E: Engine Big End Replacement

bmetcalf

bmetcalf

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This Ducati shop used an arbor press to align race cranks for Jimmy Adamo "back in the day". My Vin crank got .075" out of true and they had to put a riser block in their arbor press to accommodate the larger size and true it up.
 
Cyborg

Cyborg

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Perhaps not necessary, but not having any experience beating the daylights out of crankshafts, I'm thinking that using the mill to set the initial alignment will mean less aerobic exercise.

Crank jig.jpg
 
Bill Thomas

Bill Thomas

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Cyborg said:
I was also once young and silly trying to drill everything in sight.
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For me it was a case of have to, I got a bloke to take 3/16" off the side of each wheel, I said now I will have to rebalance them, He said no it will be fine, At the end of a 1/4 mile sprint I could not see the horizon !, So took it apart and did the deed. Cheers Bill.
 
G

Garth Robinson

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timetraveller said:
The way that I true flywheel assemblies as follows. It presupposes that the ends of the main shafts are in good order. If you do not have them then you should be able to use your lather to make a couple of centres. One centre in the chuck and the other in the tail stock. I fit the crankpin into the drive side flywheel, which already has the main shaft fitted. I then fully tighten the crank pin nut. I then assemble the big end, con rods etc. and then press on the timing side flywheel and loosely fit the nut to the timing side of the crank pin. Put it in the centres, making sure that they are tight, but not too tight, and then measure the out of round near to the flywheels on the main shafts as the whole assembly is rotated on the centres. You will now need a block of hard wood, oak is good but other woods might do, and an immovable object. I normally use the concrete foundation of any local structure but others might have access to a large anvil etc. This is when you do your body building exercises. Having worked out in what direction the timing side flywheel needs to move, pick up the whole assembly by the drive side main shaft, taking care to hold the small ends loosely, and hit the block of wood with the timing side flywheel. Go back to the centres and measure again. This will tell you whether you need to hit harder or more often. Start to tighten the timing side crank pin nut, but not fully, and then go back and do the body building again. If the neighbours see you doing this then explain to them that you had wood worm in the piece of wood and were trying to give the insects a headache. It might take a few goes but it is better to sneak up on the correct settings slowly rather than go too far and have to come back on it. Once you have it correct then fully tighten the timing side crank pin nut and go back and check again that nothing has moved. If this does not work then either the flywheels have different distances between the crank pin holes and the mains shaft holes or the faces for the crank pin to sit against are faulty. I have seen the former but never the latter.
To show how things have moved on; I have somewhere a 1940s or 50s motorcycle hand book which advises using a set square against one face of a flywheel and across the two circumferences of the flywheels. This was clearly considered good enough for bike used during the second World War or in the forties and fifties. In the hands of a skilled mechanic this might have been good enough. I'm pretty sure if I tried it then the bike would vibrate the flywheels to pieces very soon.
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I recall reading in a reprint of one of the first 48 editions of MPH where a Canadian member described the ease with which he replaced a twin big end.All that was needed was a suitable socket,a vice and a straightedge.I don't recall the mention of a large club hammer but I'll bet it was in his tool kit.A lot of the spares I have scrounged over the years have plenty of centre punch and cold chisel marks.Those were the days!
 
T

timetraveller

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Vincent, I measure them on the main shafts. On the drive side near to and some distance away from the flywheel. On the timing side where the timing side inner bearing goes. The keyways make it difficult on the timing side. I do not measure up on the outside of the flywheels although I know one racer who makes everything a few thou over sized and then cylindrically grinds, main shafts and flywheels so that everything is true to a few tenths of a thou.
 
Cyborg

Cyborg

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roy the mechanic said:
Something else that has not been mentioned, when fitting the timing side shaft-make sure the oilways are aligned.
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Somewhere I did some reading on aligning the pin as well, but not sure I understand the need. This is a caged bearing with two sets of rollers, so the oil hole in the pin lines up between the two sets of rollers. The ID of the thrust washer is larger than the OD of the shoulder of the pin, which leaves space for oil to circulate 360 degrees, so oil will find the gallery in the pin. I get that you want oil getting into the bearing at it's point of lightest load (thinking that is probably more important with things like cam bushings rather than roller bearings). Is it really going to make much difference.... seems to me that the oil will get into the area of lightest load regardless of how the pin is indexed.
 
Cyborg

Cyborg

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Bill Thomas said:
For me it was a case of have to, I got a bloke to take 3/16" off the side of each wheel, I said now I will have to rebalance them, He said no it will be fine, At the end of a 1/4 mile sprint I could not see the horizon !, So took it apart and did the deed. Cheers Bill.
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I actually wasn't thinking of motorcycles when I wrote that. I should probably keep my warped sense of humour to myself.....
 
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