Author Topic: Undercut Trans? (Read 6815 times)

MattFreeman · « **on:** March 15, 2010, 05:17:28 AM »

Anybody here have an undercut transmission on their CB750? Anyone not have one and have problems staying in gear?

I've got someone telling me it's a must. I'd hate to get this motor put back together and have problems, but the person telling me to do it has a 1000 kit and I'll only have a 811.

Is it really necessary?

Retro Rocket · « **Reply #1 on:** March 15, 2010, 05:20:23 AM »

I am doing the 1000 kit as well and mine is under cut,probably not needed if your bike is stock but any kitted bike would be better off with one....They like to jump out of gear with increased horse power..

Mick

cafenut · « **Reply #2 on:** March 15, 2010, 05:26:50 AM »

ive got an old wiseco 811 kit from 1976. not installed yet but i was curious where you picked yours up at? the undercut gears are not a must. better? well of course. what mod isnt? if you got the means then do it now if the engine is out. if its gonna keep you from riden for some time or put you in the poor house for the next 3 months go with what ya got. especially if it dont have any issues with it now.

MRieck · « **Reply #3 on:** March 15, 2010, 05:41:47 AM »

Undercutting works very well. The first problem with these old 5 speed transmissions is the forks have to move clusters about 1 foot to engage. Anything that helps engagement (undercutting) is a good thing. I've seen plenty of rounded, screwed up dogs. I know Gast will cut them back, hardweld the dog and recut it. Does Gast do it for free? No.
I think the only way you can appreciate the under cut transmission is to do back to back riding with one that isn't cut

MattFreeman · « **Reply #4 on:** March 15, 2010, 03:16:35 PM »

Thanks for the input all.

Cafenut, I bought the 811 kit from a member here a couple months ago. I've heard that you can use the pistons from a cb350 and get the same displacement.

MF

bwaller · « **Reply #5 on:** March 15, 2010, 05:15:54 PM »

I'm almost positive Mike is exaggerating about that foots worth of travel.

MRieck · « **Reply #6 on:** March 15, 2010, 05:49:24 PM »

Quote from: bwaller on March 15, 2010, 05:15:54 PM

I'm almost positive Mike is exaggerating about that foots worth of travel.

You use the left foot to shift Brent....1 foot.

6 speeds have much tighter spacing which is one of the benefits you'll realize with that 6 speed you built Brent.

fmctm1sw · « **Reply #7 on:** March 15, 2010, 05:57:11 PM »

I hate to jump in with a dumb question but what is undercutting a transmission? When I was working on my 175, I looked at those dogs and wished I could get them to engage further. I guess that's what we're talking about. How does one accomplish this?

edit* Ahh, a nice description on APE's site..

cafenut · « **Reply #8 on:** March 15, 2010, 07:39:38 PM »

ive got to check out the cb 350 thing. anyonelse know of this or is it common info?

bucky katt · « **Reply #9 on:** March 15, 2010, 07:42:36 PM »

Quote from: cafenut on March 15, 2010, 07:39:38 PM

ive got to check out the cb 350 thing. anyonelse know of this or is it common info?

i understand you end up with around 12.5:1 compression unless you machine a good bit off the piston crown

MCRider · « **Reply #10 on:** March 15, 2010, 07:47:21 PM »

Quote from: cafenut on March 15, 2010, 07:39:38 PM

ive got to check out the cb 350 thing. anyonelse know of this or is it common info?

Pretty common knowledge. The bike in my avatar had an early 836cc kit from Action 4s back in 1971. Working in a Honda parts dept we identified the pistons as reworked CB350 pistons.

For todays purposes, the amount of work that goes into reworking the CB350 piston is not worth it. relative to buying a ready to go forged kit from Wiseco or comparable.

RRRToolSolutions · « **Reply #11 on:** March 15, 2010, 07:48:23 PM »

MCRider · « **Reply #12 on:** March 15, 2010, 07:51:49 PM »

Quote from: fmctm1sw on March 15, 2010, 05:57:11 PM

I hate to jump in with a dumb question but what is undercutting a transmission? When I was working on my 175, I looked at those dogs and wished I could get them to engage further. I guess that's what we're talking about. How does one accomplish this?

edit* Ahh, a nice description on APE's site..

