3.5 MOD.. time to jump back in.

Intlwaters

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Grimracer

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Dec 27, 2001
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Gang, I have not raced a mod tunnel in MANY years... BUT.. 22 looks like its time to jump back in. (I have been racing my sport tunnel in mod when the class is running).. any how.

I have had this CMB RS for MANY years.. Russ raced it back in 08-09 or some time around that and won the WC race at Johns Island with it.. BUT.. I was never happy with the way the motor ran. I pulled it back out and did up a new OS lower unit setup for it.. (machined some parts).. whatever..

The bearings felt good BUT.. I ordered new ones and took the motor apart to prep for it.. and check this out.

If you look close you will see an eyebrow mark on the PTO.. its off to one side.. I started checking the clearance slot for the piston skirt was machined angle off the tangent plane.

The piston had a very small "ding" from the contact.

I tossed it in the mill and squared it up.. then deburred the piston.

I am still surprised. Not sure I would have caught that until after the motor was run some..

Grim

P.S.. I have a feeling this is going to be my Jeffy Preusse "CHASE BOAT"!
 

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Kris Flynn

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Mar 11, 2002
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4,265
WOW, lucky not to granade the motor!

I think those motors also had "issues" with the PTO pin slot not being big enough, or having a sharp edge which caused the pin to snap off the crank?
 

Grimracer

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Dec 27, 2001
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8,867
I will check that closely. I see they do have a camphor in that area. BUT. will look it over real good.

AND.. to add. I have not raced a tunnel boat I have not designed in almost 20 years.. that's about to change too! (Thanks Kris!)
 

john v

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Supporting Member
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Oct 10, 2007
Messages
693
in dist 11 namba mod 21 tunnel has always had a few boats, i think for the 22 season electric ob tunnel is going to be big. lot of guys have them and man they are fast. i have a lot of race boats but will always have a mod 21 ob
 

Geraghty

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Joined
May 16, 2003
Messages
1,636
Gang, I have not raced a mod tunnel in MANY years... BUT.. 22 looks like its time to jump back in. (I have been racing my sport tunnel in mod when the class is running).. any how.

I have had this CMB RS for MANY years.. Russ raced it back in 08-09 or some time around that and won the WC race at Johns Island with it.. BUT.. I was never happy with the way the motor ran. I pulled it back out and did up a new OS lower unit setup for it.. (machined some parts).. whatever..

The bearings felt good BUT.. I ordered new ones and took the motor apart to prep for it.. and check this out.

If you look close you will see an eyebrow mark on the PTO.. its off to one side.. I started checking the clearance slot for the piston skirt was machined angle off the tangent plane.

The piston had a very small "ding" from the contact.

I tossed it in the mill and squared it up.. then deburred the piston.

I am still surprised. Not sure I would have caught that until after the motor was run some..

Grim

P.S.. I have a feeling this is going to be my Jeffy Preusse "CHASE BOAT"!

Take a very close look at the piston wrist pin bosses.....The bosses like to suffer with this type of interference....
 

jaxracer

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Joined
Jan 17, 2005
Messages
766
WOW, lucky not to granade the motor!

I think those motors also had "issues" with the PTO pin slot not being big enough, or having a sharp edge which caused the pin to snap off the crank?
Hey Mike,

Exactly what Kris said, check the pto and crank pin alignment. I ran that motor for years and snapped a couple cranks in the process before figuring it out. The issue is the crank pin and the hole it sits in are just barely out of line. Take a dremel and turn that hole into a slot like the K&B or OS ptos and it fixes it. I never had an issue after that. Fast motor once it’s dialed in. A lot of guys gave up on them too soon. Jimmy Johnson went I think 68mph with one a few years back.

Let me know if I can help once you get it going!
 

Grimracer

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Joined
Dec 27, 2001
Messages
8,867
Thanks Jr... Looking at it.. it needs some clearance FOR SURE..LOL... That was a CLOSE ONE..LOL.. (already opened it up!)

Thanks.

Grim
 

Alan Elzer

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Supporting Member
Joined
May 21, 2006
Messages
1,395
Not to crash this thread but I have cranks, cmb bearings, p/s, rods, cooling jackets & o-rings, o-rings (pto & rear IB plates) lots of buttons for this motor. Message me if interested.
 
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