Drill Mod...Does it actually work??

**Wesman** · 11-06-2008, 04:27 PM

One of the most common mods people do to the clutch hydraulic system on LS1 cars is the drill mod. For anyone who doesn't know what that is, check out the "drill mod" on www.installuniversity.com

I actually just did the drill mod myself a few weeks ago because its so highly reccommened, but I have the trans out and I'm rebuilding it, so I haven't had a chance to drive it to see if it made any difference.

However, after thinking about the principles behind a hydraulic system, the drill mod should theoretically make no difference.

Hydraulic systems work on the principle that a fluid is not compressable, and therefore if you apply pressure to a fluid in a confined space, the other end will recieve the same pressure. There is nowhere else for the pressure to go unless something expands or breaks.

So when you push the clutch pedal down rapidly, if the fluid flow was actually being restricted by the line restriction, the pedal itself would have to decend more slowly as you push down on it, because you are trying to force a greater amount or fluid through the orfice at one time than can actually flow through that orfice. But as we all know it doesn't, meaning the force we are exerting on the pedal and into the pushrod on the master cylinder has to be getting down to the slave cylinder at the same exact time. If there was a delay between pushing the clutch down and the slave releasing the clutch, the pressurized fluid would have to be going somewhere else, such as balooning the line or gushing out of a seal.

Here is a picture just to help with the mental image of the system:

Anyone else agree with me here, or am I missing something??

**Goldcuffs** · 11-06-2008, 04:55 PM

I think the mod is for when the clutch is released. I havent done this yet so I don't know. I do know if I roast first and power shift second the pedal only comes up half way and then slowly comes up to the top. This happens EVERY time I do this. Driving it normal I have no problems. So with that said I will try the mod and if it cures the problem.

**Firebirdjones** · 11-06-2008, 05:00 PM

From what I understand,,,(keep in mind this is coming from an old school guy that is used to mechanical linkage),,,the restriction is there to soften clutch action both depression and release,,,so as to soften the blow on the drivetrain more than anything,,,,a warranty claim protection if you will.

Although in theory what you are explaining sounds correct, I'm not so sure it works in that manner. With that restriction slowing down the flow of the fluid the clutch simply doesn't react as quickly as you can move the pedal. I'm guessing there is something in the system that absorbs this difference..again this is a mechanical linkage guru thinking outloud here.

I can say for certain though that my wifes 6 speed wasn't quite right when purchased. Already having the LS6 clutch installed, but you couldn't hang gears if your life depended on it. The clutch grabbed instantly (about 1/2 inch from the floor) and caused very sticky shifts. There just wasn't enough free play. Wife couldn't even drive it.

I installed an adjustable master cylinder to try and cure this issue. Although shifting has become much easier,,,and quicker too,,,it's still not perfect and if not carefull can get hung between gears if you get real aggressive. I have plenty of free travel from the floor now though and now my wife can drive the car.

This is where I think the drill mod will come in handy. Wish I had done this while the master cylinder was out. Someday when I get ambitious enough I'll crawl back under there and do it. It's due for an oil change soon, perfect excuse.
I just hope it does the trick.

**Mr. Luos** · 11-06-2008, 05:01 PM

Worth every penny it doesn't cost.

**Wesman** · 11-06-2008, 05:10 PM

Originally Posted by Goldcuffs

I think the mod is for when the clutch is released. I havent done this yet so I don't know. I do know if I roast first and power shift second the pedal only comes up half way and then slowly comes up to the top. This happens EVERY time I do this. Driving it normal I have no problems. So with that said I will try the mod and if it cures the problem.

That would make the most sense, that it would effect the engagement of the clutch rather than the release.

Since the pressure plate fingers are the force that pushes on the slave and forces the fluid back up to the master, the line restriction would cause the fluid to flow more slowly, resulting in the pedal returning more gradually and the clutch engaging more slowly when you dump the pedal, say during a launch or after a shift. In that case I can definitely see how the drill mod would be beneficial.

**Mean Green Z28** · 11-06-2008, 05:51 PM

Think of it like an adjustable shock. You change the orifice by turning a knob and it dampens the stroke of the piston. Lesser the restriction, the faster it'll let the piston extend/retract; but if you take it slow, you wont feel the difference. Same thing with the clutch line. Take the restriction off and it'll let the fluid flow into/out of the master cylinder faster allowing you to do quick shifts at the track; you wont feel it on the road 'cos we don't all drive like maniacs. The restriction is put in and is designed so that you have a fine balance between not feeling any clutch chatter/etc. on the pedal and a nice fluid feeling in the pedal.

