Bigger sized bolts are really only of use if you have a damaged frame mount - as in, it's flogged out from round. Assuming of course, the manufacturer / designer / builder has not made a very basic stuff up.
I'd prefer to machine the frame hole to true circular, then machine a 'top hat' insertable washer /spacer.
In a perfect world, we'd have inserts like those in the cases at the swingarm pivot, in all the engine mount points . And all the mount points perfectly aligned. But it would make for very expensive bikes.
Many of you blokes that have done your own AF conversions, or purchased one, will be aware of the 'migratory tendencies 'of a frame conversion, directly refering the the engine mounts, of course.
I've never had a flogged out frame. That indicates idiocy, on the part of someone. . A bit of a harsh judgement, I know.
A very important things in relation to stopping frame cracking, especially with an aluminium frame, is correct shimming / spacing of the engine to the frame (a known, significant problem with the KX500s), correct sized washers /bolt heads (and spacers /shims) against the frame - as in large enough diameter, without going too far, and Not putting the sharp side of a spacer / washer against the frame - or engine. And Not having a too large spacer / washer going onto a radius of a mount.
Of course, the engine mounts 'face' is and important factor - flat and true, is the only way for it to be.
Proper bolt torque / making sure mount bolts /nuts don't come loose, should be a given / obvious.
'Necked down' bolts are very much a feature of a very torque specific fitting - very prominent in engine main components, and many other high stress, size minimised applications. The slightly smaller shank as against thread bolts, you so often come across, is just a by product of loose standards, though it could be argued (and is) that it is a half way house to the trick / controlled 'necked down' bolts . In normal use, with normal, non outrageous torque needs, it's a non issue, especially with correct washer / shim / spacer useage
'True Sized', non 'Engineering Bolt', bolts can be difficult to come by. In my frame making business, I use them for applications in sheer. Shock mounts, linkage mounts , axle block / plates to swingarm mounts etc, so there is no movement between any parts, full diametric contact, no weak points. Many of my frames / swingarms etc, mount / pivot points dimensions, are influenced by the bolts that I can obtain. I've recently had to make subtle changes to my DH frames swingarm pivot width, because I can no longer get the exact 'bolt' that I have used for more than 15 years. Give something a chance to 'move' in sheer, and your on your way to failure. Having the shank of the bolt either part way into the mount that it threads into, or, in the case of a nut being used, having No thread bearing on the mounts, makes for an incredibly stronger fitment point. Full shank / correct dia., shank usage can mean you can go to a smaller bolt, whilst still being a much stronger fitment than a dodgy bolt that's bigger.
I mention all that, because it is very beneficial to Not have threaded sections bearing onto frame mounts, or the engine itself, both for sheer point reasons, and point contact stress risers. That's where outer washers / spacers (and inner spacers) are such an important thing. Aluminium frames really benefit from correct fitment / bolt / shim / spacer usage, but steel frames won't complain about being set up well.
With a steel swingarm I'm doing for a 500 project, I'm currently tearing my hair out as to whether I just do a full steel rear axle mount, a steel axle adaptor plate to the steel Swingarm, or an aluminium axle plate to the steel swingarm. It would make it easier for different wheel fitments, and a tad lighter, at the 'most traveled' point of the swingarm, but it will make for a fair bit of extra machining - but, it's how I've always done my DH swingarms. Decisions, decisions
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