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First I have to apologize in case this is a double post. I am brand new to the forum here and I tried to post this about 10 minutes ago but I think I misstepped when I did not see the captcha notification.
I'm an old triumph Bonneville rider from back in the 70s. I had three of them through the 70s and loved them all.
I recently picked up a 72 Bonneville 650. Finally got it running well enough to ride and discovered that it is a 5 speed gearbox serial number with a 4 speed conversion in it. When I ride the bike the transmission will pop out of third gear if I accelerate at all aggressively while in that gear. If I ride conservatively you would never know there was anything wrong with the bike. All other gears stay in under heavy acceleration. I have pulled off the outer gearbox cover several times and inspected things as best I could.I re-tensioned the leaf spring which seems good and strong and lines up nicely with the shifting Cam. I can't see anything visibly wrong on the main shaft or lay shaft or gears , but I have to admit I really can't see a whole lot from that side . Does anyone have any experience with these transmissions popping out of third either with a 4 speed conversion or otherwise?
Thanks very much.

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Can't help with your problem. Look at your title. If it says T120RV then it was a 5 speed when sold. Why anyone would change it to a 4 speed is beyond me. That's the trouble with these old turds, there is no way to tell what some DPO did to them and why.


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Hi moto, I suppose it's a possibility that the original 5-speed transmission was one of many that failed under warranty and sat in the dealership service department for months on end while waiting for warranty spares to arrive from Meriden. Perhaps the customer just got tired of waiting and wanted to ride the bike, so the dealer retrofitted a 4-speed to get the bike out of the shop, who knows? Often times, the dogs on the gears get rounded off with wear, and this combined with wear on the selector forks can allow for gear disengagement under load as the worn selector forks fit loosely in the grooves on the sliding gears. Other times, the leaf spring gets worn and hold camplate engagement under heavy loads. You may need to go deeper into the gearbox to ameliorate this issue. Best of luck.
-Dave


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my 72 T120 has T120V on cases but has 4 speed with plunger someone in its murky past installed never missed a gear


1972 Triumph T120
1968 BSA A65
1968 MGB Roadster
1979 Chevrolet Camaro Berlinetta
1969 Honda Mini Trail
2004 Honda Shadow Aero
1949 Ferguson TE20 tractor
1975 yamaha xs650b
1971 SL 175 Honda project
2 olive drab WWII military bicycle replicas
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moto96s Offline OP
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Thanks everyone for the replies.
I was told by one of the parts houses/shops that some of the 5 speed transmissions were problematic and some people changed them over to four speeds after several attempts to get the five speeds to work properly. My cases are marked T120RV kind of like raf940's so I am assuming it was originally a five speed. My manual shows the leaf spring for that year, but I'm also told the plunger in the case was a much more positive system and better than the leaf spring. I understand it's a big job to install one though. Does anyone know off the top of their heads if it's possible for me to pull all the gears and whatnot off the two shafts without taking the primary apart? I don't want to start making a mess and tearing everything apart if I can never get it back together again. If I can pull all the gears off with the shifter forks and the shaft the shifter fork rides on, then I can inspect those dogs on the gears and see if that's where the problem lies. Thoughts?
Thanks again!

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i have a 72 T120RV that had the explosive gearbox, and it did indeed explode. i fixed it for twenty five years with a piece from another exploded gearbox and recently installed an upgraded CP1000 gearset. end of that potential problem.

switching to a four speed was a typical recommendation back when parts were hard to come by. bob raber in san jose suggested i do that, before i made him sell me used 72 five speed parts.

i switched my leaf spring to a plunger. there was no change in shifting.

the four speed is easy to take in and out, but you ought to do it all the way. dont use the manual. the competent people were on vacation when it was written. use hermits procedure instead:

https://hermit.cc/tmc/technote/gearbox/assemble_first_gear/index.htm

you need to look at the engagment dogs on the sides of the gears. theyre not undercut, as are the five speeds, and if theyre worn round you csn experience what youre describing.

i dont know how to check shifter forks or their corresponding rails without access to a NOS one to measure

