C900 Synchronizers

From Jerry: here is my follow up to my previous synchronizer article. Would I install used synchronizers? Yes! Only if checked out properly. This has been my procedure since new synchronizers are not available. With the number of used synchronizers I have available I have been very successful.


Another name for synchronizers is a brake. Just like your car brakes it stops different speeds (rotor speed to a stationary caliper) to the same speed which, in a car is the same speed when stopped. Braking action applied from synchronizers to two different rotating speeds of gears from upshifts or downshifts. When shifting takes place, pressure is applied to the synchronizer starting the braking action and the synchronizer will rotate slightly forward or backwards to brake the gears and also blocks the muff from sliding into the selected gear. Once both gears are rotating the same speed, the synchronizer will back off slightly opening the window of opportunity to engage gears without grinding.


Here are some detailed pictures and videos I have taken to explain in detail what works for me. Hope this helps. Picking used synchronizer:


Place selected synchronizer not attached to gear in muff.

A new synchronizer, the synchronizer will be almost level with the top of the muff.

The further worn the lower it will sit in the muff. • See diagram o A synchronizer is 7.75mm (.3025) thick. o Synchronizer Muff is 10.0mm (.3936) deep from top. o A new synchronizer will have 2.25mm (.091) free wearable space under the synchronizer to wear before bottoming out. o Pick your best synchronizers. o I will not use a synchronizer that sits > .030 down in the muff. This way it will have the remaining wearable space of .061 till the synchronizer bottoms out. • To insure you have the correct synchronizer / muff match… o Mount synchronizer on gear. o Using a paint pen, mop paint on the synchronizer mating surface of the muff (this is called bluing). o Engage synchronizer into the muff, rotating and pushing down hard for a half turn. o Feel the braking action while doing this. The harder you press the tighter the braking action. This is important! o Pull apart and read how the wiping looks. If it is the correct synchronizer, the wiping will be 100% across the surface of the muff. This insures you have the correct angled synchronizer for the muff. • I purposely installed a mismatched synchronizer and muff to show what you do not want to see when you use the blue paint pen. • Tips o Put the best synchronizer on 2nd gear. o Put second best synchronizer on 3rd. o Put 3rd best synchronizer on 1st o Put 4th best synchronizer on 4th. Usually when you dismantle a transmission 4th gear synchronizer is the best because of not being used during hard shifting. This usually becomes my 2nd gear synchronizer. • Things not to do.. o Do not glass bead synchronizers to clean. This ruins them. o Do not use Scothbright to make them shinny. Sanding anything on the sharp edges of the synchronizer will wear them out.



Doing some house cleaning Thursday. Decided to gather brass. A testimonial of how many transmissions I have been through over the years. Counted 438 C900 bad syncros going to the recycler for 1st - 4th gear and 74 bad 5th gear sincros. Plus shift forks are being recycled. Do not worry I still have 75 good early syncros and 25 syncros for 91-93 that I would install in a heartbeat and every part inside the C900 transmissions. Plus I have 15 complete transmissions ready to be dissassembled and inspected for rebuilding. I will do an article on "How to check out used syncros" in the future.




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