Grade 8 Bolts

[KVH]

Any harm using those in the engine block or will they stress the tapped threads too much?

 

[Roger]

Only if you over-tighten them! Otherwise, no problem.

 

[Geo Hahn]

Which bolts?

Also -- never thought about it before but since (as I understand it) a bolt or stud actually stretches with the application of torque -- are grade 8 bolts torqued to the same spec as, say, grade 5?

 

[DougF]

Grade 8 can be safely torqued to a higher level than grade 5 without distorting. If substituting a grade 8 for a grade 5, I would maintain the same torque. If the product manufacturer specifies a grade 8, I would not substitute it with a grade 5 bolt.

Likewise, i wouldn't substitute a stainless fastener for a grade 8. It is often misconceived that stainless steel fasteners are strong because they tend to be difficult to drill. This is not true. Depending on the grade of stainless, they can be very prone to distortion.

I have seen tests where, depending on the quality of the fastener, after the first use a grade 5 bolt can lose upwards of 40% of their torquing capacity, ie. your torque wrench shows 40psi but you are actually only getting 24psi of torque. Grade 8's don't lose as much capacity with each use, though imports tend to not hold torque as well. Imports can often be identified by an excessive oil coating to help curb corrosion.

 

[TR3driver]

Also -- never thought about it before but since (as I understand it) a bolt or stud actually stretches with the application of torque -- are grade 8 bolts torqued to the same spec as, say, grade 5?

It depends. Generally when designing a joint, the only reason to specify grade 8 would be for extra strength; in which case you need more torque. But if you are just substituting grade 8 because you feel like it then I would stick with the original torque; especially if there is a gasket involved (like pan bolts).

Also need to be a little bit careful about both bolt length and grip length when you go substituting bolts. My TR3 block has several damaged holes because some DPM (darn previous mechanic) used the wrong bolts.

 

[bobhustead]

If you should wring or shear a grade 8, the effort required to get the stub out can be significantly greater, particularly in getting the center hole in the right place.Bob

 

[JImEgan]

One problem is that hardware store grade 8 bolts are usually brittle. ARP makes stud kits for triumph engines if you really want to go that route. Check out Carroll Smith's book :[h=3][/h][h=3]Nuts, Bolts, Fasteners and Plumbing Handbook[/h]

 

[Hap Waldrop]

I use grade 8 bolts, washers, and nuts in many places not really needed for the reason grade 8 hardware come in yellow zinc plating, rather than white zinc (grade 5), or black oxide. I don't need a grade 8 bolt for the timing cover, or oil pan, but I like the yellow zinc color, so that's what I use.