Thread: What is a plastic head dowel?

It is pretty much the same as a steel head dowel but made of plastic.

They are there to locate the head to the block but LR decided at one stage that it would be a smart idea to make them out of platic rather than steel and apparently this has caused problems.

Someone who deals with TD5's should know more about it than I do.

Originally Posted by crl

I saw "plastic head dowel" mentioned on another post in reference to the TD5 engine. I have heard this mentioned before but don't know what this is, can someone enlighten me pease?

Most engine blocks have steel Dowels or Collars (around some bolt holes) pressed into the block, this helps locate and hold the head gasket and locate and guide the head into the correct position on the engine block. I believe that the TD5 has Plastic dowels, probably to prevent galling or corrosion in the dowel hole in the cylinder head, or it was cheaper than steel dowels, Regards Frank.

They're used to loacate the head onto the block during assembly. Normally they're not a problem, as the design of the motor uses the head bolts to maintain the head in the correct position once tightened. The head bolts can stretch in the engine due to the deformation during tensioning and then the additional strain from a good overheating, misservice, engine trauma etc and it loosenes the pressure which holds the head in position and the plastic dowels are not necessarily designed to prevent the type of movement which is generated and is where the plastic dowels get their bad name from. Some TD5's came out with plastic dowels and some came with metal dowels, though despite this you can't always replace the plastic dowels with metal ones as it can lead to the head cracking.