Refitting a Commando cylinder head

Chris

Lots of people should be able to help with their views, but you need to give a little bit of history, if you have any: i.e. have you had the head off, how many miles?

If it is blowing it is either: head face distorted, damaged, badly fitted and not re-torqued, or stainless studs might be implicated.

Most advice says to use a composite gasket and check the head is flat by testing it on a blued glass plate.

Norm

Check that the area around the studs in the head are relieved where the threads start and aren't raised above the surface. This can prevent the head sitting fully down.

Don't forget to check that the barrels are flat too!

 Remember that a  surface finish that is too smooth is worse than one that is too rough.

My tip? If you are checking flatness with 'wet and dry' on plate glass, don't go finer than 120 grit.

Regards,  George. 

Before taking everything apart, why not try replacing just the four large head bolts [the ones either side of the spark plugs] and tightening them to 30ftlbs?  Original, high-tensile bolts will seal much better than stainless items.  You should buy new bolts anyway so why not start there and see if your gasket is still blowing.  Another tip would be to buy and fit a PCV in the return feed to the oil tank; this can make a considerable difference to the ability of the head to breathe.  It is, after all, pressure that is forcing the oil out of any small clearances.  

Either way, Good Luck! 

= 'positive crankcase ventilation' - uses a one-way valve that lets air/gas out of the crankcase and stops air getting in thus creating a mild vacuum and helping to prevent oil leaks. They do help ... .

Chris

Stainless studs and bolts to secure the head and barrel are not a good idea and should be replaced with genuine HT steel items. Using them to hold on mudguards is fine. It is very probable that they have stretched in use and caused the gasket to fail. Stainless nuts on the engine shouldn't be a problem IF they are strong enough.

Colin has raised a point which catches a good number of people out. ie.......they fit a cylinder head that has been skimmed and then refit it with a dubious head gasket and dodgy headbolts. Often the latter being cheap Stainless items. 

The skimming of the cylinder head makes it thinner and massively increases the chances of the dodgy length headbolts bottoming out. Throw in the mix an iffy head gasket and you are going to get possible problems with a head leaking oil plus valve adjusters that run out of thread. 

Fitting a compression plate can also cause issues with pushrod lengths and may lead to adjusters trying poke holes in the roof of the head.Lower compression pistons are a safer route here.