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Old 02-14-2020, 04:25 PM   #1
vintagewrench
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Default B18-B20 Racing Crankshaft Balance



This B18 crankshaft which is going to be used for vintage road racing on a variety of different tracks and hill climbs after it is ground and re-balanced.

The standard rod journal dia. is 2.135 but it is going to be ground approximately .246" to 1.889" and Carrello connecting rods with a Honda Clevite CB1663 (or possibly a 1.850 Clevite CB1798) rod bearing are going to be used. The grind will remove some weight plus the rod bottom end will be smaller, narrower and lighter and will have only about 380 grams of rotating weight.

Photos found of other Volvo racing cranks show counterweights that have been lightened considerably. I would like to cut down the counter weights here on our heavy-duty lathe (balancing shop does not what to do it because of only having a light weight tool room lathe.) to above the final finished target weight before it goes out to be ground and balanced. This will end up leaving a lot less time spent at the balancing shop and in overall time it can be finished faster this way (spring is on the way). Also will round off or knife edge the weights in the milling machine.

Can anyone give some guidance on how to approach this project and would it be better to narrow the counterweights, which has been done in the past, or take it off of the OD of the weights assuming it can be balanced if it is done that way. Looking to have it run as smooth as possible in the 2500 to 7500 rpm range.
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Last edited by vintagewrench; 02-16-2020 at 11:10 AM..
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Old 02-15-2020, 04:57 PM   #2
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Originally Posted by vintagewrench View Post
(balancing shop does what to do it)
I'm a little confused, did you leave something out? They want too much money? Or they do NOT want to lighten it?

I have opinions on this, based on first-hand failures at this ordeal.
Either find someone who has lots of experience, or skip it and just balance it.
Short story, I broke a 4A-GE crank doing what you want to do, and the machine shop that was letting me use their huge lathe told me they don't see any advantage anyway (they build and race a lot of stuff, big and small). They prefer to lose the weight out at the perimeter of the flywheel, where it matters. After that I switched from the double-weight crank to a single-weight crank from a 4A-C, and felt no difference. Later I removed about a pound from the perimeter of the flywheel. I felt and measured a difference.

Always send all your stuff together for balance. Rods, pistons, flywheel, clutch cover and bolts, front balancer/pulley.
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Old 02-15-2020, 05:20 PM   #3
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Alway heard knife edge would create less windage. Then balance the crank with rods, piston, rings and it will be fine. I can’t afford to race but always balance engine that I build.
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Old 02-16-2020, 11:09 AM   #4
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Originally Posted by mikep View Post
I'm a little confused, did you leave something out? They want too much money? Or they do NOT want to lighten it?
Sorry, I meant say: balancing shop does not what to do it because of only having a light weight tool room lathe.

The 22lb lump of a flywheel is being replaced with a 8lb Fidanza piece.
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Old 02-16-2020, 01:58 PM   #5
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Quote:
Originally Posted by mikep View Post
I have opinions on this, based on first-hand failures at this ordeal.
Either find someone who has lots of experience, or skip it and just balance it.
Short story, I broke a 4A-GE crank doing what you want to do, and the machine shop that was letting me use their huge lathe told me they don't see any advantage anyway (they build and race a lot of stuff, big and small). They prefer to lose the weight out at the perimeter of the flywheel, where it matters. After that I switched from the double-weight crank to a single-weight crank from a 4A-C, and felt no difference....
^Yup. I looked into turning a b230 crank down, and all the engine builders I talked with said it’s a bad idea on a 4cyl that sees high RPMs. Secondary harmonics and crank twist are increased when weight is removed. The crank in the picture above is missing about half the counterweights a b21/23/230 crank has... I wouldn’t lighten it anymore than needed for balance.

Rotational moment of inertia formula has a r^2 term right next to the mass variable. Removing weight from a crank sees very little return, unless it’s grossly overweight.
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Old 02-16-2020, 05:43 PM   #6
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Originally Posted by culberro View Post
^Yup. I looked into turning a b230 crank down, and all the engine builders I talked with said it’s a bad idea on a 4cyl that sees high RPMs. Secondary harmonics and crank twist are increased when weight is removed. The crank in the picture above is missing about half the counterweights a b21/23/230 crank has... I wouldn’t lighten it anymore than needed for balance.

Rotational moment of inertia formula has a r^2 term right next to the mass variable. Removing weight from a crank sees very little return, unless it’s grossly overweight.
culberro thanks for your reply. Even though some b18-B20 racers are doing it I have also been advised not to do it on the Speed Talk forum.
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