FunkyFresh520 Posted June 10, 2016 Report Share Posted June 10, 2016 When i downshift from 3rd to 2nd it grinds. Why would that be and is it fixable? Some additional info but i have a 67 520 with a j16 thats been sitting for 20 years. Ive done a tranny oil change, and changed clutch slave cyl.. Quote Link to comment
kelowg Posted June 10, 2016 Report Share Posted June 10, 2016 Worn syncro and or wrong type or weight of gear oil. Maybe even have clutch issue. Should b like 70-w80 GL4. NOT GL5(eats away syncros) Quote Link to comment
metalmonkey47 Posted June 10, 2016 Report Share Posted June 10, 2016 Change fluid first. It's not uncommon for a Datsun manual to grind, as old as these transmissions are. Make sure whatever fluid you buy, it specifies GL4 yellow metal safe ONLY. Motul, Redline, Royal Purple are all good sources for awesome GL4 fluid. Quote Link to comment
banzai510(hainz) Posted June 10, 2016 Report Share Posted June 10, 2016 Most likely the syncros I had a trans that did this 3rd to 2nd. then I installed a 25$ trans I got . it grind from 4th to 3rd. FUCK!!!!!!!!!!! 1 Quote Link to comment
kelowg Posted June 10, 2016 Report Share Posted June 10, 2016 Sta lube makes the gl4(napa) 1 Quote Link to comment
Stoffregen Motorsports Posted June 10, 2016 Report Share Posted June 10, 2016 Old transmissions may require a bit of attitude adjustment to learn how to shift. Even when new, they don't shift like a new car. The oil I prefer to use on brass synchro transmissions is Redline MT90. It helps with the shifting as compared to generic gear oil. Learn to let the synchro do it's job and let the stick "fall into gear", and you'll be a much happier person. Quote Link to comment
FunkyFresh520 Posted June 11, 2016 Author Report Share Posted June 11, 2016 I did use GL4 .. i guess im just not gonna be downshifting to 2nd thanx guys Quote Link to comment
scooter Posted June 11, 2016 Report Share Posted June 11, 2016 Rev match and use 2 fingers and let it fall into gear. You can try gm synchromesh oil but you probably need synchros 1 Quote Link to comment
Stoffregen Motorsports Posted June 11, 2016 Report Share Posted June 11, 2016 I did use GL4 .. i guess im just not gonna be downshifting to 2nd thanx guys That's kind of fatalistic. How bad can it be? A rebuild on that trans isn't more than $500 or $700, and that's like new, so if it bothers you that much, either pull it and rebuild it or get another one. You could replace the synchros yourself, but there's a learning curve involved with rebuilding a transmission. Quote Link to comment
FunkyFresh520 Posted June 11, 2016 Author Report Share Posted June 11, 2016 That's kind of fatalistic. How bad can it be? A rebuild on that trans isn't more than $500 or $700, and that's like new, so if it bothers you that much, either pull it and rebuild it or get another one. You could replace the synchros yourself, but there's a learning curve involved with rebuilding a transmission. Im probably just gonna leave it because ill be swapping it in the future.. if i do plan on rebuilding it who knows a shop around L.A. that does great work? Quote Link to comment
Knugcab Posted June 11, 2016 Report Share Posted June 11, 2016 Just double clutch and you will be fine. Quote Link to comment
FunkyFresh520 Posted June 12, 2016 Author Report Share Posted June 12, 2016 Just double clutch and you will be fine. Ive tried. Doesnt work Quote Link to comment
scooter Posted June 13, 2016 Report Share Posted June 13, 2016 Get better at Rev matching and you'll be fine. I drove a car for a year with fucked 2nd,3rd and 4th synchros.. So I just didn't use the clutch and Rev matched and slammed that thing through the gears. Accurate tach will help you do this. Quote Link to comment
Stoffregen Motorsports Posted June 13, 2016 Report Share Posted June 13, 2016 To double clutch, you clutch once while putting it into neutral and then again while matching revs for the next gear. It's a trucker's method. I had a 1938 Ford 1.5 Ton flatbed (it was a movie truck) and it had no synchros on first or second so I had to learn how to double clutch. If your good at rev matching, you can shift without pushing the clutch at all. Quote Link to comment
FunkyFresh520 Posted June 15, 2016 Author Report Share Posted June 15, 2016 Thanx for the help guys Quote Link to comment
datzenmike Posted June 15, 2016 Report Share Posted June 15, 2016 I would stick with the heavier weight GL4. Synthetics are very good but thinner and extremely slippery and will bleed past old seals and gaskets. The synchro is a tiny (and likely worn) brass 'clutch' that rubs both the fast spinning main shaft connected to the rear wheels, and the slower spinning clutch disc and gears inside the transmission. The synchro's job through rubbing friction, is to speed up the clutch disc, input shaft and all unused gears to match the drive shaft so they can be mated without grinding. By shifting into neutral and letting the clutch out to join it to engine, you can rev the engine to speed up the clutch disc. By quickly depressing the clutch, to disengage the engine, you can often find the faster spinning clutch disc is easier to mate to the drive shaft speed without grinding. It takes a little practice and you have to do it quickly as the clutch disc will quickly slow down again. In almost all cases down shifting can most easily be done at very low speeds. There is seldom the need to down shift with the exception of climbing hills or maybe passing on the highway. Around town specially stop signs just pull to the stop and then down shift. Quote Link to comment
Stoffregen Motorsports Posted June 16, 2016 Report Share Posted June 16, 2016 The point of the synthetic oil is to get into the synchro. The thicker oil is actually too slippery and doesn't let the synchro grab the gear. I have done multiple back-to-back tests on this and the MT90, though it costs about twice as much, is well worth the added cost. Quote Link to comment
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