91-96 Clutch Master Issues and Maybe a Fix

[Gojira94]

TLDR...

 

I'm using a parts house version of the later master with .699" bore vs .688" on the earlier BA5/10, and AX15 with CSC. It works fine with a CSC AX15. However, I'm seeing some symptoms that have been widely reported with difficulty getting into 1st and reverse "when warmed up." Some swear by the LuK LMC206 as fixing the issue, but I have some suspicions as to why. Here's what I see going on with the parts house unit: the pushrod has a fair amount of slack between its retainer under the circlip to the piston's cup, so about 3/16" of pedal travel is lost before the plunger moves, and the piston is just a hair short of full travel moving fluid. My guess is the LMC206 has less slack in it.

 

Masters from O'Reilly and Autozone are the same. Advance seems to have quit carrying a house brand, only offers the LMC206. Napa re-boxes and sells the LMC206 as NCF 72206. O'Reilly and Autozone master are etched with either "2055 18099" or "2055 19199." These are the ones in question...

 

The fix: cut and thread the pushrod for a coupling nut. And keep that adjustable pushrod for re-use regardless of future master changes. Nice to be able to adjust clutch engagement as the disc wears away, too. I also just did clutch hydraulics on my 2003 Accord. It has an adjustable rod to dial in the correct length. I wish AMC and Chrysler had made them that way originally. I'm also considering adding a PEEK M2 washer between the pushrod and piston for good measure.

 

I have plenty other things to work on with my rig at the moment but I'm going to see if my neighbor has a left hand thread die set and try this at the earliest opportunity.

 

/end rant

[pizzaman09]

Cool thoughts.  

The only thing I'll add to this is that it's clear that the internal slave has much more piston area than the external slave.  I did am external slave conversion but kept my original master, that resulted in a clutch pedal of about double the weight.  I'm hoping some day to get the correct smaller bore master to match.

[Eagle_SX4]

56 minutes ago, pizzaman09 said:

I did am external slave conversion but kept my original master, that resulted in a clutch pedal of about double the weight.  I'm hoping some day to get the correct smaller bore master to match.

The extra force required to push the clutch comes from the use of the clutch fork. The further you are away from the clutch release bearing the more pressure required to do the same amount of work.  I used both the original ba10 master cylinder then went to the later external slave master cylinder and noticed no difference in required force to disengage the clutch.

[pizzaman09]

10 minutes ago, Eagle_SX4 said:

The extra force required to push the clutch comes from the use of the clutch fork. The further you are away from the clutch release bearing the more pressure required to do the same amount of work.  I used both the original ba10 master cylinder then went to the later external slave master cylinder and noticed no difference in required force to disengage the clutch.

That doesn't add up to me.  If one does the statics problem, the external slave should have about a 2:1 advantage to apply force onto the throw out bearing vs the internal slave design based on the lever arm length.  So to make the clutch heavier feeling, the surface area of the external slave would need to be less than half of the internal cylinder.  I'd have to get some measurements of the piston diameters to confirm.

[Eagle_SX4]

I am definitely not great with math and hydraulics. But in practice I also noticed the heavier pedal after the external slave swap. When I swapped to the newer master cylinder I did not notice a difference in force between the old an newer master cylinders. Both require about twice the pressure to disengage the clutch vs the internal slave set up.