Leaky Camshaft Bolt

The square headed bolt for securing the camshaft bearing within the crankcase is notorious for leaking. It is difficult to remove or tighten this bolt as the cylinder block traps the square head by design. I decided to remove the bolt and replace it with a socket head bolt correctly machined to the same dimensions as the original. This would enable the bolt to be tightened as required with an extended Allen key.

cam 1Removing the original bolt presents a problem, as normally the cylinder block has to be removed first.

Using an extended 8 mm diameter drill, the head of the square bolt can be drilled out. Access to this area is easier with the dynamo and its housing removed from the engine. Carefully pack rag around the camshaft timing gear etc to prevent any swarf entering the engine.

The final bolt remnants should be removed with an `easy-out.' If you are lucky the thread in the crankcase should be alright but if it shows any wear or stripping then it will have to be fitted with a helicoil.This presents a slightly more difficult problem as the old thread will require drilling out and to a depth such that the bronze camshaft bearing is entered by at least 3 mm to allow for the helicoil tap to bottom correctly. The length of the special tapping size drill and helicoil tap can be extended to clear the top of the cylinder block/cylinder head by brazing on an extension bar. I would recommend that if it is necessary to fit a helicoil, the replacement socket head bolt be machined with a slightly longer spigot on the end than the Austin original. This will ensure that there is sufficient area contact between the bolt and the hole in the camshaft bearing to take the camshaft side load. Carefully measure the depths of the thread in the crankcase and hole in the camshaft bearing etc to ensure that the socket bolt modification does not bottom out on the camshaft but should have at least 1.5 mm clearance.

cam 2

A Dowty seal, which consists of a washer with a rubber insert, on the replacement camshaft locating bolt. 

I would recommend using a 6 mm Dowty seal under the socket bolt modification to ensure a good seal (see photograph).

This special seal for hydraulic application has a neoprene seal carried in a metal washer and is excellent for sealing awkward threaded fittings.


This article, written by Steve Baldwin, originally appeared in CA7C Seven Focus in Dec 2003 pp16-17.