I recently picked up a 640-3 as part of a trade deal. It has good timing, but had a little too much endshake, about .004 which I think is way too loose for a .357.

I corrected the endshake with endshake washers and after that the B/C gap measured .010 and the headspace was .063.

I noticed that the extractor was below the rear of the cylinder and wondered what would happen if I put the endshake washers under the extractor instead of under the crane. Well I tried it and now the extractor is flush with the rear of the cylinder, B/C gap is .006 and headspace is .067, still in spec.

Now I need to ask the experts, is this a valid fix? Does anyone else do this or should I put the washers back under the crane and live with the large B/C gap?

__________________
Ron
NRA Life Member

The only way to fix a large air gap is to turn the barrel. Remove that the bearing from under the extractor.

Also, what controls head space is that raised boss on the center of the extractor. This is one spot you never rub down.

I heard extractors are matched with cylinders to set headspace. Perhaps, this isn't done any longer with MIM guns, I just don't know.

__________________
NRA Benefactor Member

Thanks 500, I already put the washer back where it belongs. This is a newer gun that has been shot very little. It probably came that way from the factory, I believe Smith says a .010 is still in spec. Not like the good old days when they were held to a tighter standard.

I guess I'll try shooting it. I don't have the tools to do a barrel turn and it's not worthwhile spending the money unless the gap was much larger.

__________________
Ron
NRA Life Member

If you sent it back they wouldn't even touch it.

I like tight gaps too, because stuff gets blown out sometimes. I've been hit by shooters in other lanes too!

.01 is ok, I've seen worse.

__________________
NRA Benefactor Member

The model 66 I'm running in IDPA currently has a .010 gap, which you would never know unless you measured it. I perhaps added .1 grain of HP38 to make power factor compared to my .006 gap 686?

__________________
Science plus Art

Before you jump to the conclusion that .004" gap alone is what causes your 66 to need .1 gr. more HP38 to match the velosity of your 686 I suggest rereading the protion of the old Speer reloading manual titled Why Ballisticians Get Grey. In short, they tested a whole bunch of revolvers from various manufacturers and concluded that identical appearing revolvers that could have rolled off the assembly line side by side vary in velosity by 300 fps when tested with the same lot of ammo. I'll dig into by old book storage latter and edit in the Speer reloading manual number.

That said, I agree with you that flash gap size gets more attention than deserved. I think that's because it's so easy to measure.

That's why I put the question mark at the end of the sentence. I don't really know why one revolver is consistently 30fps slower than the other. The only difference I can measure is the gap, but it might be minute barrel differences. Who knows? Either way, obsessing over the barrel gap is pointless unless it is way out of spec.

__________________
Science plus Art

I've been shooting the 640 a number of times since my original post with the B/C gap question.

I'm happy to report that the gun works fine. Quite accurate with a variety of .357 ammunition and no spitting or any other indication that it has a larger than normal gap.

I'm satisfied with it and it's become a favorite carry gun.

I found this old article mentioned by K22fan on the effects of the gap on velocity very interesting.
http://dwcollectors.org/dwca-content...un_article.PDF

__________________
Ron
NRA Life Member