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moto96s Offline OP
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Kevin, that link is a treasure trove of information. Thanks very much for sending it! I tried doing a search before posting but got way too many results to sort through. I'll do some research on the CP1000 gear set and see if that's available for the four speed as well. I was hoping to take third gear out just to inspect it before the end of the riding season here in Pennsylvania, but I'm guessing I'll be better off just babying in 3rd for the next month or so. Then I'll put the bike up for the winter and take the primary apart as it seems that is my only option to really get a look at those 3rd gears. In all my searching I ran across an upgrade kit for the clutch also. MIne needs some help, or maybe my 66 year old hands need help smile Anyone have advice or opinions on a clutch upgrade?
One more bit of information on my gearbox, I read somewhere that the inner cover was meant to be installed with Hylomar, not a gasket. Someone said the gasket thickness was sometimes enough to cause problems. Does anyone use Hylomar to seal the inner gearbox cover to the case, or do most folks use gaskets like me?
Thanks!

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thank hermit.

hes the guy that collects all tbis stuff in one place. he posts here

the CP1000 was just a fix for early five speeds. i dont normally use gaskets on the inner or outer covers of four and five speed 650s.

however.

after i installed a 1971 five speed that had been upgraded with tbe CP1000, the layshaft would jam as the inner cover bolts were tightened, unless i also installed a gasket as is used in tbe later motors. the CP1000 modified assembly was just a bit wider.

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There is not much carved in stone when it comes to these old turds. I have been using an inner and outer gasket on my gearbox for 35 years. It's kind of like do what you think is right and see what happens. Keep notes on the appropriate pages in the shop manual. You do have a genuine Triumph shop manual, not Haynes, not Clymer, don't you?


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Kevin,
Understood on the CP 1000 mods. I am blown away by the detail in Hermit's posts. I did really like "the competent people were on vacation when it was written." Great stuff!
Desco,
I do have a "genuine copy" of the genuine Triumph Workshop manual and it is a good copy, Still leaves something to be desired in certain sections. Maybe I need an owners manual as well to fill in some of the gaps. I don't really find the section on the Amals that great either.
After going through both hermits pictures and notes and the manual one more time, I think I'm gonna try to pull off the inner gear case cover once more and see if I can get the third gear off of both the main shaft and the lay shaft. I've had the shifter fork shaft out of the transmission accidentally before, and if I can get those shifter forks off the gears it looks like I might be able to slide third gear off of both the lay shaft and the main shaft. Hopefully I can put it all back together again and get some ride time in before the big winter tear down.
Thanks guys!

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bite the bullet and pull the clutch and gearbox sprocket

thats the only way to look at the main shaft

DO NOT TAKE THE BOX APART BEFORE YOU PULL THE CLUTCH OR YOU WLL SAY BAD WORDS

its much easier to remove the gearbox sprocket when th egearbox is still assembled

DAMHIK

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I've only had one Triumph with a 4 speed. The only trouble I had with that was the occasional missed shift because of the leaf spring. I did have to disassemble once when a broken chain wrapped around the sprocket. Everything but the main shaft and high gear can be removed without touching the primary drive or clutch.
The only bike I had to jump out of gear was a BSA A65. That was due to worn dogs, but the BSA dogs are less robust than the ones on Triumph gears.


"Gosh, it's not a 1/4 20, must be metric."

72 T120V cafe project "Mr. Jim"
72 T150V "Wotan"

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Understood Kevin!
That's what I'm here for, to get help and advice from people who know. I'll baby it for a few more rides and then take it apart proper when I put it up for the winter. I am thinking of changing my mainshaft sprocket from a 19 tooth according to my manual, to a 20 tooth when I go through the gearbox. It looks like I am reving at about 4k when doing about 50mph. Not sure if the speedo or tach are off? Or maybe some installed an 18 tooth sprocket during the conversion to 4 speed, I have no idea why. At any rate, according to the manual I should be doing about 3300 RPM at 50. I tried to count the teeth on the counter shaft sprocket the other day but my back was already sore from screwing around with other things.Maybe I'll take another look at it tomorrow .

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Desco thanks for the links!
Looks like some great reading on the Amals and I saw some info on the clutches as well.
Much appreciated!

