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GM Covers 4 Bolt Options


GM Covers 4 Bolt Options


Bolt Information For PML Covers on GM 7.5, 7.625, 8.5, 8.625, and 9.5 Differentials

GM has used four different types of bolts on these differentials:

  • M8–1.25 x 21mm (Metric, 8mm diameter, 1.25mm pitch, 21mm length)
  • M8–1.25 x 16mm (Metric, 8mm diameter, 1.25mm pitch, 16mm length)
  • 5/16in–18 x 7/8 (Standard, 5/16–18 threads, 7/8in length)
  • 5/16in–18 x 5/8 (Standard, 5/16–18 threads, 5/8in length)

PML includes mounting bolts with all differential covers. PML provides bolts that are longer than the stock bolts since the flange on the PML cover is much thicker than stock.

To complete your order, we need to know which bolts you have.

This can be determined by looking at one of the stock bolts or sometimes with a call to the dealer or by looking on line to order replacement bolts.

Guidelines for determine stock bolt specifications:

To definitively determine the stock bolt specifications, remove one of the stock bolts from the cover. Choose a top bolt is best to avoid leaks. Then measure the length and determine the thread.

  • A bolt head with lines or letters is typically standard.
  • A bolt head with numbers such as 5.8, 9.8, 10.9, or 12.9 is likely to be metric.
  • Bolt length is measured from the underside of the head (where a washer would sit) to the tip.
  • If the bolts are metric, the length will be 21mm or 16mm.
  • If the bolt are standard, the length will be 5/8 inch or 7/8 inch.
  • Metric bolts will have 20 threads per inch. Standard ones will have 18 threads per inch. This is very hard to count since the thread is less than 1 inch long.
  • Many hardward stores will have thread and pitch charts and you can take a bolt in to compare to the charts.
  • Use a thread pitch app on a smart phone to determine the type of stock bolt.

We know this is a hassle, but we do not want to send the wrong bolts or charge everyone a lot of extra money to cover sending all possible bolts with each cover.

Most vehicles after 2001 have 8mm–1.25 x 21mm stock bolts but PML cannot guarantee this.

Examples

Click on an image to view a larger one in a new tab or window.

Metric: These are an examples of 8mm–1.25 x 21mm stock bolts. In the first picture, the 0.8 inch length is about 21 mm. In the second picture the numbers 10.9 are just visible.
M8 bolt, 21mm long   Bolt head, M8, 21mm long  

Standard: These are examples of 5/16in–18 x 7/8 bolts. The first picture shows a thread gauge checking the bolt. The second picture shows the head of the bolt with a line on it. The third picture shows the length as 2.2 centimeters, which in inches is 0.875 (or 7/8).
Bolt 5/16, standard, thread gauge   Bolt 5/16, standard, with a line on top   Bolt 5/16, standard, 7/8 inch length

Video

This video can help with determine the size of the stock bolts.