Do not use Silicone Sealer - a warning

“It has no compression on cylinder 3 and is low on 4”, that was the information provided with the engine. The readings were 90  -   90  -  0  -  70 psi.

silicone 2

The first operation was to remove the head. This revealed that number 3 cylinder had done very little work as the piston top was clean compared to the other three cylinders. The exhaust valve on number three appeared to be burnt on one side and a coil insert on one of the head studs had worked its way proud on the surface of the block. 

The rest of the engine was fine, the bores were good and the pistons looked OK and the joints were fine EXCEPT there was SILCONE sealer everywhere. 

I know you can buy silicone gasket  solution, I know silicone has been used for years to stop leaky old engines dropping oil but please DO NOT USE IT on an Austin Seven engine to make an oil tight seal. 

Why? Well , unlike most other engine ( may be ALL other engines ! ) the Austin Seven engine uses two little jets to spray oil onto the crankshaft and into the big end bearings. These can be easily blocked by loose floating material. If they do get blocked and the driver is not observant then the end result is usually the loss of one or two white-metalled big end bearings. The cost of re-white-metalling, about £30 to £50 per bearing, cost of a new paper gasket £1 to £2. 

A very frequent cause of blocked jets is silicone sealer. In the days before I realised the cause of the problem I had used silicone but on stripping down an engine I found a ‘worm’ of silicone sealer partially blocking one of the jets. A disaster waiting to happen.

silicone 4

When a siliconed joint is tightened up you will often see the silicone oozing out at the joint. This is OK when it is coming out on the outside of the casing but the same is likely to be happening on the inside and with vibration, temperature variation and the effect of oils some of these internal bits of silicone break off and enter the oil system. 

How do you stop the leaks?  I have found that if you make sure the two mating surfaces are clean and flat, that all the studs or bolts are in good order, the threads of both parts are in good condition and the correct gasket material is used then the joint will be as good as you are likely to get it. I do use a non setting sealer like ‘Hylomar’, some use ‘Wellseal’, whichever you like the best. These do not set, allow the differing expansion rates to happen without loosing their efficiency and are easy to remove when re-making the joint. In my opinion he worse gasket sealer to use is the setting Red Hermatite, it has no benefits over the previously mentioned sealers and is the very devil to get off when you need to re-make a joint.

Getting back to the engine in question. Once the coil insert was removed and replaced, and all the valve settings corrected, compression was found at all cylinders. The likely cause of the problem? After the engine had been rebuilt and run for a short time all the nuts were tightened up, including the nuts on the studs holding the block to the crankcase. When these were tightened they had the effect of closing up the valve  clearances and some of the valves, including no. 3 exhaust valve were never closing.  

If you tighten these nuts after a re-build then do check the valve clearances.


This article, written by 'Techno', originally appeared in CA7C Seven Focus in Nov 2006 pp22-23.