I've had the trans on my project undercut by APE. Can't wait to feel the difference.

paulages · « **Reply #13 on:** March 15, 2010, 08:23:24 PM »

i'm installing mine on the 718 right now.

Swoop · « **Reply #14 on:** March 16, 2010, 03:21:57 AM »

Curious....what is the going price for the undercutting process?
Thanks
Steve

Bamboozler · « **Reply #15 on:** March 16, 2010, 08:09:01 AM »

Quote from: Swoop on March 16, 2010, 03:21:57 AM

Curious....what is the going price for the undercutting process?
Thanks
Steve

To give you a ball park, APE shows $299 for undercutting a CB750.

paulages · « **Reply #16 on:** March 16, 2010, 08:59:38 AM »

Quote from: Swoop on March 16, 2010, 03:21:57 AM

Curious....what is the going price for the undercutting process?
Thanks
Steve

around $300

KRONUS0100 · « **Reply #17 on:** March 16, 2010, 03:37:35 PM »

hey....aint there one of us here on the forums who can do this? or is APE the only ones?

MattFreeman · « **Reply #18 on:** March 16, 2010, 04:04:59 PM »

Fast by Gast and Cycle Exchange offer these services.

I'd figure that if MReick is recommending Gast then one of these three are your best bet.

bear · « **Reply #19 on:** March 16, 2010, 09:48:30 PM »

We had trouble with ripping the dogs clean out of gears.
Undercutting actually made the problem a lot worse.
We're now using gear sets manufactured by a bloke in Melbourne.
The gears have 4 dogs instead of 3 and are oblong in shape.
No more problems.

Cheers,
Brian

scottly · « **Reply #20 on:** March 16, 2010, 10:12:40 PM »

My 836 popped out of gear BADLY; I almost gave up on it until I learned about undercutting the dogs(and the holes they engage in). I used a dremel, wore out a few grinding stones, and spent some time grinding, and fitting, and grinding some more, in order to remove the least amount of material.
One note: it was necessary, after this mod, to slightly let off the throttle when speed-shifting, as the undercutting pulls the gears together under load.

KRONUS0100 · « **Reply #21 on:** March 17, 2010, 06:09:49 PM »

you did this your self? I was thinking of doing the same thing..only i was gonna use a real slow setting on my drill press with a smaller cut off wheel mounted...or father in law has a lay down drill press. my question is how hard is it to do the insides of the slots?

scottly · « **Reply #22 on:** March 17, 2010, 07:01:26 PM »

Yup. I did it with a hand-held dremel, with fine, tapered stones (bigger on the end furthest from the collet). Only the area of the dog and the hole where the thrust is applied needs attention. This was a last ditch effort, as I had already replaced 1 gear, that helped, but only for a while, and there was no way I was going to buy all new gears. I was careful, though not entirely scientific, to try to remove the same amounts from dog to dog, and hole to hole. (the dogs seemed to need more attention than the holes, if I recall; this was 10+ years ago) Transfer dye would have helped a bunch to disclose the actual contact points, but I wasn't aware of it then. I wouldn't get near it with a cutoff wheel!!!!

KRONUS0100 · « **Reply #23 on:** March 17, 2010, 07:42:41 PM »

ok..cut off wheel idea is out the window.....may just cough up the money and ship em off to be done....am getting ready to go and get 3 cb750 motors that i know are locked up...mebbe one of em has a good tranny

MRieck · « **Reply #24 on:** March 17, 2010, 07:50:01 PM »

Quote from: scottly on March 17, 2010, 07:01:26 PM

Yup. I did it with a hand-held dremel, with fine, tapered stones (bigger on the end furthest from the collet). Only the area of the dog and the hole where the thrust is applied needs attention. This was a last ditch effort, as I had already replaced 1 gear, that helped, but only for a while, and there was no way I was going to buy all new gears. I was careful, though not entirely scientific, to try to remove the same amounts from dog to dog, and hole to hole. (the dogs seemed to need more attention than the holes, if I recall; this was 10+ years ago) Transfer dye would have helped a bunch to disclose the actual contact points, but I wasn't aware of it then. I wouldn't get near it with a cutoff wheel!!!!

You guys are nuts

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Author Topic: Undercut Trans? (Read 6815 times)

Swoop