Clutch bobbin length

[Andy Antipas]

I'm restoring an 1969 titan F3 car and need a clutch bobbin. My measurements indicate there is 1.7 inches (43mm) between the throwout bearing and the clutch fingers. How long a bobbin do I need?

It is my understanding I need about .25 inches of clearance? I have a 1.3 inch bobbin on the shelf, which would give me .4 inches of clearance. Is that too much?

Thanks for the help.

Andy Antipas

[Brian]

F3 cars often used smaller flywheels and thinner (1" or so) adaptor plates, so a FFord bobbin probably won't work. If you have such a setup, a typical annular slave won't be easy either, not enough room. I typically look for about .025" of running clearance, not a quarter inch. What you do is get the basic clearance close (.25" may work fine, but I like less, and .4" is a lot more than I would choose to use) , then adjust the pushrod length so that the clearance is minimal. The return springs on the mechanism pulls the piston in the slave cylinder all the way home, so the push rod length takes up the free play. It's important to have the actuating arm on the gearbox at about 90 degrees to the pushrod so that you get the maximum leverage from the slave cylinder, which is why the short bobbin may not work. The typical old style slave cylinder likes to trap a bubble of air at the top as well, so try bleeding it off the car (making sure the piston doesn't go for a short flight...)

All this assumes a typical old style Hewland monkey-motion clutch setup

Brian

[Andy Antipas]

Hi Brian,

Its an aluminum flywheel, but the same size as a FF. Clutch is a 7.25 AP Lockheed. The adaptor plate is also thinner than a typical FF.

Yes, using hewland type slave. However, the engine lays over at 30 degrees. I'm almost thinking the bleed screw maybe in a slightly better position to bleed off air?

.025 inch clearance sounds better as well. .25 sounded too big. i would imagine a thicker bobbin could be shortened on a mill? 1.7 - .025 would be 1.675 inches.

Anybody out there have a bobbin about that thick?

Thanks,

Andy

[bob darcey]

Hi Andy. Taylor lists the standard FF bobbin length for an F3 clutch as 1.94", and the standard Hewland adapter depth is 2.15". If your adapter is not as deep, my suggestion is that you simply reduce the 1.94" bobbin by the same amount.

[Craig Taylor]

Andy,

There are a number of flywheel/clutch/adapter combinations that were used in F3. We can help you define the correct length of bobbin if you can provide the following information:

1. The distance from the friction surface of the flywheel to the back of the block. from this and the AP engineering data for the clutch itself we will determine the distance that the clutch diaphragm spring fingers extend into the bell housing.

2. The effective thickness of the adapter ring not including the centering lip that projects into the transmission bell housing.

The clearance required between the clutch diaphragm spring fingers and the bobbin when the clutch release bearing is fully retracted is to provide room for the clutch fingers to move rearward as the clutch disc wears. The fingers move back .005" for every .001" of clutch disc wear.

Using .025" clearance is very tight, only providing for .005" clutch wear. This is fine if you are very attentative to the clearance, and adjust it via the pushrod as necessary. The advantage of minimal clutch clearance is that the clutch pedal travel free travel is reduced making the clutch quicker.

We have had Tilton has manufacture very short annular slave cylinders for us that can be installed in minimal space should you want to go that route. We have the adapters on the shelf to install this Tilton annular cylinder in MK gearboxes that have the three 7mm x 1 holes around the input shaft seal. In this case, we design for an initial clearance of .150" to allow for .030" clutch disc wear - the system is self adjusting to compensate for disc wear.

If you have any questions, e-mail me at craig@taylor-race.com or call at 800-922-4327 so that we might provide answers.

Craig

[Jay Laifman]

We have a related question. We are putting together an engine and Hewland for the first time. It needs a bobbin. If we put a 1.3" bobbin, there is about 1/8" clearance with the throwout bearing all the way back.

The problem is that our input shaft is tapered. It seems to me that means there is no way a bobbin can properly slide on the shaft, and it will wobble/bang around in there. This was a custom made shaft for this installation and getting another shaft made that has a parallel side shaft in there seems crazy.

Is there an option to get a different fork? If the fork had 1.3" extensions on each end, adjusted to the right location so that the arc of the TOB stays centered, that would work. Obviously just welding on extensions is not a particularly smart way to go. We could have an entirely new fork built from scratch where the arms are one solid piece.

Do these already exist? Is this a bad idea for another reason?

Separate question, this is for a Tilton 7.25" OT-II clutch. Any idea what the total travel is from engaged to disengaged? (I found on Tilton's website that travel should not be more than .25" once TOB touches fingers.)

[pooch776]

I have never seen an input shaft with a taper where the bobbin would be. A photo would be helpful.

You could use an annular throw out bearing like the one Pegasus sells. Then you would not need a bobbin.

https://www.pegasusautoracing.com/pr...asp?RecID=4625

Good luck, Joe

[EACIII]

I put a Rally clutch in a vintage car recently. The shaft was tapered, but only at the very end by the transmission. Bobbin only needed maybe 1/4 of taper at the back side to work. Quick job on the lathe. Works fine.

Ed

[Jay Laifman]

Thanks for the info and suggestions. I'm going to take some measurements and see if they help determine how much modification I need.