"Ensure that the gasket is in good condition" is a phrase commonly found in the workshop manual. Anyone rebuilding an engine or transmission with the assistance of such wise words will probably recognise the phrase, but while we often spend much hard-earned cash buying new internals, the temptation as far as the gaskets are concerned is to try wherever possible to use the old ones. We know it's wrong, evidence of a cheapskate or even a miser, but nevertheless we still do it - and sometimes even get away with it.
But how often does that simple little gasket, costing little more than loose change, let us down and, seeing a gradual weep of oil, the unit has to be removed from the vehicle and put back on the bench?
At this point I expect to find I'm probably the only person to do this, that everyone out there uses brand-new seals and gaskets at every rebuild and that, consequently, nothing ever leaks. But somehow, on reflection, I think not. So when I met someone at a recent exhibition who talked about reusable gaskets in motorsports applications then I was immediately interested.
I have to start by saying that the idea of reusable gaskets is nothing new. Your grandfather, perhaps even great-grandfather, may have used solid copper head gaskets, which could be reused many times. Placing them on a flat steel surface and heating to cherry red with a propane blowtorch would, upon cooling slowly, soften and make them malleable enough to compress and seal the cylinder head once again - 'annealing' is the term metallurgists use here. Still used in some forms of drag racing, the galvanic potential between copper and aluminium (at 1.645 V) in the presence of the engine coolant can create severe corrosion.
However, the particular gasket design I was looking at was one specifically for a gearbox housing which, I admit, may not be the most arduous of applications. But when grubbing around in the paddock after, say, changing a set of gear ratios and trying to find a replacement gasket in the back of the trailer with oily hands, anything reusable is a godsend.
Consisting of a thin aluminium sheet laser-cut to shape onto which is bonded on either side a foamed NBR (nitrile butyl rubber) elastomer, the gasket, I was told, can be used and reused up to four or five times. When chemically blown onto the aluminium substrate and bonded to it, NBR has a temperature limit of up to 120 C. That's not particularly high as far as some elastomers are concerned, when some polyacrylates are good for 150 C and others much more, but if limited to the less arduous environment of a transmission casing then some of its other characteristics can be exploited.
In this case the property is one of recovery when, after removal of the clamping load, the elastomer rebounds back to 85% of its original thickness. Not only that but the design retains a strong torque retention in the clamping fasteners, therefore not requiring any re-torque after initial assembly.
While not always a good idea to keep on using gaskets again and again, a strong and reliable seal that does not depend on messy liquid sealants is attractive when oil is dripping down your arms.
Fig. 1 - A brace of motorcycle engine gaskets. While the copper one may be reusable, the other certainly won't be
Written by John Coxon