Notices
924/931/944/951/968 Forum Porsche 924, 924S, 931, 944, 944S, 944S2, 951, and 968 discussion, how-to guides, and technical help. (1976-1995)
Sponsored by:
Sponsored by:

How do I check 968 main bearing crush?

Thread Tools
 
Search this Thread
 
Old 11-30-2005, 09:06 AM
  #1  
gnosis
Instructor
Thread Starter
 
gnosis's Avatar
 
Join Date: Apr 2002
Location: Sydney, Australia
Posts: 174
Likes: 0
Received 0 Likes on 0 Posts
Default How do I check 968 main bearing crush?

I hope someone can help me. I have my 968 engine in bits at the moment, and before I start to put it back together I want to know if the (new) main bearings will have enough crush. This raises two questions:

1. What is considered an acceptable amount of main bearing crush for a 968 engine?

2. How do I measure bearing crush when the main bearings are held together in a cradle (which prevents me from undoing one bearing cap nut and measuring the crush with a feeler gauge, as is the accepted method of measurement with separate main bearing caps)?

Any help appreciated.

Clayton
Old 11-30-2005, 10:20 AM
  #2  
GlenL
Nordschleife Master
 
GlenL's Avatar
 
Join Date: Jul 2003
Location: Minneapolis, Minnesota
Posts: 7,654
Received 29 Likes on 23 Posts
Default

Plastigage. Little threads of plastic that you put in between the bearing and the crank. You torque it down, open it up, and measure the width of the squished pieces.

Then there's the back-yard approach of just making sure the crank moves smoothly after installation. Measure the journals is advance, of course, but it's the action of the crank that's really important. Smooth with no sticky spots.
Old 11-30-2005, 05:15 PM
  #3  
gnosis
Instructor
Thread Starter
 
gnosis's Avatar
 
Join Date: Apr 2002
Location: Sydney, Australia
Posts: 174
Likes: 0
Received 0 Likes on 0 Posts
Default

Sorry, but plastigage is used to measure bearing clearance - not bearing crush. Clearance is the gap between the torqued up bearings shells and the shaft they surround. Bearing crush, on the other hand, is the force the bearing caps exert on the bearing shells themselves. This force must be adequate to ensure the shells themselves are not loose. Too much crush and the shells will deform. Too little and the'll rotate in place. If it happens, this latter phenomenon is called a "spun bearing", which is what I'm trying to ensure I avoid. Especially since my crank and bearings are standard size, and the clearances are on the high side.

I used plastigage to find the clearances, but I need to know how to ensure the crush is okay. If the engine had single main bearing caps, the procedure would be easy - just torque up the cap, then undo one side and measure the amount the cap springs up away from the block. The problem with our engines is that you can't make that measurement because all the main bearing caps are connected together in a big one-piece cradle.

Any other ideas?

Clayton



Quick Reply: How do I check 968 main bearing crush?



All times are GMT -3. The time now is 06:07 AM.