stuffing box packing help!

[hunter323]

I am renewing the packing in the stern gland and in calculating the size of the packing required ( ie diameter of gland less shaft diameter divided by two gives me about 7mm. This is about halfway between 1/4" and 5/16" packing that I would like to buy. Do I go for the greater size or the smaller? Advice gratefully received

 

[Evadne]

The larger size is nearly 1mm too big whereas the smaller is 0.65mm too small, if your measurements with a micrometer are spot on I've only done it once and I used a piece of the old stuff to offer up to the new stuff to be sure, which is why I bought a length of each, it's not that expensive.

I would assume that it is the smaller, you just aren't going to get the big one in, not with the prop shaft as well, but the small one will squish up under pressure. I think the danger of too much friction outweighs the danger of too much drippage, within reason.

 

[Spyro]

I had a similar dilema (inbetween sizes) I bought some smaller but by the time I had put in 5 rings it took too much pressure to squish it all so ended up buying stuff slightly big and flattening it with a hammer then putting it in, friction wasn't a problem. Never had any problem with it after initial adjustment in the water.
It was PTFE I used. Got it here http://www.boatpartsandspares.co.uk/gland-packing-14-c.asp

 

[ksutton]

Probably an indication would be from what size packing is actually available to buy.

I can only find these sizes:-
Gland packing (Greasy hemp) square section
3mm - 5mm - 6.5mm - 8mm - 9.5mm -12.5mm - 16mm - 19mm

Gland packing (P.T.F.E.)
5mm - 8mm - 9.5mm Subject to availability.

I have recently used T Norris, http://www.tnorrismarine.co.uk/sterngear.php to supply mine. I will also add that I had replaced my packing during the winter and used PTFE and could only get 6mm at the time, (in Spain), correct size should have been 6.5mm, or 1/4". I had lots of problems with the 6mm and despite thinking I could compress the 6mm in the gland, it didn't work and was very difficult to adjust. After the 6mm bedded in, there was only one nut flat adjustment between running hot and leaking far too much, (even with engine off). Some of the PTFE seal material has also extruded itself out of the gland, (horrible stuff).

I have sourced the old trusted material (greasy hemp), 6.5mm and will install this shortly. I mentioned my PTFE experience to the guy at T Norris and he said they had also had problems and in fact would no longer sell PTFE.

If you are outside of UK, you will probably not find 6.5mm size.