Current performance levels achieved from the A series using modern cam designs combined with high ratio rockers have produce a demand for longer lasting valve train components. 

The valve guide is no exception and to meet this demand more modern materiel specifications have been employed. Over the past five or six years many materials have been used to try and attain the desired criteria, many of which are so hard that the valve stems are wearing out instead. Particularly since the widespread use of unleaded fuel where the upper cylinder and valve component lubrication is practically non-existent. Mini Spares has reviewed the situation with the country’s premier automotive engine parts manufacturer, culminating in the production of guides in a special manganese bronze alloy to closer matching tolerances than were previously attainable at an affordable price.

Remove the old guides, de-burr the valve guide bores in the head at the spring seat side. Fit the guides, preferably using a press – hammering them in tends to crush the top of the guide in, making accurate reaming difficult. Fitted height from the top of the guide to the spring seat surface should be 0.590” (14.98mm) in old style 12G940 heads and all pre A+ small bore heads; 0.540” (13.71mm) in later A+ type and A+ small bore heads. These are guidelines only, although fine for most road use. The fitted length may need reducing further for race use dependent on cam lift/rocker ratio/valve seat machine work etc. Your chosen engineer should be able to assist with establishing this. Once fitted the guides absolutely MUST be finish reamed. It is impossible to supply pre-sized guides because of the fairly wide standard production tolerances generally causing the guide to squeezed in slightly when fitted. The guide bores will also slacken off after several sets of guides have been fitted and removed. For excessively loose guides, it will be necessary to reconstitute the bore – by knurling for instance.

Be advised – this may cause eccentricity of the valve centre. Finished ream size is recommended at 0.28125”. This will give standard valve to guide clearances. For information purposes, dyno/race testing has shown no power loss with up to a 0.005” clearance between valve and guide, although this does cause slight smoking from start up and premature seat valve seat wear. As with all engine related tolerances, they assume a sensible running temperature is maintained. NOTE Many current automotive engineering companies use valve seat cutters that have a self centring pilot system, which does not rely on a correctly sized guide bore. After new guides are fitted, they try a valve in the guides. If it fits, they tend to ignore the reaming operation. Insist it is done to avoid valves seizing in the guides and tears before bedtime.

Importation rules regarding vehicles are very specific: Only those over 25 years in age may be brought in to the USA without compilance to Federal Motor Vehicle Safety Standards (FMVSS).