A misunderstanding not a gearing question

[Deoodles]

Interesting inspection results. Can I get some help interpreting I find it interesting that one surface is gone on 1 shift fork but the other side of the same fork is fully intact and the other shift fork is like new. what happened to the crank did I overheat this engine that doesn’t explain that shift fork. The shift forks were new and it shifted like butter always. The engine ran great no funny noise

 

[69ST]

That looks like a combination of heat treating & used motor oil on the big end of the rod. If the bearings are still tight and the clearances haven't changed since it went in, I wouldn't worry about it.

The prematurely shredded fork, OTOH...ruh-roh. Time to inspect the gear. I doubt that it's loose & wobbling on the shaft. More likely there's a rough face on that side of the gear, leftovers from a previously burnt shift fork that you replaced. It might be possible to clean & polish the contact surface on the gear. "Possible" is not the same as "easy". This is close-tolerance work, with no metal thickness to spare and next to no room for tool access. That said, the gear is made of harder steel than the shift fork, obviously, so any roughness is likely to be softer shift fork alloy, that can be removed. It may be easier to source another gear.

 

[Deoodles]

Thanks, another gear sounds like the best way to go. I hoped it wasn’t an incorrectly assembled motor that was at fault.

 

[kirrbby]

That looks like a combination of heat treating & used motor oil on the big end of the rod. If the bearings are still tight and the clearances haven't changed since it went in, I wouldn't worry about it.

The prematurely shredded fork, OTOH...ruh-roh. Time to inspect the gear. I doubt that it's loose & wobbling on the shaft. More likely there's a rough face on that side of the gear, leftovers from a previously burnt shift fork that you replaced. It might be possible to clean & polish the contact surface on the gear. "Possible" is not the same as "easy". This is close-tolerance work, with no metal thickness to spare and next to no room for tool access. That said, the gear is made of harder steel than the shift fork, obviously, so any roughness is likely to be softer shift fork alloy, that can be removed. It may be easier to source another gear.

It seems like that shift fork would have to be bent, tweaked, or cocked...or something, to wear like that..? Even if the gear was rough, why would one pad be gone, and the other looks fine...on the same side of the fork?

 

[69ST]

It seems like that shift fork would have to be bent, tweaked, or cocked...or something, to wear like that..? Even if the gear was rough, why would one pad be gone, and the other looks fine...on the same side of the fork?

I'm glad you brought up that possibility, it's logical & obvious. However, I seriously doubt that this tranny has seen any abuse...and...if it's tweakage, the entire fork would be affected, not just one side. And there are no further signs of shift fork erosion on 6 of 8 contact pads. Have to assume that the proper high-zinc oil was used, which leaves one possible cause standing proud of the rest. That's why I left this particular rabbit warren for someone else to explore.

 

[Deoodles]

Yes GN4. But not 6 of 8; 7 of 8 are good. I think I may have the answer. I looked at the fiche and have 1 to many thrust washers in here. There is 1 for a 4speed and I have 2. 1 on each shaft. I hate to say I’m the cause but it looks like I am. The secondary shaft has one but the primary does not. It simply could be a space issue.

 

[kirrbby]

The ATC70 transmission DOES use 2 thrust washers, but the H trans only has 1.

 

[Deoodles]

Well that makes it as clear as mud. I have no idea what to do on reassembly. At least it explains how I got 2 in there.

 

[69ST]

If an incorrect stack of thrust washers is the cause, it'd be good news...IMHO. You'd know what happened and how to correct it. FWIW, I agree with your assertion that this is clear as mud, for the moment. That said, I've never had a problem figuring out whether a thrust was was needed, or not. Got an extra lower end you can test? Try adding & removing thrust washers, checking endplay. I think you'll quickly see for yourself what I'm rambling on about. Excessive endplay is easy to recognize, binding even easier...and before you'd ever button-up an engine.

 

[Deoodles]

I’m interested in using an AHP 4 speed trans in this build. They say these transmissions are good up to 117cc what’s the difference in torque or HP for 124 vs 117. Is it really worth worrying about?

