Cylinder block bottom gasket.

[UKlittleguns]

Hi everybody,

Can I reject the cylinder block bottom gasket and use a modern sealant instead?

Doing so would provide a solid contact and eliminate any change in the settings that rely on cylinder height (compression ratio, ignition timing, valve clearance).  Might be very minor, but why put up with it if you can get rid of it?

Also,(and I know you people know much more than BSA), what should the nut torque be?

Best Regards

[RDfella]

The gasket is there for a reason. Takes up any imperfections without the danger of sealant getting where it shouldn't. Also, unless you have a new / rebored cylinder, leaving out the gasket will probably break the top rings. There's gaskets all over the engine, so why leave out just that one? Difference to comp ratio, ign timing would be so small as to be immesurable for our purposes. As for affecting valve clearance - that'll only happen if you keep changing the flange gasket for ones of differing thickness - but you'd be doing the tappets again anyway.

[berger]

agree with RD you are on form to break a ring, put the same thickness gasket on unless it's a fresh rebore, as for torque bloody tight without rounding off the nuts, tighten let it settle over night go round them again on separate days until they don't want to go anymore , after a few miles try them again.
 

[RichardL]

Well, I don't think I'd try the as-tight-humanly-possible approach Berger is suggesting. After all, people and spanners come in different sizes (which will sound very stupid to Berger if he is 6'6", 280 lb.). While we A7/A10 owners don't have the benefit of BSA-published torque specs, they did start publishing them for the A50/A65 models. For cylinder base studs (from that list), 18-20 ft.-lb. If that feels untight when reaching the top, one could probably add a couple of pounds if it seems comfortable.

Now, unless you own a special torque wrench that accepts Whitworth spanners, you will need a way to apply the torque with your 1/4W open end.  I do this by pulling on the box end of my box/open-end spanner with a fish scale, stopping when I reach the value I've calculated based on the length of the spanner. I will usually calibrate the fish scale before doing this.

Richard L.

[RichardL]

Oh, one more thing, and I'll make it a question before the group.

Assuming one follows a torque recommendation, wouldn't we assume that recommendaation was based on the torque required to compress the originally specified gasket material and not the metal-to-metal (approximately) contact in question?

Richard L.

[Fergie]

Hi everybody,

Can I reject the cylinder block bottom gasket and use a modern sealant instead?

Yes, you can. BUT DO NOT use a silicone sealant.
I have seen countless motorcycle engines destroyed by guys using a silicone sealant. They put it on and it squeezes out inside the engine when they tighten the bolts. Eventually, it breaks away inside and blocks the oilways/pump.   
I have for the past 30 odd years used Kawasaki sealant. No leaks and it will take up small imperfections on the gasket face.
it's so good that my BSA M20 had no leaks. 
A tube lasts a long time as you don't have to use much of it.

Torquing Bolts! Use Loctite or similar. Use the heat resistant grade if using it on engine parts.
 
Fergie.



 

[Triton Thrasher]

Silicone’s fine.   Just don’t use too much.

[Fergie]

Silicone’s fine.   Just don’t use too much.

I quite agree, Triton Thrasher. The problem is how much is too much?

However, there are better and safer gasket sealing materials than silicone. When rebuilding an engine, it's always best not to scrimp. 

[Triton Thrasher]

Yes, I suspect the newer anaerobic sealants are better.

[UKlittleguns]

Hi Everybody,

Thanks for all your comments, very interesting and informative.  Here's what I finally did and if it all goes pear shaped I'll be straight back reporting miserable failure and wearing sack cloth!

Firstly, the gasket was rejected.  I can see no point in perpetuating poor design.  Blocks and crankcases are meant to be rigid and bolting them together with a flexible element in between makes no sense unless from a 1590's cost/production viewpoint.  Now some 60 years later, I don't think anybody would be impressed about having to re-tighten a settling engine gasket on a modern engine, car, bike or whatsoever.  At least for good or bad, that's my point of view.

I note the warning about bore wear ridges and fully agree.  Fortunately my engine is a new build and so this is not an issue.

The sealant I used was Wellseal.  I think the modern equivalent is Hylomar blue.  It was developed by Wellworthy of Lymington for the assembly of military engines without the need for gaskets.  My first experience of it was back in the 1960's at Spurrier Works in Leyland where the L60 Chieftain tank engine was assembled (with practically no gaskets in sight)!

I agree that the over application of sealant is not only messy but can be damaging if lumps break away internally.  I think people might be surprised at how little is required.  Easy to check.  Just run a bead between plain clean vice jaws and note the spread on tightening.  For the size of the BSA contact surfaces it amounted to a bead the size of a thread of sowing cotton.

The nuts were tightened to 24 ft.lbs.  Simple enough to mig up a crows foot spanner for the torque wrench.  This felt very tight but was just within all the standard tables for torque values.

Job done and I think I shall now retire from this particular thread but thank you all for your comments.     

[berger]

let me know when it starts weeping please

[Triton Thrasher]

Hi Everybody,

Thanks for all your comments, very interesting and informative.  Here's what I finally did and if it all goes pear shaped I'll be straight back reporting miserable failure and wearing sack cloth!

Firstly, the gasket was rejected.  I can see no point in perpetuating poor design.  Blocks and crankcases are meant to be rigid and bolting them together with a flexible element in between makes no sense unless from a 1590's cost/production viewpoint.  Now some 60 years later, I don't think anybody would be impressed about having to re-tighten a settling engine gasket on a modern engine, car, bike or whatsoever.  At least for good or bad, that's my point of view.

I note the warning about bore wear ridges and fully agree.  Fortunately my engine is a new build and so this is not an issue.

The sealant I used was Wellseal.  I think the modern equivalent is Hylomar blue.  It was developed by Wellworthy of Lymington for the assembly of military engines without the need for gaskets.  My first experience of it was back in the 1960's at Spurrier Works in Leyland where the L60 Chieftain tank engine was assembled (with practically no gaskets in sight)!

I agree that the over application of sealant is not only messy but can be damaging if lumps break away internally.  I think people might be surprised at how little is required.  Easy to check.  Just run a bead between plain clean vice jaws and note the spread on tightening.  For the size of the BSA contact surfaces it amounted to a bead the size of a thread of sowing cotton.

The nuts were tightened to 24 ft.lbs.  Simple enough to mig up a crows foot spanner for the torque wrench.  This felt very tight but was just within all the standard tables for torque values.

Job done and I think I shall now retire from this particular thread but thank you all for your comments.   

Was that the Chieftain tank that broke down a lot?

[UKlittleguns]

Hi Triton,

Yes it was.  The L60 was a German Junkers design nicked from Germany after WWII.  Supercharged two stroke Diesel with horizontally opposed pistons and twin crankshafts.  They were multi fuel engines.  We killed them on the test bench.  Try running your bike engine on paraffin with a shovel of sand in the intake, go from cold to full power instantly and hold it there for 60 minutes (MOD spec and the estimated life of a front line battle tank).  They would actually do it although the end results were not nice.  But you know, They never leaked oil