K&B 7.5 connecting rods

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K&B 7.5 connecting rods

Postby Rocket-J » Fri Mar 06, 2009 11:49 am

I have a K&B 7.5 9100 df engine, New in Box, that I am modifying for marine use. I have alreary purchased the marine head from MECOA.
I was told the Square connecting rod, in these old engines, was too soft. I have the new Tubular Rod to install in it.
Question; The tubular rod is smaller (less mass) than the original square rod. Can this rod be used without rebalancing the engine? Also, is this new rod actually stronger than the larger squared rod?
Thanks for the info.
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Postby SteveM » Fri Mar 06, 2009 2:45 pm

When I raced the K&B 3.5cc motors we customarily replaced the stock rods with the machined version from RPM which were very square and beefy. I would think the bigger the better. if you have a scale you could weigh the connecting rods and I doubt you'd have any balance issues.
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con rod

Postby Rocket-J » Fri Mar 06, 2009 3:41 pm

I don't have a scale for that light of item. I could not find an RPM rod. I don't think they are around anymore. I agree with you about bigger is better. But I was told by another racer that the alloy they used 30 years ago was too soft, thats why I bought this newer K&B rod. It just looks small compared to the old rod. Thanks for the info.
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Postby USMC » Mon Apr 20, 2009 1:33 pm

Can you post a pic of the rods?
and give the part#
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7.5 rod

Postby Rocket-J » Wed Apr 22, 2009 8:41 pm

Hello USMC.
I have a 1970's model 7.5 Ducted Fan motor. It is New in the Box. When I say old or new rod, they are close to antique. I have installed a water cooled head and exhaust flange. I will use it in a boat.
The newer rod is installed now and I don't have the part number anymore. The original rod is crude and very squared off while the newer is well machined and round. Both are made by KB. The newer one is supposed to be stronger with a diffenent temper, but it is definetely lighter. I will try it and see what happens. I don't have a ton of money invested.
Thanks for the interest. Al
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Postby USMC » Wed Apr 22, 2009 9:32 pm

I have these con rods Part# 7982 & a Marine Head (7.5 )
take a look at the con rods are they the same as yours?

http://i707.photobucket.com/albums/ww77 ... MG0449.jpg

I live in Madera,Calif.
Take Care,
USMC & Viking...! & Little Jack...! :)
USMC
 
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Sorry for long delay.

Postby Rocket-J » Fri May 08, 2009 8:52 pm

The connecting rod that I replaced looks like yours. The newer rod that I purchased here from MECOA is tubular shaped.
The boat is done and seems to be running fine so far. I am not through with the break-in so I have not stressed it yet. We will see what happens.
Thanks for the help.[/img]
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Postby K+B.91 » Fri May 15, 2009 5:30 am

USMC, Rocket3,
I have the older 'square' rods in my engines, and yes they're softer than the new 'round' rod, but guess what, they handle the higher RPM of marine use better than the newer 'round' harder rod.
I would beusing the older style 'square' rod if it were me.
K+B.91.
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Postby USMC » Sun May 17, 2009 10:37 pm

K+B.91, Hey Thanks for the info.!

and Rocket the con rods I have are new old stock, square w/bushing
if you want one for a spare its yours.

I have other 7.5 stuff also,
I only run 3.5 outboards engines.

Take Care,
USMC & Viking...! & Little Jack...! :)
USMC
 
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