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E: Engine Big End Replacement

Cyborg

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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?
 

Robert Watson

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Checking run out. I have seen them shown on knife edge rollers on the bearing seats and the indicators on the outboard ends but I am assured that centers and bearing seat is the way to go.
Put the crank between centers in the lathe. Grease the centers and just tighten the tailstock, but not enough to press on the wheels and not too little to let them wobble on the centers.
Put the dial indicators up against the ends to make sure the centers are true. If not get the Dremel out and make them true
Put the indicators on the bearing seats.
I was told once that if one side has to be out better the drive side as it just has a chain sprocket on it, the other side has a whole gear train.

I did my thruxton crank and asked a local thruxton guys about runout, he said anything less than 3 thou was OK...... Not in my opinion. The crank in my black b was dead true maybe a 1/10 on the drive side and contrary to the above, about 6 10ths on the timing side.
 

timetraveller

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VOC Member
I have done several tens of these over the years and agree with Robert above. Centres in the main shaft ends and dial gauges on the bearing surfaces, each side of the flywheels. My own aim is to get each side down to about, or less than, one thou. If I cannot do that then I consider the flywheel assembly to need further work. One set I did seemed to have been built up from two flywheels which were not a pair. I could get one side down to less than one thou and the other side could not be made better than six thou. My conclusion was that the main shaft to centre of the crank pin was different from side to side. The owner used it but I have no idea whether it has seen significant use since.
 

Chris Launders

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VOC Member
I've not done it between centres but done several on knife edge rollers and try to get below 1 thou at the shaft end, I couldn't do this with my JAP crank as the shafts were such different diameters so I made a frame to support it on it's bearings (the bearings sitting on thin edges) and got it down to under 1 thou at the shaft end 3" outboard.
What I have found useful is to mark a point 90 degrees from the crankpin on one flywheel with a marker pen as it's easy to get lost where you are when taking it out, hitting it and putting it back several times.
None have been Vincent cranks yet.
Chris.
 

Bill Thomas

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VOC Member
The first problem I had was I was told Alpha's want to fit the bigend into your rod, I don't think they will sell you just a bigend ?. I hate sending things away so I took a chance and bought a flywheel ass'y on ebay, I may have had some luck, When I took it apart, The bigend looks like new !.
I have a new Carillo rod, 1/4" shorter than standard, So I hope to fit it to my new rod, I have a home made 10 ton press, Brother Ron made, By 10 ton, I mean that's the weight of it !!, It has took me 11 years to put it together after a house move !!, Each bit is as much as I can move !.
I have done a few cranks, Many years ago, I think people can talk too perfect !!, I bet if you check a few cranks that have been run, They are nowhere near as true as what you are aiming for. Cheers Bill.
 

greg brillus

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VOC Member
I think the reason that Alpha suggest supplying them with the rods is that the big end "Eye's" become oval. So they would press out the original big end liner, then hone the eye back to a true round size, then press in a new liner and then hone it out to the desired finish size. I would say this is the best way to do it, and it is the way my machine shop do mine. Laurie Binns who was excellent at rebuilding Vincent cranks told me that the assembly should be checked at the shaft ends between centers as has been mentioned on here. The more accurate the crank is done generally the smoother the engine will be, and the smallest of runout will aid in prolonging the life of the main bearing tunnels. Not too many old motorcycle engines had 4 main bearings .........Original flywheels are only made of Boiler plate and the press fit area minus the nut section is not overly substantial, very easy to see how the wheels can "Shift" from abuse in service.
 

stu spalding

Well Known and Active Website User
VOC Member
The first problem I had was I was told Alpha's want to fit the bigend into your rod, I don't think they will sell you just a bigend ?. I hate sending things away so I took a chance and bought a flywheel ass'y on ebay, I may have had some luck, When I took it apart, The bigend looks like new !.
I have a new Carillo rod, 1/4" shorter than standard, So I hope to fit it to my new rod, I have a home made 10 ton press, Brother Ron made, By 10 ton, I mean that's the weight of it !!, It has took me 11 years to put it together after a house move !!, Each bit is as much as I can move !.
I have done a few cranks, Many years ago, I think people can talk too perfect !!, I bet if you check a few cranks that have been run, They are nowhere near as true as what you are aiming for. Cheers Bill.
Hi Bill, my crank has .002" runout on the timing side mainshaft, it could do with being a lot smoother running. Tony Maughan always reckoned .0005" max runout for a smooth engine. Cheers, Stu.
 

