Mushroom vent screws sizes

[Dave Bebb]

Hi all got a leak on one of my outside mushroom roof vents what size screws do i need please i know when i take the old ones out they will snap off,i have got the sealant just been waiting for some good weather. do i have to re-drill and re tap a thread, or just re-drill and use self tappers thanks dave What length will 25 or 30 mm do i need thanks , going to get M5 in brass

Edited April 25, 2016 by Dave Bebb

[Guest]

Hi all got a leak on one of my outside mushroom roof vents what size screws do i need please i know when i take the old ones out they will snap off,i have got the sealant just been waiting for some good weather. do i have to re-drill and re tap a thread, or just re-drill and use self tappers thanks dave

Mine are drilled and tapped. When I removed the vent to reseal they came out no problem, but they were brass screws. If steel has been used they may well snap.

Pretty certain they were 5mm but it was some time ago.

[Paul C]

Very unlikely to get a self-tapping screw in. There's something to be said for carefully grinding off the existing bolt head, then once the mushroom vent is off grinding these flush with the roof. Then turning the mushroom vent around 45 deg and drilling/tapping new holes. I'd guess either M4 or M5.

[Slim]

Mine are 4mm brass. They were removed and some replaced during a re-paint in the last few weeks. If the old ones break off and you can't get them out consider rotating the mushroom base slighty then re-tap

 

Screws easily available on e bay and I would suggest using window tape to bed them on rather than sealant. Only problem is that Midland Chandlers only sell it in 20 m rolls. If it were not on the boat 75 miles away I'll offer to pop a couple of metres in the post.

 

 

Frank

Edited April 25, 2016 by Slim

[Flyboy]

M5 is quite a common size used. I wouldn't use self tappers.

[Dave Bebb]

Thanks i will re drill and tap was not sure thanks i will have a look on flea bay now

[Dave Bebb]

what length 25 or 30 mm

Edited April 25, 2016 by Dave Bebb

[WotEver]

I'd have thought 25mm would have been plenty but if you want to get the 39mm then brass screws are easy to cut with a hacksaw.

 

Tony

[Dave Bebb]

Thanks you have all been a great help thanks

[bizzard]

You may well have nuts inside and not threads in the roof.

[matty40s]

If re-using the same holes in the roof for your mushroom vent, make sure you make a mark on it so you remember which way it faces (ie, a scratch on the forward facing under edge).

Mushrooms may look like they are all drilled the same, but they aren't and it needs to go back down the same way it was originally.

As above, window tape is better than sealant run it round the under side of the mushroom and finish the join with a blade.

The existing bolts may well be worn. might be m4, 2bf, m5 or other. If they break or are loose when you take them out, re-tap to the next size up - you may also need to drill the holes in the mushroom vent slightly larger.

As bizzard says, quite a lot of boats have nuts on the other side, this then becomes a 2 man/person job to remove and refit - although retapping to a larger size means you don't need the nuts any more.

[Paul C]

I know it might seem obvious but if you're going to tap a (for example) M5 thread, don't start off by drilling a 5mm hole........it needs a smaller hole drilled, then the tap opens it up a bit during putting the tread onto it, and the 5mm dimensionis actually the outer dimension through which a bolt would make a clearance fit.....

 

Google around on the internet for a table of hole sizes to drill for the relevant size bolt/screws. (For M5 its 4.2mm)

 

Also, try to obtain a proper set of taps with 3 different ones since each cut will be much easier than trying to do it with just one tap, which might snap.

[matty40s]

oh, and make sure you use cutting oil - or any oil, otherwise your tap will probably snap.

[Iain_S]

I know it might seem obvious but if you're going to tap a (for example) M5 thread, don't start off by drilling a 5mm hole........it needs a smaller hole drilled, then the tap opens it up a bit during putting the tread onto it, and the 5mm dimensionis actually the outer dimension through which a bolt would make a clearance fit.....

 

Google around on the internet for a table of hole sizes to drill for the relevant size bolt/screws. (For M5 its 4.2mm)

 

Also, try to obtain a proper set of taps with 3 different ones since each cut will be much easier than trying to do it with just one tap, which might snap.

 

If just tapping the roof, a first tap will be fine, as the last bit on the tap is a full depth thread. I find I only need a 2nd tap if the hole is more than a centimetre or so depth.

[Tonka]

Mine are UNC or UNF cant remember which

[ditchcrawler]

You may well have nuts inside and not threads in the roof.

That's true, one of mine has NOW, they are all 5mm by the way

[Guest]

Good idea to fit locknuts on the inside to help stop theft.

[DHutch]

Ours are clearance holes, with a nut a bolt. Faster than tapping if there two of you installing it and your doing it before the lining goes in, but otherwise a bit of a pain in the arse.

 

Oddly, the roof hatches are drilled a tapped, which is better in most ways although it does technically allow you to remove them from the outside which given the size would allow you into the boat. In practice however they are also stuck down with PU sealant and even if you took all the screws out, good luck. A brick to a side window would be much faster!

 

As said, worse case, drill them out, grind flush, rotate a bit, start again!

 

Cutting fluid is ace, both for drilling holes and cutting threads, game changing.

 

 

Mine are UNC or UNF cant remember which

 

If they are brass, almost undoubtedly UNC as you wont get the fine thread in brass. If stainless or mild steel, could be coarse of fine, I would use course.

 

 

Daniel

[Paul C]

Ours are clearance holes, with a nut a bolt. Faster than tapping if there two of you installing it and your doing it before the lining goes in, but otherwise a bit of a pain in the arse.

 

Oddly, the roof hatches are drilled a tapped, which is better in most ways although it does technically allow you to remove them from the outside which given the size would allow you into the boat. In practice however they are also stuck down with PU sealant and even if you took all the screws out, good luck. A brick to a side window would be much faster!

 

As said, worse case, drill them out, grind flush, rotate a bit, start again!

 

Cutting fluid is ace, both for drilling holes and cutting threads, game changing.

 

 

 

If they are brass, almost undoubtedly UNC as you wont get the fine thread in brass. If stainless or mild steel, could be coarse of fine, I would use course.

 

 

Daniel

 

You could use security screws, or just one (or two so you can't slide it sideways) security screws. Of course, then you're committed to replacing those screws if ever it needed removing.

[Sir Nibble]

I know it might seem obvious but if you're going to tap a (for example) M5 thread, don't start off by drilling a 5mm hole........it needs a smaller hole drilled, then the tap opens it up a bit during putting the tread onto it, and the 5mm dimensionis actually the outer dimension through which a bolt would make a clearance fit.....

 

Google around on the internet for a table of hole sizes to drill for the relevant size bolt/screws. (For M5 its 4.2mm)

 

Also, try to obtain a proper set of taps with 3 different ones since each cut will be much easier than trying to do it with just one tap, which might snap.

For metric tapping drills, take thread size and subtract pitch. So for a 5mm x 0.8 screw drill size is as you say 4.2mm

[WotEver]

For metric tapping drills, take thread size and subtract pitch. So for a 5mm x 0.8 screw drill size is as you say 4.2mm

Oooh, that's cool

 

If you just buy an M5 or M6 bolt though, how do you know the pitch? Is it fixed?

[Sir Nibble]

Oooh, that's cool

 

If you just buy an M5 or M6 bolt though, how do you know the pitch? Is it fixed?

There is a standard, 6mm is a 1mm pitch, 8mm 1.25 for instance. The tap will be marked.

[WotEver]

Thanks

[Dave Bebb]

Thanks for all your advice got m5 fit perfect had to re tap as the old bolts snapped off had mushroom vent off cleaned up re sealed no leaks now perfect