Grinding gears 4th and 5th - Manual Transmission

on Wednesday 14 November 2007
in A-Z Index - Everthing is listed in here > Transmission

My transmission grinds from 5th to 4th, anything I can do?

    The 4th gear grind is a common problem and it is caused by the soft metal that the syncros are made of. The rest of the trans parts supposedly are made of a harder metal. Nissan supposedly acknowledged this according to a post a long time ago here, but has no recall for it. What I am doing is geting the Neo 75W90 HD gear oil(search to find web sellers) for my 5 spd. I hear that it alleviates the grind as well as the Redline MTL, but it is the correst weight of 75W90 and not the bearing killing 75W80 of the Redline MTL(and this is confirmed by not only Z32 owners and Stillen's race team, but Supra owners and Rx7 owners too). Another option is to put one quart of auto trans fluid(Redline or Neo synthetic) in place of the manual trans gear oil since it flows better and can better let the syncros work, but this may be bad for the bearings. This has worked for Supras. Just knowing that this is not exclusively a Z problem helps calm me down a bit, but there really is no excuse for it, especially when it occurs on stock cars as much as modded ones, so increased power is not the culprit here.


    NISSAN calls for a GL4 75W-90 oil in the tranny and this is what MT-90 is. MTL is a lighter wt oil. So if you are changing out your tranny oil, use MT-90. If it still grinds or is balky, check the fluid level before doing anything else! I had an occasional clunk shifting into second, and it went away when I added more MT-90- it was a bit underfilled. I used MTL in the past and in just a year it was black w/ metal shavings on the plug magnet. MT-90 has not done this. Spoke with a STILLEN team member at an IMSA race long ago, and he said the same thing, that MTL will wear the tranny in their racing experience. Of course, Redline products are not magical miracle cures so don't expect too much of it. Use MTL as a last resort, as some have posted good (short term?) results with it.