**Firebirdjones** · 11-09-2008, 05:47 AM

Hey Wes,,,did you drill the orifice to 1/8 inch??? I'm told thats where it needs to be.
I'm changing oil in my wifes car today and thought I'd pull it down and check it. Not sure if it was drilled or not when the LS6 clutch was put in,,,so I'm going to pull it down again to be sure.
Wish I would have thought to check while I was installing the adjustable master cylinder

**Wesman** · 11-10-2008, 01:16 PM

Originally Posted by Firebirdjones

Hey Wes,,,did you drill the orifice to 1/8 inch??? I'm told thats where it needs to be.
I'm changing oil in my wifes car today and thought I'd pull it down and check it. Not sure if it was drilled or not when the LS6 clutch was put in,,,so I'm going to pull it down again to be sure.
Wish I would have thought to check while I was installing the adjustable master cylinder

I was going to...but I tried the 1/8" bit and it wouldn't even fit into the orfice of the line. So I went down one size to a 7/64" bit, that slid right in, so thats what I used.

**Aetos** · 11-12-2008, 05:52 AM

Originally Posted by Wesman

I was going to...but I tried the 1/8" bit and it wouldn't even fit into the orfice of the line. So I went down one size to a 7/64" bit, that slid right in, so thats what I used.

I used a 1/8", it didn't fit into the actual orfice... so I made the orfice bigger, lol. I held the drill center to it and made the hole straight in the center, seems to be working for for the past 2 weeks.

**Wesman** · 11-12-2008, 10:06 AM

Originally Posted by Aetos

I used a 1/8", it didn't fit into the actual orfice... so I made the orfice bigger, lol. I held the drill center to it and made the hole straight in the center, seems to be working for for the past 2 weeks.

Yea I'm sure that works as well. I just didn't want to modify the orfice, since I figured it has to seat in the master cylinder properly. I guess I could have just used the drill press to make it perfectly straight, but the 7/64" bit fit perfectly thorugh the orfice (barely touched the sides) so thats what I went with.

**Firebirdjones** · 11-23-2008, 11:34 AM

Well Wes, I went to that website you posted and I can't find anything on there about a drill mod.

I'm in the middle of it now. You are right, a 1/8 bit will not fit passed the orifice but a 7/64 does.
As a matter of fact that 7/64 bit falls down into the line about 1 1/2 inches,,,,yikes,,,,how far down into the line is this restriction????

**Wesman** · 11-23-2008, 04:17 PM

Originally Posted by Firebirdjones

Well Wes, I went to that website you posted and I can't find anything on there about a drill mod.

I'm in the middle of it now. You are right, a 1/8 bit will not fit passed the orifice but a 7/64 does.
As a matter of fact that 7/64 bit falls down into the line about 1 1/2 inches,,,,yikes,,,,how far down into the line is this restriction????

Try this link:

http://www.installuniversity.com/ins...r_cylinder.htm

And yes, the bit will fall into the line about 1-1.5 inches, the restriction isn't right at the top of the line like you'd think it would be. The restrictor itself isn't very long though, once you start drilling you should be through it pretty quickly.

**95camaroZNC** · 11-23-2008, 04:27 PM

Originally Posted by Firebirdjones

Well Wes, I went to that website you posted and I can't find anything on there about a drill mod.

I'm in the middle of it now. You are right, a 1/8 bit will not fit passed the orifice but a 7/64 does.
As a matter of fact that 7/64 bit falls down into the line about 1 1/2 inches,,,,yikes,,,,how far down into the line is this restriction????

McLeod Master Cylinder and Drill Mod Install is what its under in the library.

**Firebirdjones** · 11-23-2008, 05:36 PM

Okay thanks, I got through it with the 7/64 bit then went ahead and drilled with 1/8 inch.
Put it back together and bled the system. Seems okay with same pedal feel. I may adjust the Ram master cylinder again for a little more free play.
Didn't get to hammer on it and see if it improved anything,,,maybe tomorrow.

**Firebirdjones** · 11-24-2008, 12:47 PM

Well I did hammer on the car today. I really don't notice any clutch action difference, the shifter still feels sticky in all gears when I get in a hurry. I think the adjustable master cylinder made the most improvement in the car when I installed that a few months back.
I don't think it's going to get any better for me unfortunately. Shame that I don't feel comfortable hanging gears in it,,,makes going to the track questionable for me.

The car drives around town just fine and effortless though, and my wifes happy so maybe I should leave it well enough alone.

