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DOWELS in the Timing Cover and Engine Front Plate - TC-303D
This article contributed by Mick Anderson in Australia
MGA Twin Cam front engine plate and timing chain cover dowels
There is some misunderstanding of where the dowels that locate the Twin Cam front engine plate and timing chain cover are fitted.
Firstly, what is the purpose of the dowels?
To aid efficient assembly of the engine on the production line the bolt holes for the bolts locating the front plate to the engine block, and locating the timing chain cover, are drilled oversize. With so many bolts over such a large area, if the holes were drilled exactly to the bolt diameter it would be impossible to fit all the bolts due to tolerance variations in the hole placements.
However, accurate fitting of the plate and timing chain cover assists the front crankshaft pulley oil seal to be concentric with the pulley even though the seal can cope with some variation. But, more importantly on the Twin Cam, accurate placement is essential for correct meshing of the ignition distributor drive gear with the gear on the half speed shaft. Also, the front plate position effects the oil sealing where the cylinder head rests on the plate, but this is not as critical because the compressible seals on the head can also cope with some variation. Pushrod engines do not use dowels, but only use a centering tool to center the oil seal on the pulley.
To obtain this accuracy at least two dowels must be fitted to both the plate and the cover. Holes are drilled accurately and dowel pins are fitted with a tight fit in the hole. Note that this also applies to the engine rear plate and the clutch pressure plate.
How many dowels are there and where are they fitted?
There are three dowels as shown in the Service Parts List (SPL). There are not four dowels (two each) for the plate and cover, as one dowel is used for both the plate and the cover. SPL details are:
|Illustration # ||Part # ||SPL page||28 ||AEH307 ||Facing page A.1 (2nd. And 3rd. editions)
||29 ||AEH308 ||Facing page A.1 (2nd. And 3rd. editions)
||82 ||AEH307 ||Facing page A.6 (2nd. Edition) and
A.9 (3rd. edition)
For the rest of this article the dowels will be referred to by their illustration number as only that number is unique to each dowel, as each dowel has a different description in the SPL, even though two dowels are physically identical.
SPL dowel descriptions
28 Dowel for engine front plate
29 Dowel for engine front plate and timing case
82 Dowel for timing case to engine front plate
From the description it can be seen that dowels 28 and 82 locate two items together and dowel 29 locates three items together. Dowel 29 protrudes much further out of the block than dowel 28.
For clarity, the positioning of these dowels is shown below in photos and diagrams.