Author Topic: cb750 under cut trans (Read 3253 times)

Mitchell · « **on:** June 03, 2016, 08:23:47 AM »

I was have been talking to a local guy i deal with sometimes that back cuts transmissions. I was just wondering what the normal costs is for this type of service and which gears to get done. He mostly does 6 speed transmissions so he quoted me 200 bucks to do the whole thing but 5th he said he thought it would be really hard to back cut. So im guessing 2nd, 3rd, ad 4th get cut normally? Any input would be greatly appreciated because ive never had this done and im about to build an 836 for my turbo bike and feel like it will be a must

MRieck · « **Reply #1 on:** June 03, 2016, 08:36:57 AM »

Correct. 5th can be cut be cutting down every other tooth though I don't recommend it. It is a necessary mod if you are making any power.

Ilja · « **Reply #2 on:** June 03, 2016, 01:14:46 PM »

Well to be honest... I dont know if its really that necessary.
My bike apparently makes 111nm of torque but didnt jump a gear a single time during the first 1500km of testing it. Offcourse thats not a long distance but it shows that the gearbox is actually pretty good..

Only when my tarozzi rearset was getting jammed and it didnt go into gear correctly it jumped out of gear.
It shifts a bit rough but def not jumping.

I think if you are using a quickshifter or doing dragraces which is hard on the trans it could be important.
So it all depends on how you use the bike I think.

Jerry Rxman Griffin aka MuthaF'er · « **Reply #3 on:** June 03, 2016, 01:32:34 PM »

Seems to be a good price! We may need his information.

SOHC4 Cafe Racer Fan · « **Reply #4 on:** June 03, 2016, 03:32:37 PM »

Quote from: Jerry Rxman Griffin aka MuthaF'er on June 03, 2016, 01:32:34 PM

Seems to be a good price! We may need his information.

Yeah, Cycle X charges $290. $200 is a great price if it's solid work.

MRieck · « **Reply #5 on:** June 03, 2016, 03:46:59 PM »

Quote from: Ilja on June 03, 2016, 01:14:46 PM

Well to be honest... I dont know if its really that necessary.
My bike apparently makes 111nm of torque but didnt jump a gear a single time during the first 1500km of testing it. Offcourse thats not a long distance but it shows that the gearbox is actually pretty good..

Only when my tarozzi rearset was getting jammed and it didnt go into gear correctly it jumped out of gear.
It shifts a bit rough but def not jumping.

I think if you are using a quickshifter or doing dragraces which is hard on the trans it could be important.
So it all depends on how you use the bike I think.

May I ask where you are from from a geographical perspective. You talk about metric measurements so I am curious.

Bill/BentON Racing · « **Reply #6 on:** June 03, 2016, 03:52:58 PM »

Stock trans like stock rods. ....one day they will decide they don't want to play no more! And yes they will work for a while but eventually the wear happening inside catches up with you! G'luck, Bill

Mitchell · « **Reply #7 on:** June 03, 2016, 05:24:18 PM »

Thanks guys! The trans in it now has been going strong for almost 2 years but has slowly got worse, its to the point now i have to drive it easy or it will jump out of gear every time in 2nd 3rd or 4th. 1st and 5th seem fine but im new to this back cutting thing so i wanted to hear from the pros! The guy's drug of choice is a kz900 but he has worked on quite few 750 sohc's over the years, i only talked to him for a few minutes about it so im going to call him and try to set the priabo in stone on monday. When i was in his shop he was about to back cut a few transmissions i know one was for a gsxr 750, he works on alot of drag bikes and had a kz powered rail car he was trying to get ready for rockingham......i think this weekend or the next but not sure if they got it ready

Mitchell · « **Reply #8 on:** June 03, 2016, 05:26:10 PM »

And by priabo i mean price

PeWe · « **Reply #9 on:** June 04, 2016, 05:00:29 AM »

Under cut is a correction of Hondas design. It should have been there from the very beginning. 2:nd and 4:th gear have always been easy to shift wrong, not getting it to seat correctly when shifting gear.
I learned how to shift back in the days to minimize the gears to not jump that often. This was before increase of the power. Boots under the shifter arm when accelerate can end up in the foot going upwards just a little so gear will jump out to the "false neautral".

I have under cut now and feel no difference except for no jumping gears so far.

