Clutch Hydraulic TECH - Concentric slave and clutch master setup
AIR GAP, FREE PLAY, AND CLUTCH MASTER CYLINDER SIZING
Want to know the easiest way to burn up your clutch and/or release bearing?
LACK OF AIR GAP!
WHAT IS AIR GAP?
When it comes to clutch setups with a concentric slave cylinder, air gap is space for the clutch room to grow as it wears. Your clutch discs get thinner and thinner as they wear, when this happens the diaphragm fingers get "taller" and move towards the bearing more and more. The air gap you setup allows the bearing to move backwards and away from the clutch to allow for this growth/wear over time.
There is no set time or mileage on how long this takes to lose all your air gap, but it depends on the clutch friction material, your driving style, and overall abuse and amount of slip the clutch sees. Riding the clutch from a stop will reduce the clutches life and reduce the air gap at a faster rate.
Manufactures differ on their recommended air gap distance, with Tilton's recommendation being 0.125" which is about 3mm. I like to set my gap between 3 and 4mm, usually closer to the 4mm as it buys a bit more wear time before needing adjustment.
WHAT HAPPENS WHEN THERE IS NO AIR GAP?
The clutch loses its clamping force, because now the diaphragm cannot fully release and clamp the discs as tight as designed. This can make the clutch slip and will burn up the clutch very quickly if not addressed. It will also burn up the release bearing. The bearing is only designed to see pressure when the clutch pedal is pressed down, not a constant load like it would see when the diaphragm applies force to it.
WHAT HAPPENS IF TOO MUCH AIR GAP?
The more air gap you introduce, the less amount of sleeve is left available to stroke the clutch. This can cause an overextension of the bearing, and can cause failure. Usually complete loss of pedal, and most of the time it tears the seal and causes the bearing to need to be rebuilt.
INITIAL AIR GAP SETUP
When you take your initial air gap measurement, the gap is the distance between the bearing and the clutch fingers. BUT, after you BLEED the clutch, now the bearing will be in contact with the clutch fingers. The bearing will not apply heavy pressure to the fingers but it will constantly ride on the fingers. So, after bleeding your AIR GAP now lives in the sleeve of the bearing. After you bleed your clutch, double check by looking in your bellhousing at your bearing to make sure you see the 3-4mm of air gap in the bearing sleeve. Below are pictures illustrating how the gap should look.
FLAT FACE AIR GAP:
RADIUS FACE AIR GAP:
TILTON BEARING SPECIFICS
The Tilton 6000 series have two sleeves. On the flat face bearing the back of the bearing will sit flush with the top of the housing. The radius bearing has a stepped lip sleeve. The gap will look a little different on radius bearings as it will be that 6mm long stepped section + 3-4mm of air gap. I attached pics here so you can see it just like it should look when you look inside the bell from the side of the bearing.
LOOKING IN AT FLAT BEARING THROUGH BELLHOUSING:
Flat bearings are used on almost all the twin discs I sell, any pressure plate with the rounded tips/flare at the inner ring of the diaphragm fingers. The radius bearings are used on the single discs and the smaller ST215 twin discs, as well as OS Giken clutches and other "straight finger" clutches.
I've talk about this in the older youtube video but sometimes I feel like its not something people grasp well without pictures.
AIR GAP SETUP VIDEO:
CAUTION: OVEREXTENDING
Your release bearing has a travel limit. Take caution when setting it up! If you have 1" of total travel, and your air gap is set to 0.25" that means you only have 0.75" of travel distance left before the bearing overextends. When this happens you usually lose all your clutch pressure, it tears the seal, and you will be pulling down your transmission to fix this. Always make sure you do not have too much air gap.
Your clutch pedal stroke and master size also plays a big part in the amount the bearing will move. So, if you have a large bore master, and a lot of stroke in your clutch pedal, you may need to use a pedal stop, or what I do, is adjust the clutch pedal using the master rod downward towards the floor, and use the actual floor in the car as the pedal stop. The way I set these up, is about 1/2" of movement off the floor before the clutch starts to grab. This way, you have wiggle room but not too much to where you overstroke the diaphragm or overextend the bearing.
MASTER SIZE
Your master size should be determined based on the amount of distance (stroke) the clutch diaphragm requires for full release, along with the relation between your master bore size and the clutch pedals stroke amount. For example, if your clutch requires .440" of travel, then you would want to get a master that is just above that in total stroke amount.
Here is a chart showing Tiltons release bearing + Tilton master when getting a full 1.1" of stroke from the clutch pedal.
|
Bore Diameter |
Max HRB Stroke |
|
Fractional (in) |
(in) |
|
5/8" |
0.265 |
|
7/10" |
0.333 |
|
3/4" |
0.382 |
|
13/16" |
0.448 |
|
7/8" |
0.520 |
|
15/16" |
0.596 |
|
1" |
0.679 |
|
1-1/8" |
0.859 |
The good thing is, I have done all this work for you, and when i sell a clutch, I set it up with the proper master sizing to make it work properly. When you buy a clutch from anywhere else, like Summit Racing, TURN14, etc... do they do that R&D for you? Hell no! But, that is why you buy from me instead of those guys right? From an actual guy who makes stuff, a person with hands-on experience, not just a website selling you a product and making money on it. I am not trying to be cocky, but you will see a lot of other vendors selling products and using my recommendations, riding my coat tails on this work that I have done.
CLUTCH MASTER - FREE PLAY
Since we are talking hydraulic system, lets also talk about the master. In your master cylinder there is a bypass valve, this valve releases the pressure from the master, if the piston in the master does not return back past this bypass port, the master will continue to build pressure like a pump. This will cause HAVOC, and can destroy a clutch, destroy your bearing, etc. Basically what will happen is your release bearing will push in more and more and more on the clutch diaphragm, reducing its clampign power, and will burn up the clutch, burn up the bearing. Catastrophic stuff... MAKE SURE you have free play at the top of your clutch pedal where the clevis attaches. If you pull the pin out of the clevis and the clevis shoots backwards towards the seat, you do not have free play. Watch this video for more info on that.
CLUTCH MASTER TECH VIDEO:
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Joel Grannas