 

[69ST]

I'm still on the fence when it comes to the AHP tranny. Behind-the-scenes, word had it that ~120cc displacement was pushing the limits. However, I never was able to get any kind of serious, objective, specs of any kind...keep the mud goggles handy. Some riders feel that the aftermarket gears are significantly noisier than OEM. All of which having been said, my reservations are based on one, largely objective, factor...lack of real mileage. Longevity remains the unknown country. Seriously, I'd like to see how the assembly holds up after 10-20,000 miles, before passing judgment.

I've gone through a number of CT70, CL70 & SL70 motors over the years; those latter two models typically had 15,000+ miles on the odometer. Rarely saw one that needed more than forks & pins, at that stage. OTOH, a few have raved about the AHP being a "great" transmission with most having 3-digit mileage. The highest mileage total I can recall, offhand, was ~3K. So, rather than badmouthing what may be a good product, I try to remain objective. Thus, IMHO, it's inconclusive, a.k.a. "the jury is still out", due to insufficient field service history.

Best I can offer is the fact that it's an inexpensive setup. With just a little finesse, which I think is well within your skill set, I doubt that you'll break parts. Further, a road bike is probably going to use 4th gear more than the other 3 combined...which bodes well for clutch, shift fork and gear life. Think of it as the small bike equivalent of "highway miles".

I believe that Honda used an updated version of the 4-speed gearbox, up to 2012. What I don't recall is the model designations (C70, C90?) as none were sold in the US. If I had the time, I'd search through CMSNL listings and contact David Silver Spares (UK), for the details...and the parts.

Wild-assed parting shot...
The 3-speed Honda gearbox has wider gears and known metallurgy. It can also be easily paired with a manual clutch. If you can live with a 4-speed that lacks second gear, it's the small Honda analog of a GM Powerglide...a.k.a. "the poor man's Lenco".

 

[Deoodles]

Ohhhh you didn’t just say what I think you did I don’t like the idea of replacing a shift fork and the associated gear when I can’t find the cause of the failure and, reinstalling it. My guess is I’d spend close to a buck and it could happen again. If there isn’t a big difference in power between the 117 and the 124 I could do it. It is fun to think about a 124H 3 speed though.

 

[kirrbby]

It's only 7 cc's more

Same size piston

 

[69ST]

It's only 7 cc's more

Same size piston

There's only 1CC difference between a 52x51 stroker, 50x55.5 stock Nice, and a 52x52 stroker...108, 109, 110cc, respectively. The torque and powerbands are wildly different. All else the same, nothing affects torque output like stroke length. That is the point of this thread, in a nutshell. Is that enough to push one of these transmissions beyond mechanical limits? There's no definitive answer, at this time. Best we can do is make an educated guess. So, now is the time to raise all of those painful issues that most would rather avoid. IMO, better to "overthink" and agonize now. Once the machine is buttoned-up, going back in would be a lot more agonizing.

The one thing going for OEM transmissions is field service history...closing in on a half-century now. That's unparalleled depth and something that could reasonably substitute for lack of engineering data which we will likely never have. IMO, engineering and manufacturing a transmission is the most difficult & costly part of these bikes, bar none. So, if there were an inherent weakness with the OEM 4-speed, we'd know about it by now...burnt shift forks, as you experienced, would be as common (and widely known) as tuliped intake valves. But they aren't.

If one is really uneasy about these two tranny choices, there is a 3rd option...Takegawa 4-speed.

 

[Deoodles]

I really had to know what happened to this transmission I spent a good bit of this afternoon really looking at it. The shift fork seems slightly bent meaning one of the contact pads is lower than the other and it wore into the gear. I’m not sure what bent shift forks really means but I do think if I replaced the gear just so I have a clean one and put a new fork in it it will be fine. There is no indication that the gear has any damage to the naked eye but it’s hard to see that surface. I don’t know how to find that gear it’s 31T it’s countershaft second gear anybody have any cross reference or part numbers that might help me locate it.
23441-065-010?
23441-065-901?

 

[69ST]

Here ya go. FYI, I used your PNs and did not verify them using a Honda parts book...because that's back in my office. HTH

To the best of my knowledge, if the gear looks fine it probably is. Shift forks get bent: in-transit/storage/industrial accident, from bang-shifting, dumping the bike on its LH side. Of course, defective product is a possibility. Who knows, WGAF, at this point? As long as you get good parts, it's "problem solved...world saved...crisis averted".

