I am replacing my 4.0, going through everything at the same time. I have a lot of rusty bolts in the engine bay, and would like to replace the non-essential (so not grade 8) bolts to stainless. I have misplaced my bolt guide, but was wondering if someone already had put a list together. I can't seem to find anything other than a company that makes a kit for the CJ. I'll go buy another bolt sizer and do a write up if nobody knows. Thank you,

 

No list for your sizes,however, 300 series stainless has a higher tensile strength than carbon steel, this is good for bolts applications, stainless is comparatively tougher, not harder, tougher. Stainless is not hard and can gall (metal to metal transfer) when tightening, so use a lubricant! For our work we order our fasteners in "waxed" to resist galling. At home use a good anti seize, it's the favour you do for yourself in the future.
An 18-8 Stainless is a very common grade fastener (basically a grade 304 stainless). It will provide you with a little better than an equivalent steel grade 5 bolt. For applications requiring more strength, use a 316 grade stainless bolt, that will give you an equivalent to a grade 8 steel fastener.
I transition from stock metric to imperial inch sizes when I can, easier to find inch fasteners variety in stainless. In applications where you are screwing into existing holes, get a set of thread gauges to measure you pitch and a caliper to measure your diameter.
You can be both for under $50 easy. Then Google standard fastener sizes and print off a chart from your search. To find fasteners, sometimes Farm supply can have some of the odd sizes on hand, also McMaster Carr is an excellent source and has huge catalogue on line.

 

www.boltdepot.com

 

18-8 or 304 hardware is really soft. There's dozens of time when I broke off the heads while tightening and I wasn't even torquing a lot either.

 

Thanks, great info!

I actually have a complete metric assortment. Was really bummed to learn that the engine is all standard. Oh well.

 

I think you may want to re-check your specifications...
The properties of Grade 5 and Grade 8 Hex Head Cap screws for automotive applications fall under SAE J429. Stainless fasteners fall under ASTM F593 which has serveral specifications for 304 alloy.

By "several" I mean 4 different specifications covering different size ranges and different min. tensile strengths. (ie. F593A is 65 ksi, F593B is 75ksi, F593C is 100ksi and F593D is 85 ksi. ALL of which are less than the MINIMUM tensile strength of SAE J429 Grade 5: 120 ksi.) Grade 8 MINIMUM tensile strength is specified at 150 ksi. (1 ksi = 1,000 psi)

That said, the greater issue is that of minimum yield strength. That is the point at which a material deforms plastically. Translation: it loses its elasticity. 304 stainless has a minimum yield strength anywhere from 20 ksi to 65 ksi. Grade 5 is 105ksi and Grade 8 is 120 ksi.

In other words, 304 and even 316 stainless is considerably WEAKER than Grade 5 and Grade 8 carbon steel bolts. This is evidenced by the simple fact that if you torque stainless nut to stainless bolt it galls and then friction welds itself. Which is why, as you said, anti-sieze is needed.

There ARE higher grades/classes of stainless available that have more suitable mechanical characteristics for critical applications but they are harder (and more expensive) to source.

BooGTS. You have the right idea in not using stainless where Grade 8 bolts exist but make sure you are careful which Grade 5 bolts you replace as well. Just because it's a Grade 5 bolt does not necessarily mean it's not a critical point.

Another important point to note is that all torque specifications are given for CLEAN, DRY threads. As soon as the threads are not clean and dry the torque specification changes. With anti-sieze on the threads the torque specification reduction can be as much as 65%. as in you only apply 35% of the amount of torque you are supposed to. Take my word for it: I learned this one the hard way and did the research after the fact to understand where I went wrong.