**Rob00Taws6** · 12-16-2008, 04:45 PM

usually i do all my own work on my car, but when i decided to change the clutch on my car i decided to hand my car off to a local shop that i trust deciding that dropping the transmission on my car without a lift and by myself would just be too much. when i saw 4 guys struggling to get the T56 out of my car i felt that it was definitely money well spent. I asked the owner of the shop wile he had the transmission out of the car to do the drill mod, he said that he was going to flush out the fluid in the clutch hydraulic system, and that i wouldn't need it. i trust him and he was right. The clutch i put in my car wasn't drastically different, i went with an LS6 clutch. when I have the tools, and the money to make the changes that i want to make, i'll put a better clutch in the car. that when I'll make sure the clutch disengages as quick as it should, but for now i'll keep on top of the fluid in the clutch master cylinder!

**Aetos** · 12-16-2008, 05:02 PM

Originally Posted by Rob00Taws6

usually i do all my own work on my car, but when i decided to change the clutch on my car i decided to hand my car off to a local shop that i trust deciding that dropping the transmission on my car without a lift and by myself would just be too much. when i saw 4 guys struggling to get the T56 out of my car i felt that it was definitely money well spent. I asked the owner of the shop wile he had the transmission out of the car to do the drill mod, he said that he was going to flush out the fluid in the clutch hydraulic system, and that i wouldn't need it. i trust him and he was right. The clutch i put in my car wasn't drastically different, i went with an LS6 clutch. when I have the tools, and the money to make the changes that i want to make, i'll put a better clutch in the car. that when I'll make sure the clutch disengages as quick as it should, but for now i'll keep on top of the fluid in the clutch master cylinder!

The owner of the shop is not correct and topping of the fluid in your cylinder won't solve anything. The line restriction does not make your fluid magically evaporate. The line restriction does not allow fluid to flow quick enough to disengage a clutch at high RPM which results in sloppy clutch feel/slipping. Do yourself a favor and either buy a Tick master cylinder or do the drill mod.

**Firebirdjones** · 12-16-2008, 05:12 PM

Originally Posted by Rob00Taws6

usually i do all my own work on my car, but when i decided to change the clutch on my car i decided to hand my car off to a local shop that i trust deciding that dropping the transmission on my car without a lift and by myself would just be too much. when i saw 4 guys struggling to get the T56 out of my car i felt that it was definitely money well spent. I asked the owner of the shop wile he had the transmission out of the car to do the drill mod, he said that he was going to flush out the fluid in the clutch hydraulic system, and that i wouldn't need it. i trust him and he was right. The clutch i put in my car wasn't drastically different, i went with an LS6 clutch. when I have the tools, and the money to make the changes that i want to make, i'll put a better clutch in the car. that when I'll make sure the clutch disengages as quick as it should, but for now i'll keep on top of the fluid in the clutch master cylinder!

My wifes car has the LS6 clutch in it as well. It's fine for moderate power. I recently did the drill mod on it and don't notice any difference in the car or shifter/clutch feel at all.
The biggest difference I made in the car was installing an adjustable Ram master cylinder. That was wonderfull in that I could adjust the free play and get the pedal feel that was necessary. My wife couldn't even drive the car until I had that in there and adjusted. Now she loves it. The stocker just wouldn't disengage the clutch far enough to be comfortable,,,it grabbed right off the floor, it was rediculous...

**Rob00Taws6** · 12-16-2008, 05:26 PM

Originally Posted by Firebirdjones

My wifes car has the LS6 clutch in it as well. It's fine for moderate power. I recently did the drill mod on it and don't notice any difference in the car or shifter/clutch feel at all.
The biggest difference I made in the car was installing an adjustable Ram master cylinder. That was wonderfull in that I could adjust the free play and get the pedal feel that was necessary. My wife couldn't even drive the car until I had that in there and adjusted. Now she loves it. The stocker just wouldn't disengage the clutch far enough to be comfortable,,,it grabbed right off the floor, it was rediculous...

Mine didn't grab right off the floor, but the pedal moved so incredibly easy that i thought that there was a problem at first. after 500 miles of breaking the clutch in the pedal firmed up and i couldn't be happier with how it feels. I'm reluctant to do this mod because everything is working just great right now, but I'm still reluctant to horse third gear, i don't think that going to change. i can see the theory behind the mod, and im sure it works better as the hydraulic fluid ages and becomes more contaminated, or shifting the car when its cold. But, i know enough not to bang third, and not to beet on my car when its cold.

**Rob00Taws6** · 12-16-2008, 05:34 PM

thats another thing too, i was having minor problems with my stock clutch, and the LS6 clutch was worth changing into my car, but anyone thats having the same problems with a lightly modified car. if they want to try saving a buck i would try this mod as well as flushing the old fluid out, just for the hell of it i would like to see if it works. sounds good in theory and it cost a lot less than a new clutch.