Distributor Gear Clearance

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   In a precision industrial application using gears like this the two shafts are mounted on bearings; they cannot move radially and can't move more than a few thousandths of an inch axially.  Next, the mesh will be set up with shims or a grind spacer for a backlash of say .005-.010" (depending on gear size and pitch); think ring and pinion installation and set up.    

Now think of all of the machining tolerances that affect this ideal gear mesh I described above in this application.  I easily come up with 10 between the block, timing cover, two mating gears, cam timing gear thickness, distributor housing, distributor shaft....  With all of those tolerances, lets say +/-.005" for each, and with a camshaft that can walk forward, setting this tight doesn't seem to be a good idea unless it's required for the engine to run properly at the expense of the gears.

What do you gain with a tight gap with possible accelerated gear wear vs a wider gap?   If it has to be tight, so be it, but I wouldn't do it voluntarily.

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Steve_P Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Charter Member Joined: Jun/28/2007 Status: Offline Points: 3803	Post Options Post Reply Quote Steve_P Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 3:04pm
	In a precision industrial application using gears like this the two shafts are mounted on bearings; they cannot move radially and can't move more than a few thousandths of an inch axially. Next, the mesh will be set up with shims or a grind spacer for a backlash of say .005-.010" (depending on gear size and pitch); think ring and pinion installation and set up. Now think of all of the machining tolerances that affect this ideal gear mesh I described above in this application. I easily come up with 10 between the block, timing cover, two mating gears, cam timing gear thickness, distributor housing, distributor shaft.... With all of those tolerances, lets say +/-.005" for each, and with a camshaft that can walk forward, setting this tight doesn't seem to be a good idea unless it's required for the engine to run properly at the expense of the gears. What do you gain with a tight gap with possible accelerated gear wear vs a wider gap? If it has to be tight, so be it, but I wouldn't do it voluntarily.

Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6877	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 5:33pm
	OK, let's try a different explanation for this particular gear arrangement. The shafts are fixed on their respective centers. The gears are cut at 45 degrees - very poor power transmission but not required. If the cam walks forward or back think wedge - the other gear will rise, fall or rotate. The amount of rotation is directly proportional to the clearance of the shaft to move up or down. The amount of rise or fall is directly proportional to the clearance of the the shaft to move up or down. The mesh of the two gears is only related to the gear cut, design clearance and respective shaft centers. Adding more clearance for the distributor shaft to move up and down only changes rotation play and axial play - nothing to do with the mesh of this type of gear. Just try it if you like. On a rebuild move the cam forward and see what happens to the distributor shaft. It will rotate or move up - nothing else it could do. Ask why there are so many posts on AMC's and distributor gears having to be matched or being chewed up. Not shims, miss matched gears or bad timing cover (shaft) alignment. Shims or clearance won't fix either one.

billd Members Profile Send Private Message Find Members Posts Add to Buddy List Moderator Group Forum Administrator Joined: Jun/27/2007 Location: Iowa Status: Offline Points: 30894	Post Options Post Reply Quote billd Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 6:16pm
	Look at the many many thousands of distributors in all engines running .030-.040 with no troubles. Taking it less won't gain anything. I don't like too much more than what I mentioned because you get more hammering up and down but it's not because of any troubles. I just prefer to not have .090 in there. The end play doesn't change dwell, doesn't impact points, and in a sense is going to be controlled by the pressure of the distributor shaft driving the oil pump. In other words it can't just jump up and down in there - it's not going to rattle or bounce freely because it's driving something offering resistance. Put a clear cap on (if you can still find 'em - they used to be out there because they were cool) and watch things that way.
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Trader Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Addicted Joined: May/15/2018 Location: Ontario Status: Offline Points: 6877	Post Options Post Reply Quote Trader Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 7:23pm
	Totally agree with a little more clearance won't matter squat with many older ignition systems. Now when you have HEI or MSD or ... with multiple spark options and they use the distributor for crankshaft/camshaft location you are looking at a different animal. So it really depends on what is installed and following the manufacturers recommendation. For stock AMC go with more clearance - not a big issue. But make sure when you change from stock the gears match and you follow the new manufacturers recommendations or it may not turn out as well as you hope.

Bandana Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473	Post Options Post Reply Quote Bandana Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 7:39pm
	Some great thought and tips guys, thanks. I guess since I am running an MSd Billet distributor I should probably try to get it down to the 10-15 range. My additional shims will be here tomorrow so I can at least get the distributor set up and ready. Bulltear said they'd have the new timing cover out on Monday so maybe it will get here in a day or two. Would be nice to get this back together....

Airdrie AMX Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Aug/08/2010 Location: Alberta, Canada Status: Offline Points: 329	Post Options Post Reply Quote Airdrie AMX Report Post Thanks(0) Quote Reply Posted: May/22/2018 at 10:11pm
	Personally I would leave it where it is and forget about it, there is no gain to make it any tighter or chance of damage leaving it where it is. I have 2 MSD distributers with the oem gears on them and similar endplay with many troublefree miles on them. No need to "fix" something that doesn't need it.
	72 amx javelin 401 4spd

Bandana Members Profile Send Private Message Find Members Posts Add to Buddy List AMC Nut Joined: Jul/26/2016 Location: NC Status: Offline Points: 473	Post Options Post Reply Quote Bandana Report Post Thanks(0) Quote Reply Posted: May/23/2018 at 9:08am
	Ok, for what it’s worth, I called MSD this morning and for this distributor at least, they recommend 10-15 mils.

6PakBee Members Profile Send Private Message Find Members Posts Add to Buddy List Supporter of TheAMCForum Charter Member Joined: Jul/01/2007 Location: North Dakota Status: Online Points: 5457	Post Options Post Reply Quote 6PakBee Report Post Thanks(0) Quote Reply Posted: May/24/2018 at 7:57am
	Just two comments. 1) Causes of spark scatter. 2) Reasons for installing crank trigger ignition.
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