kylecycler

I've been using Blackburn bar end mirrors for a while and they work exceptionally well - a well engineered piece of kit. Provided you tighten the pivot bolts every once in a while (they tend to loosen when you swivel them in and out to get through doorways when storing the bike), and keep the knurled plastic knob tight against the mirror itself, they don't go out of position even on a fast, bumpy descent.

However, I've found that when I tighten the pivot bolts, the allen key I've been using isn't quite a perfect fit and over time I 'round off' (internally, of course) the button head allen bolt (if you click on the link above you'll see the bolt I mean).

I eventually took one of the bolts out (while I still could - just) and measured it, to see what to get as a replacement. It measured ~ 4.5 mm - neither M4 nor M5 - and that's when it dawned on me that I'd been using a metric allen key (I live in the UK, and although I ride miles, not kilometres, every other bicycle fitting I've come across is metric), but Blackburn is a US company and although the mirror is manufactured in China, I guess Blackburn might have specced the bolts for their home market (either that or they're M4.5, if there is such a thing)? I wouldn't know if that even makes sense, but if it does then that's why I've been rounding the bolt heads.

That might be wrong, but can anyone please tell me what size button head allen bolt I should buy to replace the originals?

The mirror's bolts are 4.47 mm - 0.176" (whatever that is in fractions) - in width and 10 mm - 5/8" - long. The heads, FWIW, are 8.82 mm - 0.358" - wide. I think I count 11 or 12 threads over the 5/8" but it's hard to tell (let me know if I need to try harder).

I've found these on ebay uk:

https://www.ebay.co.uk/itm/UNF-BUTTO...1ARdOk-s2XvLOQ

https://www.ebay.co.uk/itm/1-4-UNC-S...-/122455801252

Sorry if all that's a bit of a faff to read (I might be making something simple sound stupid-complicated) but I'd appreciate anyone's help.

Thanks.

HillRider

There are indeed 4.5 mm bolts and they measure 0.177" in diameter so that is what you have. If the nut is molded into the mirror base, you will have to get the proper size bolt from a specialty fastener or industrial supply dealer since M4.5 is not a common size.

Alternately, you could drill a clearance hole completely through the base and use a longer bolt and external nut. Then you can choose what ever size bolt you wish.

Another alternative is to buy a new mirror.

alcjphil

Another possibility is that Blackburn, as an American company chose to use Torx bolts which are an American invention. Don't get me wrong, I think that they are great but only if you use the correct tool. As an English speaking Canadian, I think that all tools for bicycles should have Robertson head bolts(look it up). But then, my French speaking friends in Montreal will often refer to me as a Tête carrée

Bill Kapaun

Since I have an external/internal thread gage, I grabbed the caliper-
A 10-24/10-32 screw measures .180".

dsbrantjr

#8 is 0.164", #10 is 0.190 major diameter, so your measurements are right in the middle.

There is such a thing as 4.5mm screws.

There are two 4.5mm thread pitches, coarse at 0.75mm and fine at 0.5mm.

A call to Blackburn will likely yield confirmation of the size, and possibly some replacement screws. I would suggest other than button head screws if they will serve; the drive sockets are smaller in relation to the screw size, regular socket heads take a larger, stronger driver and are easier to obtain.

You also need to make certain that the drivers fit correctly and are not worn or they will damage the sockets.

kylecycler

Many thanks for the replies, knew I could rely on you folks, this was the right place to ask the question and I'm sure I'll find the answer.

Over here it's 3 a.m. so I'm off to get some shut-eye and I'll get back to you in the morning (for me!).

dedhed

Do they not have hardware stores across the pond where you can bring things in and try different bolts?

kylecycler

Sorry I didn't get back to this sooner, I've been awful busy.

I'd ordered up a bag of M5 bolts before I posted the tread but as expected they turned out to be too big, M4 being too small.

I should have tried a hardware store, right enough; they don't tend to stock button head bolts that small, but they could have helped establish the size. I should still follow dsbrantjr's suggestion, email Blackburn and see what they say, that would be the sensible move (so sensible I never thought of it!).

They're definitely allen heads, not torx, although torx fittings are the best, I think. Curiously, on Tuesday I was helping a pal fit a bike rack to his car, a big allen key wasn't working and it turned out they were torx bolts so I'd to use a torx bit.

Actually I'd been coming around to thinking they were what Bill Kapuan suggested - 10-24/10-32 - but this afternoon a pal with a set of metric and imperial allen keys let me try both - I'd been using a 3 mm allen key, which fits pretty snug, but it turned out that there wasn't an imperial allen key that would fit - the closest sizes were either too small or too big - which would suggest the bolts are metric after all, so I guess they're M4.5.

I think the reason I've been turning the heads is because the bolts aren't great quality, plus I've been using blue Loctite to keep them in place - they tend to loosen as you fold the mirror back out after folding it in for storage (over your way, you'll have the mirror on the left, not the right, so it would loosen as you folded it in!).

Anyway, if I don't get any joy with Blackburn and can't source M4.5 bolts I'll try tapping the threads out to 5 mm for an M5 - there seems to be scope for that, thread-wise, and the head will still fit. Or if that doesn't work out, drill straight through and use a bolt and nut as HillRider suggested.

All this fuss about a wee bolt, but I've got three flat-bar bikes with these Blackburn mirrors, and most of my riding buddies with flat bar bikes also use them (when I started riding with the group three years ago only a few had a mirror; now only a few don't). Those with drop bars almost all use the Cateye BM45 bar end mirror, btw (I use one on my road bike), which is also excellent.

I'll let you know how I get on.