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| Joined: | 03-12-2005 |
| Location: | Kingman, Arizona USA |
| Posts: | 532 |
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All of the original suspension hardware is supposedly Grade 5. I have no clues to the design standards used by Vauxhall suspension engineers in the 1960s, and would therefore presume, for safety's sake, that these bolts are just barely strong enough to avoid failure during service.
What this means is that you may be able to replace the original bolts with smaller diameter Grade 8 bolts, or their metric equivalents. I'd use smaller bolts only if they had a yield strength at least equal to that of the original parts. You might need to consider using super-strong bolts from vendors such as ARP Fasteners. Remember that a suspension collapse will usually result in a catastrophe.
In general, drilling or reaming out the metal core of a bushing is fine, provided this leaves enough metal in place to give a reasonable service life. A bushing failure merely requires replacement before the bolt starts to be abraded, so from a safety standpoint there's a lot more leeway here. If you do drill or ream out the bushings, be sure to use a good anti-sieze compound on the freshly exposed metal.