Gasket "squishing out" - what's normal?

[skyflyer]

I've installed a new fuel sender unit and added some fuel tank inspection ports. I have used 1mm gasket material for the ports (Viton material) and the sender unit had a 2mm gasket supplied.

I have also used hylomar non-setting gasket sealant.

Not having looked too closely at the old gasket when i removed the broken sender, I was surprised to find the extent to which the gasket material compresses and squeezes out to a diameter greater than the plate. I dont want to over tighten the screws (on a test piece when I did a destructive test the screws (stainless) sheared before the threads (aluminium) stripped, to my surprise).

On the other hand I dont want to leave any possibility for leaks.

can anyone give guidance on how tight i should be doing up these bolts - (no 10 self tapping flanged hex headed) either in terms of torque or in terms of how much gasket squish!

Cheers

PS Please let's not revisit the choice of materials, screws sizes, etc etc. I have what i have and its not changing!

 

[PowerSlapper]

1) I would not use Hylomar or any other sealant on a rubber gasket unless specified by the EOM as these sealants will act as a lubricant and assist in squeeze out.
2) It is not possible to specify a torque setting for self tappers as they are cutting their threads as they go unless you set the screw fully then remove and then fit gasket and fitting even then a bit hit and miss.
3) I would (and do frequently) nip up all the screws evenly then tighten another ¼ to ½ turn to pinch the gasket. i have found ½ turn usually sufficient to bed the (dry rubber) gasket preventing leaks with out distorting the flanges, stripping threads, breaking screws or damaging the gasket through excessive squeeze out.
I am sure an expurt will be along shortly to tell you how it should be done after all what do I know, I am "only" a girlie with a passion for thing mechanical. Ooops, that didn't come out right.
Having built and repaired engines and equipment over the years with out leaks i think my method works although i am sure others have their fav method.

 

[pvb]

Can't comment on torque, but I'd have thought compressing the gasket by no more than 20% should give a satisfactory seal.

 

[macd]

Second those.

Providing the pressure applied to such a gasket is consistent and even, it should require very little pressure at all (nor any goo). As it happens I've just had the spigot off one of our taps. It seals (against mains pressure) with an O-ring, the whole being secured in place by a single grub screw which applies no pressure at all to the spigot, which is just a push-fit. OK, an O-ring is a slightly special case, but a viton gasket isn't altogether dissimilar. The worst thing you can do is screw it down so hard that the mating faces deform. Any observable amount of gasket squish, provided it's uniform, should be enough.

 

[RichardS]

1) I would not use Hylomar or any other sealant on a rubber gasket unless specified by the EOM as these sealants will act as a lubricant and assist in squeeze out.
2) It is not possible to specify a torque setting for self tappers as they are cutting their threads as they go unless you set the screw fully then remove and then fit gasket and fitting even then a bit hit and miss.
3) I would (and do frequently) nip up all the screws evenly then tighten another ¼ to ½ turn to pinch the gasket. i have found ½ turn usually sufficient to bed the (dry rubber) gasket preventing leaks with out distorting the flanges, stripping threads, breaking screws or damaging the gasket through excessive squeeze out.
I am sure an expurt will be along shortly to tell you how it should be done after all what do I know, I am "only" a girlie with a passion for thing mechanical. Ooops, that didn't come out right.
Having built and repaired engines and equipment over the years with out leaks i think my method works although i am sure others have their fav method.

I agree with all that.

When I refitted my fuel senders an 80mm diameter gasket was supplied by VDO. The tech at VDO said not to use any gasket compound unless the surfaces were very poor, which they weren't. I nipped them up with just a thin smear of silicone grease and have had no leaks after 4 years.

Richard

 

[PaulRainbow]

1) I would not use Hylomar or any other sealant on a rubber gasket unless specified by the EOM as these sealants will act as a lubricant and assist in squeeze out.
2) It is not possible to specify a torque setting for self tappers as they are cutting their threads as they go unless you set the screw fully then remove and then fit gasket and fitting even then a bit hit and miss.
3) I would (and do frequently) nip up all the screws evenly then tighten another ¼ to ½ turn to pinch the gasket. i have found ½ turn usually sufficient to bed the (dry rubber) gasket preventing leaks with out distorting the flanges, stripping threads, breaking screws or damaging the gasket through excessive squeeze out.
I am sure an expurt will be along shortly to tell you how it should be done after all what do I know, I am "only" a girlie with a passion for thing mechanical. Ooops, that didn't come out right.
Having built and repaired engines and equipment over the years with out leaks i think my method works although i am sure others have their fav method.

You're advice looks good to me Jules, even if you are a "girlie" :encouragement:

 

[skyflyer]

Thanks one and all - just the advice I was looking for.