500T head?

Hi Martin,

There seems to be quite a few Model 18 engines about with a "T" suffix after the model year/model number. The 500T was first offered to the public late in 1948 at that years motorcycle show. The engine was a Model 18 fitted with an aluminium head and barrel. The original head was a sand cast all aluminium part with a cast iron insert which only provided the two valve seats and the spark plug hole and looked like a pair of glasses and became known as the "spectacles" head. In 1951 the process of casting the head was changed to the die-cast method and these have the whole of the combustion chamber in a cast iron skull with the aluminium cast around it. This was the Wellworthy Alfin process and both head and barrel have this name in the casting. This was the same head thatwas used on the 1955-6 OHV singles.

The valves are the same as the standard cast iron ES2. The valve springs, caps and colletsare the same on the early heads, but the Wellworthy type top caps are not dished and the springs are slightly shorter.The standard cast iron ES2 heads use cast iron guides but the aluminium heads should have phosphor-bronze guides.

Regards, Richard.

Hi Martin,

I don't think I explained the number stamping very well, what I meant to say was that there seems to be more engines stamped with a "T" suffix than there were 500Ts made! There is no difference between the 500T cases and the ES2/Model 18 cases apart from the "T" stamped on. Yes the engine number looks original but you will need to check the factory records to be sure and include the shop build number which is nearer the front on the same side.

I haven't used Colsibro guides myself, but I know Mike Pemberton swears by them. Do they makea specific guide for your head? as the standard valves have .375" (9.525mm) stems so you need to check if you need valves to fit. If you decide to go for them you would be best to check with Mike for application and clearances. The valve guides in these heads become loose after being changed a couple of times so there may be oversize ones available or you could use Loctite valve guide adhesive if there is a problem. Be careful if you use Colsibro measurements that they are correct for an air cooled engine.

Regards, Richard.

Hi Martin,

I don't think I explained the number stamping very well, what I meant to say was that there seems to be more engines stamped with a "T" suffix than there were 500Ts made! There is no difference between the 500T cases and the ES2/Model 18 cases apart from the "T" stamped on. Yes the engine number looks original but you will need to check the factory records to be sure and include the shop build number which is nearer the front on the same side.

I haven't used Colsibro guides myself, but I know Mike Pemberton swears by them. Do they makea specific guide for your head? as the standard valves have .375" (9.525mm) stems so you need to check if you need valves to fit. If you decide to go for them you would be best to check with Mike for application and clearances. The valve guides in these heads become loose after being changed a couple of times so there may be oversize ones available or you could use Loctite valve guide adhesive if there is a problem. Be careful if you use Colsibro measurements that they are correct for an air cooled engine.

Regards, Richard.