TR4/4A Clutch Slave Size?

[RJS]

Earlier this morning I posted about dark clutch fluid. That got me investigating my slave cylinder. My research turned up that 1" bore is original to TR4A slave cylinder but, that 7/8" were all that were available for many years. Since my clutch pedal has always been very heavy, I began to wonder if I had a 7/8" and not a 1" bore.

The side of the slave cylinder is stamped "Girling 64670739". But, an internet search for that turns up zero, nothing, nade. Sooo, I went back into the garage, pulled the rubber end cap off and measured the internal bore using calipers. I came up with not 1", not 7/8" but, 13/16". Ugh!

For $35 I am tempted to simply order a new 1" bore slave cylinder, install it and hope it solves my heavy clutch. Going up 3/16" in bore size is nearly 20% which I have to believe will have a significant impact/reduction on pedal effort.

Appreciate your thoughts on slave bore sizes and/or Girling part numbers.

Bob

 

[TR3driver]

Makes sense to me. Except the part about 1" slaves not being available. 4A uses the same slave as TR3-4 (with Girling hydraulics) and I've never heard they were NLA.

Kind of makes me wonder if someone wasn't trying to solve something else, like lack of travel at the slave. Going from 11/16" to 1" will reduce travel (for the same MC motion) by about 1/3.

 

[RJS]

Randall,

Agreed. If master cylinder bore stays constant, by increasing slave cylinder size will have two effects: 1) reduce pedal pressure and 2) reduce travel of the slave piston. Both are acceptable in this situation. The reduced pedal pressure will be welcome and, since the 1" will be returning to stock, I am not concerned by the reduced piston travel. With the 13/16" SC bore now, I was clearly getting too much piston travel.

I am not a math genius but, I think the following calc is correct. Bore diameter will be increasing by 3/16" which is 18.75% based on the straight measurement. But, I believe the correct way to do it by calculating the area of a circle with A=pi*r(squared). On that basis, the area is increasing by 51%. I suspect there is more accurate math to be applied to this situation but, regardless, I am fairly encouraged the new slave cylinder will significantly improve clutch pedal feel.

And the more I think about it (which is always dangerous), the symptoms of my clutch exactly mimic a SC which is too small (super heavy and engagement is like an on-off switch).

I'll post back my results after I install the new SC in the next week or two.

Bob
PS: Randall, re trying to solve something else, the current SC was installed by a less than qualified British Car mechanic when I had only owned the car a few months and wasn't yet doing my own work. I truly believe he just grabbed any old SC off his shelf and threw it in there. It looks totally correct other than the bore size is 13/16" rather than 1". That was back in December 2000

 

[TR3driver]

Right, ratio of the piston areas. New piston has roughly 1.5 times as much area, so 50% more force; but 1/1.5 (which is 2/3) as much movement.

 

[RJS]

OK, for all you TR4 and TR4A owners out there - WOW! I finally got my TR4A out for a test drive with the new slave cylinder installed. If you see my original post, the SC in the car for the past 14 years had a bore diameter of 7/8". The clutch was extremely heavy and acted like an "On-Off" switch making it extremely difficult to engage the clutch smoothly. I always attributed this feel to the clutch itself as there are many articles from TR6 owners with similar complaints about heavy clutches.

WELL, my new SC has the factory original 1" bore and it transforms the car! Pedal effort is probably 50% less AND since the slave pushrod travel is relatively less than the old, it is much easier to modulate clutch engagement for a smooth take off.

Boy, anyone who has a heavy clutch pedal simply must check they have the correct size SC. For reference, the body circumference on the old 7/8" slave was 4.25" and it was stamped "Girling 64670739". The body circumference on the new 1" slave is 4.5".

Cheers and good luck

Bob
PS: unbelievable such a simple fix could make such a difference: $35 and one hour to install, bleed and adjust. And to think I was contemplating a tranny out job to replace the entire clutch

 

[titanic]

Interesting discussion. TR6s (same clutch and hydraulics as TR4A) suffered clutch release problems when the clutch master cyls. were reduced to .70" dia. from .75". This was done to address complaints of heavy pedal effort. The reduction in master cyl. dia. did decrease the pedal effort, but also reduced the stroke at the slave cyl. and seem to cause clutch disengagement problems if there was any wear in the pivot points or air in the system. AFIK, all slave cyls were 1" from the factory. BPNW does sell a "long throw" slave cyl. to address the release problem,the dia. isn't stated, but would have to be smaller than the 1" stock ones.
TR4A drivers must have been more macho than TR6 people, as I don't remember there being any complaints of excessive pressure with the stock set up
Berry