Synchronizer Operation Explained

You are absolutely correct.

Makes me appreciate that big truck transmissions generally do not have synchronized gearsets. I had to rebuild an entire transmission when someone had broken the main output shaft in one. No books, no drawings, nothing but brute competence. I will say, the way that they are made with dual or triple countershafts, the design is ingenious. The input and output shaft are only connected by a pilot bearing. The fact that the main output was snapped into two pieces, and the transmission didn’t fly apart is testament to the designer! The countershafts held the gears in their proper location, even though the shaft was toast! No synchronization there, just a good gear matching hand was required!

I have a Kawasaki motorcycle and the factory service manual chapter on repairing the transmission has one line about " paying attention to the mating dogs ". Made me chuckle.

Not if you have old stuff with slider gears

@Franky997 grinding gears on old stuff like '47 chevy, farmall tractors, or cub cadet lawn mowers does damage the actual gear teeth. The whole gear slides to engage

Works just like a timing light

Really sir you said true thing

Given the hard job, its a reasonably good explanation.

Thanks for the compliment.

Thank you for your good comments. You are correct. One note. although these blocker rings are brass there are other blocker rings made of other materials and sometimes either lined with a friction material similar to brake lining and others that use a bi-metal design with a sintered metal inner cone that is soft with a high coefficient friction. I will consider a followup video with this additional information.

We had some Mustangs rings made of paper! Replaced many.

​@@Franky997Can confirm that because some years ago I worked for a company that made synchromesh rings for many of the major European car manufacturers. Selector forks were also made from phosphor bronze, but of a different grade.

Thank you for your comments. Please note that the back cut is not present on all synchronizer sleeves and gears. I see it missing mostly on older transmission designs. There are several reasons why a transmission will jump out of gear and worn out back cut is one of them. Normally what happens is the oil is not changed when it should be and or the driver is rough on the transmission and the blocker rings start to wear causing grinding (gear clash) which in turn chews up the clutching teeth on the gear and on the sleeve which wears away the back cut. It's often a domino effect.

Thank you

Yes. He really has all the fine details down. Gear boxes for Spec Miata racing are rebuilt all the time. However the re-builds tend to have problems. If a rebuilder replaces the bearings, the brass synchro rings, and the seals then the gearbox will shift into gears easily and the box will hold oil, but will it stay in gear? Apparently if the back-cut (2:15 in the video) on the engagement teeth between speed gear and synchro sleeve are worn, these parts need to be replaced too. Someone who is simply installing a 'rebuild kit' will miss this.

I'm glad this worked out for you.

Why the test, if you don't mind me asking???

Your Welcome

Thank you. This is one of my top videos.

Glad you enjoyed it and found it of value.

John Smith from Plymouth, MI?

They are used in all my classes.

A couple of questions when you have problems going into 1st. Is this when the vehicle is not moving or when downshifting while moving? or both? Have you tried replacing the lubricant with the recommend fluid? Because you said that you have to wait a good 10 seconds to shift into reverse makes me think you might have a spin down problem. This is where power from the engine continues to send power into the transmission input shaft even if the clutch pedal if fully pressed. What can happen is one of a few things. The clutch disk can stick to the flywheel, or the disk could bind on the input shaft and not slide away from the flywheel quickly enough, or the input shaft could be binding on the pilot bearing. All of these will delay clutch disengagement and cause hard shifting. I'm not sure what vehicle you have but I have had good luck using Redline Manual Transmission fluid solving shifting problems.

They are not locking keys. They are synchronizer keys. They transfer the force between the sleeve to the blocker ring.

I don't think I have ever seen a synchronizer with 1st on one side and 3rd on the other. Normally 1st and 2nd and 3rd and 4th or I have seen 1st and Reverse and 2nd and 3rd and 4th and 5th. So a worn blocker typically will not cause jumping or popping out of gear. Normally worn clutching teeth on the gear and on the sleeve cause this. Not those teeth normally get worn down because the blocker is worn and the driver is grinding the gears during each shift. Other things like worn shift forks or loose shifter mechanism. More rare would be week or worn detent springs or misaligned bell housing (older classic cars). He is an odd ball one that I had one time. I had an older VW with a plastic rather stiff shifter boot. The boot was off center and when installed upside down it would exert force on the shifter in the direction of neutral. Under heavy acceleration the boot would exert enough force on the shifter that the vehicle would jump out of gear. Imagine resting your hand on the shifter al the time and while in 1st or 3rd the force of you resting your hand on the shifter was actually putting a slight amount of pressure on the shifter toward the neutral direction. Under heavy acceleration this will cause the transmission to jump out of gear. It's just one of those odd ball things but it happens, so I mention it to you.

@@tb-skyline-auto9836 Thanks for the reply, when i wrote ''on both sides of the shifter'' i meant 1st gear being Top left and 3rd gear being in Top Middle (both gears are in the same area) but its a normal 5 speed gearbox in a UK Ford Fiesta mach 8... So your saying gears popping out is, in some cases is due to the clutch?

It's extremely unlikely that replacing the clutch will solve a popping out of gear problem. I'm sorry to say that this is typically due to a problem inside the transmission if it's not shifter related such as worn shifter linkage or a improperly installed stiff shifter boot. I recommend that you have a qualified mechanic look at it for you and cross your fingers that its a linkage or shifter problem. One last possibility might be worn out or broken engine and or transmission mounts could do this.

