Replacing Worn Synchro Rings

By Olin Kane

Originally published in the Austin-Healey Magazine, February 1996

Note: Better, harder synchro rings are now available but were not at the time of this writing.


Once you've fixed all the big things that are wrong with your Healey, the little things start to gnaw at you. In my case it was the crunch when I shifted from 3rd to 4th and 3rd to 2nd. If I timed it just right I could do the 3rd to 4th without a crunch but I had to do it just right. If I was in a hurry or forgot, crunch. There was no way to avoid the 3rd to 2nd crunch at any speed over 20 mph so I just quit shifting from 3rd to 2nd unless I was at a crawl.


The problem was worn balk rings (synchromesh rings in yank parlance). These are essentially brass clutches that as you shift from one gear to the next, bring the two gears up to the same speed before you actually engage them. When they are worn, the two gears aren't rotating at the same speed when you engage them and, crunch. There is only one solution, replace them. There is a significant amount of labor involved and many of us live with the crunch rather than pay someone $500+ to fix it. However, it is possible to do the job yourself. It's not easy and should not be undertaken lightly but if you've got time, tools and a fair amount of mechanical experience, read on. I just finished doing mine and it cost me about $80 and took maybe 15-20 man-hours.


First, a couple of assumptions. My car is a BJ7 and looking at my Bentley manual, it appears that BN6 and earlier cars had a different transmission/overdrive. So what follows may not apply to them. Speaking of Bentley, I’ll assume you have such a manual or a similar one and won't cover issues addressed in there. I’ll concentrate on what Bentley doesn't tell you.


First steps


The first step is to remove the clutch housing, transmission and overdrive assembly. Remove through the interior of the car as per Bentley. Roger Moment has written articles for both Austin-Healey Magazine and Chatter (See AHMAG November 1995) on how to do this. I removed the propeller shaft completely but otherwise followed Bentley. The transmission assembly is quite heavy (probably around 100 lbs.) and one trick I used to make it easy to handle was to support the assembly from above. I have two hydraulic floor jacks (on wheels) so I placed them on the car floor on either side of the transmission, placed an oak 2 X 2 across their lifting pads and used a strong rope to suspend the transmission from the 2 X 2. I could then take up the weight of the assembly by cranking up the jacks and then moved the jacks towards the rear of the car to disengage the transmission from the clutch. Once free of its shackles, my daughter helped me haul it out of the car an up onto some concrete blocks.


At this point remove the clutch housing and OD unit. Plan to replace the oil seal in the clutch housing and perhaps the release bearing (throw out bearing) if it's not relatively new. Check other parts for excessive wear. Clean and lubricate as appropriate. At the other end, pull the driving flange (connects to the propeller shaft) off the OD output shaft with a gear puller so that you can replace the oil seal there. Examine the rubber mounts and plan to replace them if they're torn or squished flat. That's all you need to do with these two components.


Now the beast itself. Take your time disassembling the transmission. There are more parts in there than in the space shuttle. Follow the Bentley flow. Every time you remove a part, draw a little diagram of where it goes if it's not abundantly clear in the Bentley diagrams, clean it and stick it in a baggie with its near neighbors. The fork selectors are brass and will surely show some wear, especially where the selector lever engages them. You'll have to decide if the wear is not excessive and is acceptable to you. I decided to live with mine. The biggest chore I ran into was removing the mainshaft assembly. Its bearing housing is a press fit into the rear of the transmission case and is a challenge to remove.


Mark the housing


There is a little peg on this housing that has to go into a hole in the OD unit. Therefore it is critical to make a mark across the housing and case so that you can reassemble them correctly later. Locate this peg and hole before removing. I made a little gear puller of sorts out of some strong #8 screws and used two little holes in the housing to pull it out of the case. Once it's out, you can drive the drive gear shaft assembly out of the front of the case easily with a piece of broom stick and a hammer.