MRieck · « **Reply #10 on:** June 04, 2016, 07:29:55 AM »

Quote from: PeWe on June 04, 2016, 05:00:29 AM

Under cut is a correction of Hondas design. It should have been there from the very beginning. 2:nd and 4:th gear have always been easy to shift wrong, not getting it to seat correctly when shifting gear.
I learned how to shift back in the days to minimize the gears to not jump that often. This was before increase of the power. Boots under the shifter arm when accelerate can end up in the foot going upwards just a little so gear will jump out to the "false neautral".

I have under cut now and feel no difference except for no jumping gears so far.

....as it is with modern transmissions. It would have involved extra machining and would have added 20.00 to the cost of the 750.

MessnerMoto · « **Reply #11 on:** June 04, 2016, 09:07:49 AM »

I did race cut 1,2,3,4 for 80eu on CNC. Special tool was made to make 2 degree angle on every tooth... Gears are really hardened. The tool had to be sharpened 2 times..... It was done precisely. At the bottom of the tooth 0.2mm was taken off... I t is important to respect that because you don't want to take off hardened layer of metal. It is hard to see on pictures...

dragracer · « **Reply #12 on:** June 04, 2016, 10:49:52 AM »

Quote from: Mitchell on June 03, 2016, 05:24:18 PM

Thanks guys! The trans in it now has been going strong for almost 2 years but has slowly got worse, its to the point now i have to drive it easy or it will jump out of gear every time in 2nd 3rd or 4th. 1st and 5th seem fine but im new to this back cutting thing so i wanted to hear from the pros! The guy's drug of choice is a kz900 but he has worked on quite few 750 sohc's over the years, i only talked to him for a few minutes about it so im going to call him and try to set the priabo in stone on monday. When i was in his shop he was about to back cut a few transmissions i know one was for a gsxr 750, he works on alot of drag bikes and had a kz powered rail car he was trying to get ready for rockingham......i think this weekend or the next but not sure if they got it ready

My guess is you visit Donald Shaw near Augusta. If so, Donald does great work but be warned, he's slow as molasses on a cold day. Good guy but he's not in a hurry to get things done. He has done boring and case work on a KZ engine for me before.

Mitchell · « **Reply #13 on:** June 05, 2016, 06:36:17 AM »

Yes thats him lol and yes he is slow as christmas. But im not in a real big hurry and his shop is only about 20 minutes from my buddies shop where i work on my bike. Messnermoto how did your transmission hold up after your race cut

dragracer · « **Reply #14 on:** June 05, 2016, 10:08:12 AM »

Oh yeah, and that motorcycle based dragcar belongs to my buddy Keith. He's also a forum member under the name, Voodoo Racer.

Mitchell · « **Reply #15 on:** June 05, 2016, 12:04:08 PM »

Sweet dude that thing was some kinda nasty. Was my understanding right that they were tryin to get it ready to take it to the rock

dragracer · « **Reply #16 on:** June 05, 2016, 02:51:31 PM »

Quote from: Mitchell on June 05, 2016, 12:04:08 PM

Sweet dude that thing was some kinda nasty. Was my understanding right that they were tryin to get it ready to take it to the rock

I doubt they'll let him race it at Man cup, motorcycle engine or not. In my opinion it'll set a bad precedent.

Ilja · « **Reply #17 on:** June 10, 2016, 03:04:13 AM »

My Kawasaki 750 turbo also has a undercut gearbox from factory. :-)
Its basically an upgraded z650 64hp engine from factory (same power as the CB750 stock), same gearbox but I think the z650 didnt have the gears undercut.
Gear shafts 2mm thicker, conrods are like z650 only the wristpin is 1mm thicker, cast flattop pistons, stronger clutch, same hyvo chain and thats it. People have been pushing 200hp with the stock cast pistons and 270hp with 810cc on that stock bottom end with oem rods and the stock undercut trans. gears.

As long as you don't machine the hardened layer of the dogs, it's a good idea to have the gearbox undercut if you race the engine. For touring around on your caferacer.. don't bother and do it when it finally skips after years of use but thats my opinion.

Mitchell · « **Reply #18 on:** June 28, 2016, 07:17:33 PM »

Well i have yet to talk to mr donald about the back cutting, i will up fate as soon as i do.

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Author Topic: cb750 under cut trans (Read 3253 times)