Cyborg

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VOC Member
Well checking it on the lathe would certainly be easier and cheaper given the fact that I don't have the knife edges. Interesting subject the "between centres vs knife edges". TP told me when discussing one of his new cranks that "the crank cannot be checked between centres on a lathe, only on knife edges close to the inner bearings as the grinding process does not use the centres and they could be out". I mentioned this to one of the local gurus (who is retired and only works on his own projects) and he just looked at me like I had two heads. He said just set the crank up in the lathe and if the centres aren't centred, then just centre them with the grinder. Not sure why I was leaning towards knife edges. Maybe it was the fact that they made a special tool just for that purpose.... or I'm just a shitty student and wasn't listening to the guru.
 

Cyborg

Well Known and Active Website User
VOC Member
I have access to a big end complete with liner and I have a few rods to choose from. Hopefully one of them will be ok. Again, I haven't done any measuring yet, so not sure if the new big end liner is large enough to allow for some honing in the rod. I'll have to find someone I can trust to hone it.

As far as press fit area goes... if one side or the other is borderline, would it be reasonable to use Loctite (just in the one side). I know that probably depends on my definition of borderline. BTW, what would be the minimum allowable interference fit?
 

vibrac

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VOC Member
Many years ago when I bought a comet engine literally off a scrap heap for a fiver and stripped it, the flywheels were within half a thou on circumference and a thou everywhere else! it all looked original so I guess that's the standard. I must admit I have always subbed out flywheels it does seem a lot of effort and equipment for a very infrequent task but I do check them in my lathe when they return I suppose now the Scottish option is out I shall have to rely on Maughns
 

Cyborg

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VOC Member
My clan is from the Isle of Lewes and were worshipers of the Callanish Stones (too cheap to build a church). I have two other cranks (Norton) waiting in the wings, so with the combination of my thrifty nature and my proximity to Maughns, at least in my mind, at this point it makes sense to take a stab at it. It may all come to a crashing halt once I get out the micrometer and bore gauge.
 

timetraveller

Well Known and Active Website User
VOC Member
Yes, and a decent spring balance or equivalent which is accurate to a gram or so. I have always tried to balance to Vincent's original figure for twins of 46 % but I know that some of the racing boys go for 50% or more.
 

Bill Thomas

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VOC Member
Hi Bill, my crank has .002" runout on the timing side mainshaft, it could do with being a lot smoother running. Tony Maughan always reckoned .0005" max runout for a smooth engine. Cheers, Stu.
Ah the real world ! Stu, I always aim for perfection, But never get it.
The standard Vin crank is not the best in the world, People have been trying bigger pins and press fit with no nuts for years, My L/ning was so smooth when I first built it, But after it's first race, Not any more !.
If I can get .002 on my 560 Comet, I will be thrilled to bits !!.
I know you liked Bird Watching Stu, But what about Fishing !!, I asked Dear old Big Sid from USA about vibration, And he told me a trick about putting 1/4"or was it 3/16 ths, lead weights up the handle bars, He could tell a good story but I thought it's worth a try, My vibes were around the 65/ 70 mph ish and it's much better now, I think it was before you were on the Forum.
Another good trick is to drill and fit a small roller, Half in the flywheel and half in the main shaft, So the shaft can't turn. Cheers Bill.
 

passenger0_0

Well Known and Active Website User
VOC Member
I've done a few built-up cranks (Indian, Vincent twin etc. ....) in the past and personally wouldn't align them on centres unless these have previously been ground true to the main shafts. I used a tool room centre-grinder as you don't know how much the shaft centre has been pushed around by over zealous use of an extractor. I've also seen a drive-side main shaft that was twisted with 0.006" run out on the main bearing surface when run on centres. Mentioning this to the owner he told me several years ago he had locked up the engine at revs when the main shaft bush seized in second gear. Strange it had been vibrating a bit since then and pulled out its main bearings from the cases.

There are roller type set ups out there for truing cranks but I prefer plain knife edges located on where the main bearings are located.

All to their own I suppose. I now don't have access to specialised equipment so entrust my cranks to others.
 

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