At this point, the 3rd and 4th gear balk rings are free. Locate the 3rd/4th gear synchro hub assembly. You don't have to take this apart. Just note which end faces the front of the car and stick it in a baggie. If you are compelled to take it apart, mark the parts for later reassembly and watch the three spring-loaded ball bearings fly across the garage as you pull it apart. Disassemble the driveshaft assembly to get at the 2nd gear balk ring. Again, don't mess with the 1st gear assembly unless you need to replace part of it. Same deal with the flying ball bearings.


I bought my balk rings from a supplier in England just to save some money but they are readily available stateside. I replaced all three even though the 3rd gear ring looked OK. Get the two oil seals, gaskets and any other parts you've decided to replace.


Putting it back together


Now the fun part, reassembly. Some general tips:

  • Get a good night's sleep.

  • Don't put anything back in place unless you're absolutely sure it goes there, that way. If there is any doubt in your mind, study the parts, the Bentley diagram, and your notes some more.


When reassembling the needle bearings, use a real stiff grease like wheel bearing grease to keep them in place. Make sure the two shaft assemblies are complete including the 4th gear balk ring. To ease refitting the two shaft assemblies back into the case (remember they are a press fit), stick both shaft assemblies in your freezer for a few hours and stick the transmission case in your oven and heat to 150 degrees (F) for an hour or so. The shaft assemblies will go in so easy you can even rotate them for a few seconds. This may be necessary on the mainshaft assembly to line up the marks you made on the bearing housing and the transmission case. You only have a few seconds to do this rotation thing until heat transfers and things get tight. Leave the fork selectors and related hardware off for now.


Next, attach the OD unit. Suspend the transmission between a couple of concrete blocks with the mainshaft sticking straight up. Now there are a couple of things we have to do to the OD unit before we attempt to replace it. Look down into the bowels of the OD unit with a flashlight. Note the roller on top of the oil pump plunger. If you take some measurements, you will discover that this thing has to be retracted about 1/8" to clear the oil pump cam that is on the mainshaft. I don't know if you can force the cam over it but I didn't take the chance. I looped a foot of.020" aviation safety wire over the topmost loop of the spring and ran it out through a hole in the strainer cavity. I then compressed the spring and tied the safety wire around a wooden dowel placed across the opening of the cavity. This safety wire is stainless steel so it is very strong. The .020" stuff can be tied just like fish line. Next look all the way down to the bottom of the OD unit and notice a splined receptacle that receives the splined end of the mainshaft. If you see what appears to be a single receptacle, you're OK. If you see two splined things, you have a small chore to do. There are actually two splined things down in there: the planet carrier and the roller clutch. They need to be perfectly aligned with each other for the mainshaft to slide through them. If you see that they are not, you must align them. The bottom-most one, the roller clutch, will easily rotate counterclockwise. You must make a tool to stick down in there that will slide past the planet carrier and engage the roller clutch. I made a little T thing out of a piece of.050" sheet steel and a steel rod. Twist the roller clutch CC~~ until it is aligned with the planet carrier.


Replacing the overdrive


Now you are ready to place the OD unit on the transmission. It should slide on relatively effortlessly all the way except maybe the last 1/8". This last bit of distance is the mainshaft bearing fitting into the OD unit (very tight fit). If the OD unit doesn't slide on easily, take it back off and reexamine. When you're sure it's on correctly, draw the OD unit down tight with the eight nuts that attach the OD adapter plate to the transmission case. Finally, cut the safety wire on the pump spring and withdraw it.


We're in the home stretch now. Replace the selector forks and their shifter shafts (lubricate the five ball bearings). Safety wire the striking fork screws. Insure that you do this in such a manner that the screws can't back out. On my transmission, the Ist/2nd shifter shaft was able to travel too far forward which allowed the 1st/2nd shifter fork to hit 2nd gear (generates brass shavings). To limit the forward travel of this shaft, I replaced the buffer pad which resides in a recess in the clutch housing with a thicker one. Replace the three ball springs and the cover and you're done with the transmission. Replace the clutch housing and release bearing and have a beer. You're two to three hours away from driving again.


That's it. Nothing to it was there?


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.


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