Clutch Pressure Plate Bolts

[A26B]

Maybe I shouldn't be concerned about it, since I've not heard of any catostrophic failures, just loose bolts about to cause problems.

Changing out flywheels from the nodular iron, dual mass to aluminum flywheels, Fidanza in my case, is where problem manifests, at least in my mind and a few others. The problem is...using stock bolts in an aluminum flywheel and the lack of sufficient thread length engagement.

Now, there is a new set of circumstances with a potential problem also pertaining to pressure plate bolts. The replacement pressure plate being sold today is the same height (thickness) as the LT1, which is thinner than the original LT5 part. Stock bolts are too long and are being replaced with "off the shelf" bolts of varying grades. This concerns me.

The following photos show the difference between "off the shelf" bolts and the OEM design. Its necessary to add a dimension to thinking about the job bolts do when it comes to clutchs. Mostly, bolts are stressed axially. Clutch bolts are stressed axially to, but also in shear. The OEM bolt is designed to cope with shear, "off the shelf" bolts are not. The difference is shown in the photos. OEM bolts have more beef where shear stress occurs and no stress risers. Shear stress is imparted to the counterbore in the flywheel as opposed to "open" threads in the counterbore area and pressure plate bolt hole.

Ron Woods offers the ideal replacement bolt for use with the aluminum flywheel. I don't know if he has made one for the thinner, replacement pressure plate or not. It may not be necessary. The stock LT1 bolts may be the ticket if used on the irn, dual mass flywheel. If used on an aluminum flywheel, I hope Ron makes a new bolt for that application.

I'm old school and have seen the aftermath of flywheel & clutch explosions at high RPM. It ain't pretty. Such a small item, often overlooked but very important. Let's discuss & see if the result is worthy of a "sticky" recommendation.

Jerry

[Kevin]

great thread jerry, thanks for keeping an eye out for this. this is great info to have

[rhipsher]

Hey jerry. I went to Fastenal and bought 9 7/16-20 X 1.25L and 6 3/8-16 x 2.25L grade 8 preasure bolts. The thread engagement is much deeper than the stock bolts with the same shear engagement as the stock ones. They just have 1/2 inch more thread length. As we speak I've got the Fidanza mounted and just bolted up the clutch/preasure plate. Getting ready to put the bellhousing on. So don't scare me like that man. I don't need anymore ZR-1 heartaches this year. I'm trying my best to get it ready for Thunder II.

[A26B]

Originally Posted by rhipsher

........and 6 3/8-16 x 2.25L grade 8 preasure bolts. The thread engagement is much deeper than the stock bolts with the same shear engagement as the stock ones. They just have 1/2 inch more thread length........

I'm not sure what you are referring to as "shear engagement." Please provide a bit more info about this.

The following comments pertain only to the OEM pressure plate, NOT the new, "thinner" replacements.

You can see that the STD bolt will not work. The threads are long enough to provide the additional engagement in aluminum, the shank of the bolt is too short @ 1.32". The shank is not long enough to extend below the pressure plate, into the flywheel counterbore. It leaves threads exposed above the counterbore and less resistance to shear forces. There are other deficiencies with the STD bolt. (1) The head does not have a flange that will prevent digging into the alum pressure plate, (2) the diameter of the shank is 0.004" smaller and (3) the shank is not machined square at the transition to threads. Such a transition would make it difficult to obtain sufficient shank penetration into the counterbore and not bind on the thread transition before the bolt clamps the pressure plate to the flywheel.

This is waaay long winded but there is simply no good "off the shelf" substitute for the OEM design. It was engineered with a purpose, from the flange, to the shank dia, length & thread juncture.

The OEM bolt is marked as a Grade 5. Grade 8 has a higher tensile strength. Grade 5 will bend or stretch more before it breaks, than a Grade 8 will. Grade 8 can be torqued higher, providing more clamping force, but it is less resistant to breaking from flexing & stress risers. Grade 8 offers some compensation to the "off the shelf" design issues, but doesn't solve them.

I'm not trying to scare you, just want to make sure this subject reaches it's proper resolution for everyones safety.

See you at Thunder II!!

[rhipsher]

They may not be the perfect replacement bolts but what other choice do we really have other than machining your own and heat treat them to factory specs. Or drill and tap the Fidanza for a 3/8-16 helicoil which could use the stock preasure plate bolts and have stronger thread engagment. Those are the only alternatives I see. And I am also using the factory preasure plate. Not the thinner replacement.

Originally Posted by A26B

This concerns me.

Jerry it is of concern, but even after telling people, they just don't pay attention or care. The proper thing to do is to call up your local dealership and order the bolts for the LT1/4 pressure plate. It's 6 bolts!

The use of the fidanza flywheel however is a different issue. You going into alum rather than steel. What Randy/Ron is trying to do is get more thread engagement into the alum. With that being said, I'm running the factory bolts with my fidanza and have no issues and I beat my car.

[A26B]

rhipsher: The only correct option at present is the bolts Ron Woods makes.

http://www.zr1products.com/products.html



Jeff: Right on about buying the LT1/4 bolts!

New flywheel/threads, proper use of thread lock compound, correct bolt tightening sequence & torque certainly reduces the liklihood of failure. You know & practice proper technique. Not everyone does. Those are the fellow ZR-1ers we are concerned about. Thanks for chiming in on this thread.

[A26B]

Pressure Plate Bolt Application Guide

Combo 1: OEM Dual Mass or Rippie Iron Flywheel with thinner, replacement pressure plate (LT1/4)

How to tell? On the workbench; Place a bolt in the new pressure plate. If the shoulder (disregard the threads) extends through the pressure plate more than 0.090" (3/32") then you have the thinner replacement unit. If shoulder only extends through 0.090" (3/32") then you have the OEM thick unit.

Remedy: Obtain new bolts (6) from GM for LT1/LT4 clutch pressure plate.

Combo 2: Aluminum, Fidanza type flywheel with OEM thickness pressure plate.

How to tell: On the workbench; Place a bolt in the new pressure plate. If the shoulder (disregard the threads) extends through the pressure plate more than 0.090" (3/32") then you have the thinner replacement unit. If shoulder only extends through 0.090" (3/32") then you have the OEM thick unit.

Remedy: Obtain new bolt kit from SRP, "Extended Length Pressure Plate Bolts" web link is http://www.zr1products.com/products.html



Combo 3: Aluminum, Fidanza type flywheel with thinner than OEM replacement pressure plate.

How to tell: On the workbench; Place a bolt in the new pressure plate. If the shoulder (disregard the threads) extends through the pressure plate more than 0.090" (3/32") then you have the thinner replacement unit. If shoulder only extends through 0.090" (3/32") then you have the OEM thick unit.

Remedy: Ideal solution is a bolt not yet available. Until such time, obtain new bolts (6) from GM for LT1/LT4 clutch pressure plate. Use thread lock compound, following preparation instructions, cross tighten bolts evenly and do not over-torque.

[Jerry#397]

Jerry, I and others want to thank you for all the work put in on this post, one of the best; detail is just outstanding and the pic's are worth a thousand words. I am in contact with Randy and will post after talking to him and Carolina clutch on Monday.

Again, your efforts are much appreciated.

Jerry
93 ZR-1

[jonszr1]

having seen a buddys mustang litterly cut in half by a clutch explosion . my toes want to thankyou and both my zs thankyou too