Why You Have Broken Flywheel Bolts
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(all information below is a recommendation from Race-1 not necessarily from GM)
We have seen several engines with broken flywheel bolts in recent years. The blame is immediately placed on the bolt, and that is partially correct. The actual cause in most cases is the length of the bolt and not the grade or quality. A common length bolt to be used is 1¼ inches in length and when used with a Bert flywheel and no shims—it will be too long. Even with a washer under the head of the bolt, in most cases this will still be too long.
In ALL installations the bolt length should be verified to be correct.
- 1) begin by measuring the depth of the hole and the thickness of the flywheel—add those two dimensions and subtract .100” (little less than 1/8”) and verify the total thread length from your calculation.
- 2) Double Check:
- Install the flywheel and torque the bolts to 65 ft. lbs.
- Remove the bolts and inspect the first thread on the bolt.
- If the bolt has a drag when being removed—chances are it is too long.
- If the first thread is mushroomed chances are the bolt is too long.
- A bolt that is too long by 1/16 to 1/8 of an inch will appear to be tight but in actuality is not, and it will shear (break) because of constant side loading.
NOTE: Manufactures have many different tolerances and styles—the example given above is what we have found for a BERT flywheel used on a GM crate engine that is externally balanced (602–604). ALWAYS VERIFY YOUR PARTICULAR APPLICATION!!!!