These front engine plates are for Pre-Featherbed 500cc 30M/Manx models, using magnesium crankcases, and are fabricated using sided jigs (see pictures), to ensure they have the correct offset, form and hole location as original engine plates. All dimensions are copied from an original SOHC 500cc Manx engine plate
Background information: Genuine Pre-Featherbed Norton racing models (from approximately 1935 onwards) used magnesium crankcases and had a different front engine mount on the frame downtube on both rigid and plunger models. The front downtube engine mount on these frames sat approximately one inch (1") higher than their aluminium crankcased road going (International) SOHC counterparts. Therefore these different type engine plates were required.
Added to this, the magnesium (30M or early Manx) crankcase also had wider engine plate mounts at the front of the crankcase than aluminium International crankcases - that necessitated the engine plates being manufactured with a 'kink' in their sides, so they came in at the front to match the frame downtube lug.
We have our engine plates lasercut - having used an original 30M (SOHC Manx) engine plate as the template - but we then have specially made jigs to allow them to be formed to the correct shape and profile - before the front engine bolt hole is then bored to the correct dimension on the jig. This ensures these plates are always the correct dimensions (see accompanyning photos).
Finally, we lightly linish the edges (to remove any lasercutting marks) and lightly oil them to protect them from rust, before packing. Price is for a pair of engine plates.
Final point for owners of 1948- 1950 350cc (40M) models - the 350cc 40M model used a crankcase that was physically slightly smaller than the 500cc 30M model (those using engine plates - Item 0202) - But sometime around 1948 Norton standardised the racing models on the 500cc type crankcase for both capacities - just having a smaller cylinder hole bored in the 350cc crankcases. If you have a later set of pre-Featherbed 350cc Manx crankcases and are not sure which type they are - email us on firstname.lastname@example.org and we can help confirm for you - but if post 1948 350cc, then they are likely to require this type of front engine plate.
In the accompanying photographs is a picture of one of my own Manx Nortons (a 500cc plunger model) and you can see the distinctive engine plate fitted. Note the top line of the plate is horizontal, where the 350cc model would have the top line sloping down towards the engine - one of the easiest ways to identify a 500cc from a 350cc model at a glance