Elongated rockerbox bolt query

[t20racerman]

I'm rebuilding my alloy A10 after 18 months off the road. Unfortunately one of these elongated bolts got mislaid, so I bought another. However, the shank was too wide to fit inside the head.
I phoned the supplier who was adamant that his part was correct. He said that he had two heads to hand, and that his pattern part fitted both heads perfectly. Said it should be 7/16" diameter, which it was.
Mine is 0.406" which admittedly, is an odd size.

Any ideas why the bigger one doesn't fit my head? I bought the ones on my bike about 40 years ago when I picked up an alloy head for it.

Puzzling

[berger]

do you mean it won't fit the hole in the head or won't start to screw on the stud from the rocker box . when these nuts won't start on the thread i have known them to be machined down a tad to give a bit of slop.

[t20racerman]

I mean that it won't fit into the head. Just turned the shoulder down a bit to match my old one and they are both now fitted. I'm just puzzled as to why my old ones have a smaller diameter and the 'correct' ones don't fit.
Could be my head is burred over a bit, but I can't tell now as the head is fitted.
Problem was easily solved but I just wondered if anyone had come across this before.

[RDfella]

Have you tried all the holes? Check the hole hasn't been squeezed at the top by the old bolt. Holes like this - especially in aluminium - should have a slight chamfer. Maybe needs a pass with correct drill (assuming the correct size).

[Svend]

Could it be that you have those for the dynamo strap or battery holder ? They are quite similar. Just a thought. Svend

[muskrat]

G'day T20.
Sometimes if the washer that goes above the nut is omitted the hex of the nut can rub on the head and burr the hole.
I have made thicker washers to remedy that. Also I get those nuts started before tightening the other r/box bolts.
Cheers

[CheeserBeezer]

This will be a pattern parts issue.