What is a bob weight and do i need?

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Model T Ford Forum: Forum 2015: What is a bob weight and do i need?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Tyrone Thomas - Topeka KS on Wednesday, October 28, 2015 - 01:21 pm:

went to a machine shop to balance crank. The guy said he would have to use "bob weights." what are they and is this an expensive deal?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Wednesday, October 28, 2015 - 01:27 pm:

That must have been a machinist used to V type engines. When balancing a V8 you need bob weights to simulate the weight of a certain percentage of the pistons and rods, but that shouldn't be necessary when balancing a 4 cyl engine.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tyrone Thomas - Topeka KS on Wednesday, October 28, 2015 - 02:03 pm:

Roger, yes, he is a modern engine guy. I ask him to balance the crank. He did say bob weights and that I needed to bring him a piston/ring/rod setup. So do I need to tell him rather than this bob weight stuff, to shave material off? Or maybe I need to find another shop. One that is run by an old timer? I want to do this right.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Wednesday, October 28, 2015 - 02:31 pm:

If you have a regular T crank without counterweights, then there's absolutely no use with the bob weights, talk him out of that or use another shop.

Though I'm no authority on the subject, just repeating what I have been told by people I respect. Here's an old discussion on counterbalanced cranks where Les told us he doesn't use bob weights when balancing - and his engines are fast :-)
http://www.mtfca.com/discus/messages/29/21552.html?1166114824


Top of pagePrevious messageNext messageBottom of page Link to this message  By John T. Tannehill III, Hot Coffee, MS on Wednesday, October 28, 2015 - 03:46 pm:

Maybe ask him about a Hen weigh. If he ask you what's a Hen weigh, tell him about 2 1/2 pounds. Go to another shop. LOL


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan McEachern on Wednesday, October 28, 2015 - 07:57 pm:

A 4 cylinder 180 degree crank shaft does not require bob weights for balancing. Find a shop that knows what they are doing. If you don't plan on balancing your rods and pistons, don't bother with having the crank balanced.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Wednesday, October 28, 2015 - 08:05 pm:

New aluminium pistons are usually very good in balance - all four within a few grams. Original rods is a completely different story - the more to choose from the better, some sets can't be balanced with just grinding, the heavy ones would have had to be ground until their strength was compromised.


Top of pagePrevious messageNext messageBottom of page Link to this message  By David Stroud on Thursday, October 29, 2015 - 03:07 am:

Tyrone, if you can't find a good shop, let me know. Dave


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