Sunday, 6 January 2019

Engine Strip Down

Having got the engine out of the car the remaining ancillaries and intake manifolds etc were removed from the top of the engine, leaving us with what is known as a long block.  It was pretty dirty with oil residue from various leaks over time and a nice coating of Castle Coomb grass! for good measure.

We borrowed an engine crane and loaded the engine into the car, taking it over to Archer Motorsport Services in Hertfordshire.  With some much appreciated help we were able to  dismantle the engine in just under three hours.   The aim was to identify the root cause of the failure, determine if the engine is even rebuild-able or just scrap and form a plan on what to do next.  

Working on cylinders 1-3 first the cam covers and camshaft assembly came out pretty easily.  Next the cylinder head bolts are removed and binned.  These are single use so have to be replaced regardless. 

Inlet and Exhaust camshafts above connected by timing chain.

Pistons exposed - some carbon build up as expected

Cylinders 1-3 pretty clean and unmarked

The valves and pistons on this side of the engine looked pretty good.   Also there is no sign of bore scoring in the cylinders and only light carbon build up on the pistons. 

Moving to the sump we found evidence of a destroyed bearing shell. A lot of fine particles in the oil residue and some larger pieces of metal on the magnetic oil pickup - which isn’t good.  

On the cylinder 4-6 side, again the valves and camshafts were the same as 1-3, but removing the cylinder head we could immediately see the piston no.6 was loose with no big end bearing left.  This was the cause of the knocking when the engine ran.  

Crankcase split exposing the crankshaft box and pistons 4-6

The lack of a bearing shell has caused the conrod to damage the crankshaft surface. This is beyond the amount that can be ground so its scrap.  

Crankshaft removed - spot the damage!

No.6 con-rod damaged the crankshaft surface after the bearing shell disintegrated

It has also caused some scoring of the piston in the cylinder bore as there was too much play going up and down in the cylinder.  

Cylinders 4 (least) to 6 (most) marked/scored

Cylinder 6 is renowned for oil starvation issues in these engines.  It is the furthest from the oil pump.  This cylinder was showing low compression when we tested the engine a year ago.

It’s a marginal call as to whether to rebuild the engine or replace it with a used engine.  With a used engine likely to have at least 70-80,000 miles there are two considerations.

  • The engine will be starting ti suffer from wear such as our engine
  • There is a higher risk of a catastrophic problem since the engine history is likely to be unknown

We think therefore it is worthwhile trying to rebuild our engine.  To do this we need to source a good crankshaft first.   Then we can get cylinders 6 (and possibly 5) re-lined using either steel or Porsche Nikasil liners.  The head will need to be skimmed and the valves lapped, then it can be re-assembled with new consumables, bolts, gaskets and bearings.  Re-assembly should take a day.  

Through careful sourcing of the parts we need this should be only marginally more expensive than a used engine - but giving up the piece of mind and greater assurance over the condition of the engine at the end of the process.