Downshifting a '39 transmission

F

Ford Freak

Well-known member
Hey guys - what would be the (aprox.) max. speed I should be able to downshift ( no double clutching ) from 3rd to 2nd a '39 transmission , without getting the dreaded crunch ? Trans. is rebuilt , and clutch & pressure plate are new .
 
I don't ever get a "crunch" when downshifting 3rd to 2nd, at any speed. Sure you have everything correct?
 
It really should go in smoothly - that is the whole purpose of the synchro assembly. I have an old 39 trans in my 32 Cab (build it when I was 16 years old - 40 years ago). It has been drag raced and had some abuse and now my second synchro is going out - have to be really careful in downshifting (double-clutching usually), etc.. Due to this, I'm rebuilding it from one end to the other - as I don't want to damage the Zephyr gears in it.

If your ****** was freshly rebuilt AND your clutch is correctly adjusted, you should have no issues with downshifting. You probably need to have the transmission looked at. Hopefully it was a recent rebuild and you know the vendor.

Some guys know how to rebuild these (and put in good quality parts) - and some do not. I've seen a few 'messes' with ones that were claimed to be 'rebuilt'. :) They are a very simple transmission - actually quite easy to rebuild. If you're a newbie, get Mac Van Pelt's book - is a good reference.

D
 
Thing to remember is that these transmissions still shift slowly; ie, they are not as slick as modern stuff.'Racing shifts' will still produce a crunch. The synchro's need time to spin the selected gear to the matching RPM's.
Personally, I seldom downshift, I utilise the low down torque these engines produce and stay in top gear coming to a stop for example right down to about 5 MPH.
 
Provided all of the transmission's innards are in good condition you're shifting too fast. Press and hold for a second before completing a gearchange to give the synchronizer a chance to do its thing; these old toploaders aren't Tremec TR-6060s that you can jam into gear as hard as you want. Also, try blipping the throttle a little as you're executing the shift. In my car this seems to improve the synchronizer function, I suspect by spinning the input shaft slightly through the pilot bearing/bushing.
 
Well , I depress the clutch , put the shifter in neutral , (from 3rd.) wait 5 seconds , then attempt to get it into 2nd . If I am going over 10 mph. , it grinds .
Upshifting , 1st to 2nd , & 2nd. to 3rd. , it is fine . Since this is supposed to be a synchromesh trans . , I would think something is a miss with the downshifting .
 
In order for the gears to mesh properly they need to be close to the same RPM. When they are close enough the syncros will do their job. There seems to be a level of wear on your second gear syncro, that leaves you wanting.

However while you wait 5 seconds the engine RPM drops to idle speed.(the clusters find their own RPM) The main shaft - driveline - RPM is road speed.

If you think about it, while under acceleration, we come out of second gear after having worked the engine to a higher RPM. So when expecting to down shift, check road speed, and imagine what the engine sounded like. During the five seconds get OFF the clutch, zip the throttle, bring the motor to a noise level a little louder than what it sounded like before upshift. Now find second, clutch in, engage driveline.

This is a successful double clutch. I understand you don't want to do this. I also understand you should not have to do this. In its present state your trans is asking for it, you are already pausing too long to have the RPM match up. The thing about taking your time means giving the syncros time to match up the RPM

As stated by Brian - New Zealand
"The synchro's need time to spin the selected gear to the matching RPM's."

(or in other words find the gear - not only the lever position but tooth to tooth RPM) - Which happens best on a down shift if, engine RPM is slightly higher than required by road speed.....Or... the second gear syncro is healthy.

To recap what all this means, There are three revolution speeds involved when shifting gears. The driveline (rear wheels, drive shaft, trans main shaft) This speed can be found with the speedometer or, if you're good, judging the road. The engine speed (plus flywheel, pressure plate) can be judged by ear. The third revolution speed is, the input (front) shaft with clutch disc and the cluster gears (first and second). The RPM of these parts can neither be determined, or controlled, when the gear selector is in neutral and the clutch is depressed . They are on their own-(neutral clutch in)-. So....Fine when syncros work well its not an issue. Depress clutch select a gear and go.

Since you already take five seconds to attempt a down shift, I figured I may as well give you the process of successful double clutch. It can't hurt when done properly. Just be certain that when you gas it to control engine speed that there is no pressure on the clutch pedal. Otherwise the trans won't receive the message.

I wish you luck in getting your trans sorted out.:(

The link below is by Mac V P explains synchro wear with pictures.

http://www.fordbarn.com/forum/showthread.php?t=163599
 
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Food for thought on this issue. If the trans is driven a lot with the worn synchro rings, and many crunchy shifts have been made into second gear, there is a good likelihood that the synchro sleeve (7106) has ground down the little peaks of the synchro teeth on second gear. Now you've got a larger problem because you're now going to need a fresh second gear......or a main drive gear.....or a synchro sleeve, or all three.

The lesson here is that if your transmission is just starting to crunch when shifting, think about replacing the synchro rings sooner than later. It might save the cost of the more expensive gears.
 
Mac's post is exactly why I am rebuilding a second 39 trans to replace the one in my 32 Cab - with parts from Mac of course!
 
And to add this...I would only get early Ford transmission parts from Mac Van Pelt. I rebuilt the 39 Trans in my 5w with parts from Macs Auto Parts and had problems shifting. With guidance from both Mac Van Pelt and Bruce Lancaster, we finally realized the inside bore of the synchro rings that I had bought were machined oversized making them useless. New synchro parts parts from Mac Van Pelt fixed the problem...Sam
 
Mac VP, RE: 'Food for thought'. Right you are. Sooner is always better than later!

Thank you, I just have too much time on my hands.:o
 
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