Head Insert Fitting

A few notes on setup & fitting of an inserted Cylinder Head...

There have been a few different ways of doing these heads over the years - although in recent years there has been one quite common one - the head casting is machined to take a removable "Skull" or insert (although we have seen them described as "Scholes" - there is no such thing)

Accuracy of machining is very important - for the cylinder head - and for the insert to avoid any problems.

We did have one supplier send back some heads that were not anywhere near accurate enough - the recess in some of them was too deep - and the diameters were not concentric on some of the inserts - which - if we had used them - would have caused many problems.

We now have the inserts made with excess material on the face - to allow adjustment to suit the cylinder you are using - it's much easier to remove material - and impossible to add it...

What we are aiming at:

The head inserts MUST be sitting approx 0.04mm ~ 0.05mm (0.0015" ~ 0.0020") proud (or above) the head surface when the head is tightened down on the engine. (we call this "Pinch") to be sure the inner O rings are compressed - and combustion chambers are sealed.
(The top of the cylinder also needs to be flat - and no detonation erosion on the top of the bores)

We check this by sitting the up-turned head on 4 x screwjacks on a surface plate - and checking the step from head surface to insert face in several positions - it's not vitally important how level the head is - only the "step" value is being measured - at several points...

Before we do  this though - there is another vital step that is quite important...

When the head is tightened down on the engine - the head inserts are pushed tight into the recess machined in the head casting - so when we check the head "off the engine" - we need to make sure the inserts are tight in the head when we are checking off the engine - so the inserts need to be pulled into the head - you can use an old spark plug & big washer to pull the inserts into the head casting.

When we have the inserts sitting approx 0.04mm ~ 0.05mm  (that's 1.5 ~ 2.0 THOU)  proud of the head surface while locked in place in the head casting - we now want to make sure the 2 inserts are parallel with each other - we do this by rubbing the insert faces on some abrasive paper on a surface plate - it usually only takes a very small amount off the insert face - but it ensures they are sitting parallel & will clamp on to the inner O rings properly.

Note: It's important that you leave the inserts clamped in the head until it's finally fitted - also - whenever you remove the head - clamp the inserts in place before you undo the head from the cylinder. 

It is also quite likely you will need to adjust the squish gap during final assembly - we can machine the squish gap if you are unable to do it - but it does need some accurate measurements off the engine you are fitting to - or the engine on the bench in front of us....

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