but we have the odd one that is out of round or too tight (funnily we don't seem to have any too loose)
When this occurs we have a Delapena bench honing machine like this we use to hone them out.View attachment 17578

That's interesting. I have a couple of Norton cranks that need new big ends. They use Manx rods, so perhaps less likely to go out of round. The machine shop that I'm considering for the honing uses a similar machine. I do wonder how accurately things turn out. It must take some experience and skill to hang onto that rod and keep it at a right angle to the hone so you don't end up with a bell mouth on either side of that narrow race. I see that when some shops are sizing rods for multi cylinder engines the do two rods at a time so it's easier to keep them straight.

 

[Cyborg]

As I have said before, I once took my crank to one of our top blokes, And watched him "Hone", The big end eye by winding the rod round a large pin in the vise by hand, With grinding paste, Others on the Forum said this was OK, But it didn't impress me, Maybe I will have to do it at home, But my hole 560 crank will be a Bodge, Fingers and everything else crossed !!. Cheers Bill.

That is interesting as well. It actually sounds to me (remember I'm a big end virgin) like the large pin in the vise might be a reasonable way to go depending on the circumstances. If there isn't much material that needs to be removed, perhaps it would be ok? This engine is (which is the only part of the Comet I have) destined for a some sort of special. It will be in more or less stock trim and not expected to be a 100k motor. I know if I'm going to do it... it should be done right and I tend to be on the fussy side of the spectrum, but maybe a crank that lasts 30k is alright in this case. When I croak, some subsequent owner can can curse me on this forum when the crank gives up at 31k.

BTW, I don't think I would want to use a vise. Perhaps the large honing pin held in the mill chuck? I have lots of different grades of grinding paste and a can of that stuff to harden the pin. Anyway I should stop thinking out loud or all y'all are going to think I'm a little odd.

 

[Bill Thomas]

That is interesting as well. It actually sounds to me (remember I'm a big end virgin) like the large pin in the vise might be a reasonable way to go depending on the circumstances. If there isn't much material that needs to be removed, perhaps it would be ok? This engine is (which is the only part of the Comet I have) destined for a some sort of special. It will be in more or less stock trim and not expected to be a 100k motor. I know if I'm going to do it... it should be done right and I tend to be on the fussy side of the spectrum, but maybe a crank that lasts 30k is alright in this case. When I croak, some subsequent owner can can curse me on this forum when the crank gives up at 31k.

BTW, I don't think I would want to use a vise. Perhaps the large honing pin held in the mill chuck? I have lots of different grades of grinding paste and a can of that stuff to harden the pin. Anyway I should stop thinking out loud or all y'all are going to think I'm a little odd.

I like it when people think out loud, Can't stand B.S.
I worry if I don't use my short conrod, After I have gone, Some poor sod may put it in a standard crank, Then find the piston don't come to the top of the barrel !!, We are only talking 1/4 ". Cheers Bill.

 

[Chris Launders]

When using the honing machine the pressure on the stones is finely controlled, very little pressure is used, you can hold the rod with two fingers and you run it left and right along the length of the stones to make sure wear on them is even, as I say I don't normally do this but I have once, the lever at the bottom of the machine shown controls the expansion of the stones once you have selected the appropriate set up and installed it, ours has a rope and treadle on it so you can apply tension with one foot while holding the rod with one hand and providing lubricant via an oil can with the other. Slow and steady, making sure not to put any side pressure on the rod is the way.
I'm not sure if the speed is variable or not, and I'm off sick at present so can't check.
Chris.