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E: Engine Big End Replacement

Cyborg

Well Known and Active Website User
VOC Member
Now that the B twin is more or less in one lump after more than 40 years in boxes..... I need to start organizing things for the next project. So... I have most of a Comet engine, but it needs a new big end bearing. There aren't many shops around this area that I would feel comfortable handing over a crank to. I'm thinking of taking a stab at doing it myself. Haven't started yet, so need to clean things up and start measuring to determine if I actually have something to work with. I have a press that should be sufficient for reassembly, dial gauges etc. Also have a lathe, but I'm thinking knife edge rollers would be better for checking runout.
Has anyone on here been through this process before? Advice?
 

Cyborg

Well Known and Active Website User
VOC Member
Well.... I chucked the flywheel and shaft into the lathe and it didn’t look all that promising, so I removed the chuck and then put it between centres. It’s indicating about .003 runout on the OD of the flywheel so will use it for a door stop and continue with the pair that came as one lump.... (at least until the pin broke). Could be that one of the centres is off, but don’t feel like spending any more time on it.
 

Bill Thomas

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VOC Member
This is the trouble with mine, If I can't get a good reading in the lathe,
With the old stuff, What chance do I have after welding and stroking !.
I see now it's been said the centres could be out,
This is why I sawed a mainshaft in half, To put the 2 wheels together with most of the balance holes,
also with bits of tubing, To try and set it up for welding the wheels together,
Ready for boring, Scared it will be miles out !,
But must give it a try, S--- or Bust !.
Looking at some youtube stuff, They seem to hold a lot of faith with the old ,
Straight edge across the outside rim.
Some of the youtube stuff with the H.D. s is rubbish,
One bloke bashing hell out of the wheels while still with the dials on !!.

I still have in my " Head " a view of a friend of Bob Dunn's,
Who worked in the next shop, Putting together a pair !! of flywheels,
Fully together they just spun in the hand, ie loose on the taper,
So would just rely on the nuts and bearings to hold them true :D ,
Wonder what happened to that bike ?, Not a Vincent, Thank God.
I will try John's idea of checking the chuck,
Good Luck, Bill.
 

Bill Thomas

Well Known and Active Website User
VOC Member
Talking of straight edges, I have just bought a pair of cheap Lidl ,
Right angle Magnets, Used for welding, I already had a few smaller ones,
Thinking it might help if I position 4 of them around the rims, Together with the tubes
in the balance holes, How could it go wrong :D .
 

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greg brillus

Well Known and Active Website User
VOC Member
Instead of welding up the holes Bill, Why don't you use an oversized big end, say one that is 1 1/4" diameter pin and then just offset bore the hole outward by half the amount.........Depends on what length rod you've got I guess........Cheers.
 

Bill Thomas

Well Known and Active Website User
VOC Member
That's what I was going to do Greg, But then I thought Alpha's wanted the conrod,
And it got to be more money, I don't like sending stuff out,
Alpha's messed up my Twin Bigend years ago, 1. 1/4" and it snapped !.
I have a short rod 1/4" shorter Carillo.
I don't ride fast now, Lucky to ride at all,
It's just something I always wanted to do, I may not ever do it,
Just a bit of Fun.
I have a spare pair of wheels and shafts, So could always do a 500.
 

oexing

Well Known and Active Website User
VOC Member
No need for a lathe for trueing a crank, in fact this "between centers" logic may look impressive and scientific - but isn´t . That way you can only guess what the crank ends may do once installed in the case but you do not know . Just get two alu plates or even two plates of plywood and cut a Vee in both to lay the crank in there. So this is like spinning on the inner bearings and you check with clock at the ends. The readings will be EXACTLY like in the engine, no guesswork from numbers you get from between questionable centers and runout on bearing seats.

Vic
 

Bill Thomas

Well Known and Active Website User
VOC Member
I agree Vic, I have a set of bearings like we had photos of a few months ago,
On legs, I also have V blocks I used years ago on my Goldstar,
But because I was just a mechanic, This is a bit tricky for me,
You can see my spare set of wheels and shafts, While they are apart it would be nice to have something to compare for after I have welded and bored the other wheels,
I bought another dial gauge,
But I have not found it easy, I must try what John said,
I don't think the centres are good on the shafts, It's all a bit of a pipe dream,
I am getting a bit old for this, I have been playing with Bikes for many years and have other Bikes that are running, So don't NEED another , But it would be nice,
You never know with me, I could suddenly get keen and get stuck in, I am hoping anyway.
Cheers Bill.
 

Pushrod Twin

Well Known and Active Website User
VOC Member
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
Thank you for posting that Chris, I have an identical machine which I bought as a kitset. It had the appendage to the right of the adjustment handle, but I couldnt figure out what it should do or where it should go. I had never seen one on the American Sunnen equivalents I used. It appears to be a holder for tools. The holder is threaded and attached by a bolt from the inside of the housing so mine will remain unattached until such time as I have to dismantle the machine for some unforeseen maintenance.
 

oexing

Well Known and Active Website User
VOC Member
Ahmmm, what sort of runout can you check with half a crankshaft ?? Concentricity of outer dia. of webs relative to mainshafts ? That is no point at all, you want no runout at shaft ends, in case of Vincents at least the timing side -and that with complete assembly I´d say.
For lapping the i.d. of big ends to size and roundness I fabricate an alu taper round with max. 1 degree taper set on lathe. The lapping material/ring is cast iron, same internal taper and ring slotted and grooved so it can be expanded by lightly knocking it along the alu taper. For lapping apply diamond paste , 5 micron grit is allright for finish. With very slow speed on lathe you can get the big end perfect by feel for resistance when going along the cast iron ring. You cannot get any better perfection than by lapping, even block gages are lapped to size.

Vic

P1070397.JPG



Pleuel u. Läppring.JPG


fertiggeläppt.JPG
 

Pushrod Twin

Well Known and Active Website User
VOC Member
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 "hone" process you refer to is more commonly known as "lapping" and is quite acceptable for removing small amounts of metal, tenths of thous, provided the lap is soft enough to hold the abrasive and kept parallel.
Like Oexing so graphically demonstrated above. :oops:
 
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Cyborg

Well Known and Active Website User
VOC Member
Ahmmm, what sort of runout can you check with half a crankshaft ?? Concentricity of outer dia. of webs relative to mainshafts ? That is no point at all, you want no runout at shaft ends, in case of Vincents at least the timing side -and that with complete assembly I´d say.
Vic



Well... I’d be checking to see if if the mainshafts were still straight. In a previous post I mentioned the crank had a broken crank pin. I checked the drive side shaft and flywheel and it was ok, I also checked the timing side and the shaft was bent. The point is.... I want to find out what if anything is bent before I assemble the crankshaft. There is no point in assembling the crankshaft, trying to align it and then discovering the shaft is bent.
 

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