window screw sizes? BA or metric?

Birdseye

Birdseye

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Helping a crew member reseal the windows in her Contessa 28 and the screws are an absolute pig to get out. Have succeeded with one window but one or two of the screw heads have been damaged in extraction. Given that the boat is from the 80s its possible that the screws could either be imperial ( BA even) or metric. How do I tell?

The thread diameter is 3.55 mm which means that it fits an M4 nut OK but is a bit loose.
 
I have this problem from my 1970's boat .... they are BA .... and I was able to replace them with metric ...

I could have used BA - as the shop in town specialises in all sorts of nuts / bolts but decided to push out the dome headed BA and replace with Metrics.
 
I think a 3.55mm thread diameter is likely to be BA. Metric interscrews are either 3.5mm or 4mm nominal size, with the actual thread either that size or slightly under.

BA4 are 3.6mm nominal size (which could account for your 3.55) and 0.66mm thread pitch. That pitch is close enough to M4's 0.7mm that, combined with the BA thread's smaller diameter, it will "fit" - but wreck the bolt thread if it is put under any force.

BA5 are 3.2mm nominal size and 0.59mm thread pitch. That pitch is so close to M3.5's 0.6mm that it also appears to "fit" (but, again, will wreck the bolt thread under any force at all).

It's still possible to get both BA and metric interscrews through a small number of niche suppliers... I only know this because a friend of mine was incorrectly supplied BA by accident and it caused us some real trouble. They're expensive. But nearly all boat window screw options are expensive. And new ones are stainless which may be saving up problems with aluminium windowframes.

It is hard to avoid damaging old interscrews, the brass under the chrome is quite soft and they are often rather stuck together after 40-50 years. The date I need to do mine is creeping forward and I'm casting around for other options.
 
Strangely enough, only an hour or so ago I came across a box of old interscrews from when I redid the windows in a boat maybe ten years ago.

Those in the boat were Imperial (BA IIRC), and like you some were damaged or destroyed trying to take them out. I was unable at the find exact replacements at the time, so replaced the whole lot with the metric equivalents.

I obtained those from Hadlow Marine, who I found very helpful. (Other suppliers available.)
http://hadlowmarine.com/Pages/Interscrews.html
 
justanothersailboat said:
I think a 3.55mm thread diameter is likely to be BA. Metric interscrews are either 3.5mm or 4mm nominal size, with the actual thread either that size or slightly under.

BA4 are 3.6mm nominal size (which could account for your 3.55) and 0.66mm thread pitch. That pitch is close enough to M4's 0.7mm that, combined with the BA thread's smaller diameter, it will "fit" - but wreck the bolt thread if it is put under any force.

BA5 are 3.2mm nominal size and 0.59mm thread pitch. That pitch is so close to M3.5's 0.6mm that it also appears to "fit" (but, again, will wreck the bolt thread under any force at all).

It's still possible to get both BA and metric interscrews through a small number of niche suppliers... I only know this because a friend of mine was incorrectly supplied BA by accident and it caused us some real trouble. They're expensive. But nearly all boat window screw options are expensive. And new ones are stainless which may be saving up problems with aluminium windowframes.

It is hard to avoid damaging old interscrews, the brass under the chrome is quite soft and they are often rather stuck together after 40-50 years. The date I need to do mine is creeping forward and I'm casting around for other options.
Click to expand...
measured more carefully the diameter is 3.56 and the pitch is 0.8/.85 Difficult to measure pitch reliably but definitely not 6 or 6.6. M4 metric but if so why would they be loose in an M4 nut? maybe its just best to junk them and replace with M4 pan head screws and dome nuts
 
Birdseye said:
measured more carefully the diameter is 3.56 and the pitch is 0.8/.85 Difficult to measure pitch reliably but definitely not 6 or 6.6. M4 metric but if so why would they be loose in an M4 nut? maybe its just best to junk them and replace with M4 pan head screws and dome nuts
Click to expand...
Good check, thanks for the updated measurements. OK, they could also be either weird, or some really Imperial thread rather than BA. Surely not M4 at that diameter. .8ish pitch is steeper than M4, could be 32tpi?

Nothing's quite as neat as an interscrew but they're also a bit of a pain, I can see the appeal of changing to dome nuts. I probably will do the same in a year or so.
 
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