1) the small clutching teeth are already 95% in alignment with the teeth on the gear before the sleeve is allowed to slide onto the gear so there is no grinding. This is accomplished by the proper operation of the blocker ring. When the blocker ring becomes worn out and can no longer perform properly that is when the clutching teeth start the grind during the shift. This is the grinding noise that can be heard by the driver between shifts and is called gear clash. The reason these tiny teeth can handle the torque comes from the total surface area contact between the gear and the sleeve. The number of teeth and the length of the teeth determine the total contact surface area. I'm not an engineer but this is something that has to be engineered into the design. They must determine the maximum torque that the synchronizer and gear would have to be able to handle and how much contact is needed.

Between two gears, only one tooth on each gear is engaged with the other gear at any time. Between the parts of the synchronizer assembly, all of those small teeth are engaged at the same time, spreading the load out between them. Many small teeth are as strong as one large tooth.

Try Changing the oil and put Redline Manual Transmission Fluid (MTF) in it. This stuff does wonders sometimes.

@@DrivelineMaster I just have changed the incorrect 90 wt for 50 wt motor oil. That by its self worked wonders. It made it possible to where the gear shift would actually move the gears in and out. Before that I could take it out of gear and it would stay in gear. It did that one morning until the oil got warm. I ran it all day, came in and drained that 90 wt while hot and its been better ever since. Everything but the synchroniser. So mine is not slowing the gear down when all that noise is going on? I wonder if the air shifter could be sticky somehow?

Unless the manufacture says you can use motor oil instead of GL4 or GL5 gear oil I wouldn't leave the 50st motor oil in your transmission. 90wt gear oil is not the same as it would be if it was 90wt motor oil. The way they measure thickness for gear oil is different than motor oil. Once again I would recommend the Redline MFT. I know it cost more but it is worth it. Unless your transmission was designed to work with motor oil (like some honda's) you will end up damaging your transmission and costing you more $$$ in the long run.

@@DrivelineMaster It calls for 50 wt engine oil. Granted Synthetic would be better but we just had a cracked head replaced at 6,389.00 on the truck and now the compressor has blown apart on the other truck leaving us stuck with a core fee. Cat wants 1,500.00 just for the core fee alone. Fleet Pride got us a refurbished for little over a grand and no core charge.

As long as the factory specification is 50wt engine oil then that is what you should use and have confidence that you won't cause any damage. This is the first trans that I know about that uses 50 wt straight motor oil.

Unless a specialized fluid is specified and the spec is 75w-90 or 80w-90 GL4 I really like Redline Manual Transmission Fluid (MTF)

Good ole stinky 80-90 gear lube is the highest temp rated fluid

Up here in MN maybe a 75-85 would be a good choice. I think up in the cold states an ATF is hard to beat. @@DrivelineMaster

I don't want to give the wrong information. Most manual transmissions can not and should not use ATF. ATF can only safely be used in a manual transmission if and only if the service information or owners manual says so. Never use ATF in a manual transmission unless it has been specified by the manufacture as an acceptable winter oil option.

The synchronizer hub which is splined to a shaft, the sleeve and the blocker ring all rotate at shaft speed.

@@DrivelineMaster ok thanks is the key connected rigidly to the blocker ring? and how are they connecting? and how does the blocking ring return its original position?

You do not need to use the clutch when shifting from any gear into neutral. There is a small amount of stress taken off of the components if you do use the clutch but not enough to cause any damage doing this. My sister was so superb at speed matching all the gears by just using the gas pedal that she could up-shift without the use of the clutch and as a passenger you would have never known. This too does not hurt the transmission or the clutch but keep in mind it is far and few people who can pull this off, not even me.

@@DrivelineMaster Then there was the time my clutch cable broke when I was driving on I-5 in Orange County. I had to learn how to shift without the clutch real quick. It can be done but I'm sure I didn't do the transmission any good.

You do not need to use your clutch to shift from gear to neutral and no harm will be caused. In fact if you are good with speed matching you can upshift and downshift without the clutch and without grinding the gears. I have seen it done by some very talented drivers. If you can do this without grinding the gears there is no harm in it and in fact your clutch will last longer. I too had a clutch cable break while driving in downtown San Francisco in semi rush hour. This was a very interesting drive into work but I did it without using the clutch.

See my reply

@@DrivelineMaster I'm a stick shift driving instructor, in the advanced course I try to teach people how to clutches shift (floating the gears), I think, if done properly, it's the best and most comfortable way to shift a manual. However, someone is arguing with me on facebook saying that this will wear down the syncros. I'm sure they are wrong and I think you have already answered this question in your replies but have you any specific input?

Pretty much correct, the synchronization only takes place when the gears are spinning. Even though the car is not moving if your engine is running and the trans is in neutral but vehicle is not moving some of the gears inside the transmission are spinning. If you push the clutch pedal to the floor and move the shifter quickly some synchronization might take place. If on the other hand after you push the clutch pedal to the floor you wait about 30 seconds before moving the shift lever then all the gears will most likely be stopped.

The speed gear appears stationary thanks to the use of the strobe light. The counter shaft is in mesh with input not output.

Here you go. The sleeve pushes on the keys which push on the blocker ring. The job of the blocker ring through friction between the ring and the gear is to grab the gear and make it go the same speed (synchronize) so the sleeve can slide over and lock the two together. If the blocker ring is unable to synchronize the gear to its speed a lot of friction is taking place. With more friction more heat is generated. With more heat there is expansion of components. The heat and expansion is a counter force pushing back on the blocker ring. That force on the blocker ring is transferred to the keys and then onto the sleeve. The counter force prevents the sleeve from moving close enough to the gear for the clutching teeth to touch. If the clutching teeth are allowed to touch before synchronization a grinding noise (gear clash) will be heard. Hope this helps.