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Hi David,
Thanks for chiming in! You have given me hope. It kind of seems like it should work, but I am sure it will be a major pain in the a##$. I need to get a bike lift or build a stand out of two by fours and plywood to put the bike up on while I work on that transmission.The bending over to get down to it is killing me smirk

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https://photos.app.goo.gl/XxnfSikWMzyjFcWS8

Folding metal sawhorse, straps from handle bar bar mounts to eye bolts on sawhorse, 5 wheel office chair to sit on and roll from one side of the bike to the other. Push the bike up a ramp that you probably already have. Takes some juggling to strap it down put I do it myself.


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I'm curios as to how one retrofits a 4 speed into a 5 speed case. One of the first modifications to fit a 5 speed where a 4 speed was original is to enlarge the case opening for the high gear. How does one make it smaller?
Let us know what you find for a sprocket. Most stock Triumph gearing results in about 60 mph at 4000 rpm. The stock sprocket for '72 was 19 tooth. I actually put an 18 on my Bonneville, but I only ride it on back roads so I wanted a bit more drive out of corners. The only gauge on that bike is oil pressure, so I can't comment on rpm vs mph.


"Gosh, it's not a 1/4 20, must be metric."

72 T120V cafe project "Mr. Jim"
72 T150V "Wotan"

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The enlarged hole in the bearing shield has no effect (the inner gearbox cover is also modified).

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Desco, that is a great setup! And you get that up by yourself?? I am impressed! I like the metal sawhorse idea and I will probably steal that from you. Beautiful bike! I do use a rolling office chair in my shop as well, works great.
David, I've thinking about checking my oil pressure with a gauge. What a great addition it would be to my gauge set to add a permanent OP gauge. I'll count teeth on the sprocket today and get back.
Hi TinkerToo, thanks for joining. During my original searches on Google I saw something about the plungers in the gear change mechanism being different between the 4 and 5 speed. I'm referring to the plungers that actually engage the shifter quadrant when you operate the shift lever. I believe I checked them at that time, but that was a while ago. Do you happen to know if they are different and if So what the difference is?
Thanks

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Both the plungers and the quadrant that holds them are different. John Healy wrote an excellent article in Vintage Bike Magazine on 4 to 5 speed conversion with pictures. Not sure I can link it on my iPhone, but a search on John Healy 4 to 5 speed will get you there. I believe there is a mistake in the article on the round nosed 5 speed plunger, but that is no interest to you.

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Against stupidity
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Here's what happens when you don't do it right;

https://photos.app.goo.gl/uNnzSdHqSd9u5Ch1A

Other than a tiny rip in the seat cover all was well.


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Tinker, thanks for the info!
Hugh, Thanks for the link!
Desco, Holy Crap man! That looks like a disaster. Glad you walked away from it and only minor damage to the bike!
OK, I had some more time to play with it today. Reset the float levels on my new AMAL Premiers one more time. Think I got it right this time.
I counted the teeth on the countershaft sprocket and it definitely is a 19 tooth. Took it out for a short ride and I'm definitely seeing 4K on the tach at 50 miles an hour and it sounds to me like the engine is revving that high. Going to get my better half to help me calibrate the Speedo tomorrow, but I have a feeling it's right. If the speedo and the tach are right, something is wrong.
I have been reading some of the information from the links that you all have posted and I am starting too wonder if this isn't one of those bikes where the 5 speed exploded and took out the case. I noticed when I first started tinkering with the bike there was a bad mismatch between the engine casing and the transmission casing where they come together underneath the pushrod tube. I'll post a picture when I can. Between the speed and RPM mismatch, and the way the bike pops out of 3rd, I wonder if somebody didn't cobble together a 4 speed the best way they could with whatever parts they could find to get it running, and then sell it . Certainly just a theory at this point, hopefully that's not the case.

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The early 5-speed didn't necessarily explode, but the first gear dog was weak. Many times that dog broke and sometimes took out the case. The one on my Bonneville probably broke at some time, there are scars inside the case as evidence. I did need to replace the lay shaft after mine broke between the fourth and top gear. I did the same on my '72 Trident just to be certain it would not happen on that bike.

GBbooboo.jpg

"Gosh, it's not a 1/4 20, must be metric."

72 T120V cafe project "Mr. Jim"
72 T150V "Wotan"

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