 

[Deoodles]

Thanks, OEM fork and gear on the way. I think I’m about ready to move forward.

 

[Deoodles]

Next up is cam chain tensioner. There are a few different versions. Will the one I have in my old style case work in the new style or will I have to get a new style? I just realized the case bolt I use for my fuel valve was removed in the 12v cases. Not happy about that.

 

[Roy Chambers]

Thanks, OEM fork and gear on the way. I think I’m about ready to move forward.

TRANSMISSION CAUTION : Hi, I have been following this post from the beginning!
Because i am new to the forum, i hesitant to help.

racerx, suggestions as to what might have happened to the Shift Fork was very good ones to start with, but i am pretty sure that it is going to do it again!

After working on BSA 250 Transmissions, and their known problems, i was able to get them into a usable condition.
These Transmissions have many problems, one built in problem is the fact that you are trying to shift from First Gear, which has a Ratio of 2.65 to 1 in to Second Gear, which has a Ratio of 1.60 to 1. The Gears a not spinning at the same speeds. That's when you hear that audible crash sound from the gearbox.

Buying the Revised Shift Forks, Gears, and Cam Plate, did nothing to correct the problem.
( Cam Plate ) , does the same thing as a Shift Drum, just in a different way.

So, That's when i put a old Crankcase up into the Milling Machine and Milled a 3" square opening in the back of the Crankcase, and 1.5 X 4" opening in the top.

After trying 14 new and used Shift Forks, i could see that only 2 of them were sliding the the Gear into the correct positions both ways. Most of them, would move the Gear into an OK position one way, BUT not enough the other way OR not enough one way and way too much the other way, still putting pressure on the side of the Gear!
That's a formula for a BURNED UP FORK.

Keep in mind, we are talking about British made Machine, and this is a Honda forum and i am not going to list all of the reasons for BSA demise, way too long. Honda holds Very Good tolerance on the parts and workmanship.



This is what i would do!
1,Forget about what the Shop Manual OR the Parts book say's about how many shims go on each end. The Bike is 45 years old, you don't really know if they used 0,1,2,or3 shims, in production. Things could change on the production line because of this part or that part, they are not going to mention it in the book.

2, Set the Left Crankcase up on 2x4s, and use a level to level it up the best you can on the gasket free mating surface, drop the Shift Drum in the Crankcase along with the Counter Shaft. Make sure you hear the drum clunk down in the Case.

3, Rotate the Shift Drum to First Gear. Did Shift Drum come up off of the Case SLIGHTLY?, NO, go to step 4!
If YES? Make Shim for Shift Drum Spud. ( raise Shift Drum up )!

4, Rotate the Shift Drum to Second Gear,,, turn it left and right slightly, making sure that the Fork is at its highest spot.
This is the big question,,, at this point,, did the Counter Shaft, OR mainly 3rd Gear raise up ???

If it did, then that's why your Shift Fork BURNED UP!!!

To correct this, you must move 3rd Gear up. Thicker Spline washer OR shim under Counter Shaft OR different Shift Fork.

Remember what ever you do, you must make sure that you have some end play on both the Counter Shaft and the Main Shaft when the whole thing is together and the Gasket is in place and all the Screws are torqued up.

You can also check the Main Shaft and Gears in the manner, and i think that you should. It's a good practice to check everything that you can possibly check.

I hope that everyone understood what i was trying to explain, not to good at writing it down.

Thanks Roy.

 

[69ST]

Excellent info!

The one place where I have to disagree is the C/S & M/S shims. The numbers used were either 1 or 0, with absolutely no doubt. That eliminates the question of whether two might have been factory-installed. Adding a second shim, where one had been used, or one where none had been used, will result in the gear cluster/shat assembly binding.

Interesting solution, changing the thicknesses of various parts. There's very little room to work with, on the shafts, I'm talking thousandths of an inch. It'd take some high-precision, trick, machining and levels of expertise that are beyond me. But, before considering exotic machining solutions...which may turn out to be THE answer...I'd want to know what was/is out-of-parallel. This kind of unequal fork tip erosion is atypical for these transmissions.

IMHO, if you are correct, the most likely solution will be re-machining the shift fork. That's a whole can of worms, in and of itself...