4.2 Con Rods

[gmunro]

HI Guys. I am aware that when rebuilding the engine, it is essential to have the main bearing caps align with matching letters/numbers on the block.

Also, when refitting the con rods, it is also necessary to align numbers and to fit them into the corresponding bore.

The workshop manual does not state the rotation of the con rods save that the pistons have an arrow pointing to the front of the engine. Does it matter if the numbers on the con rods are on the exhaust side of the engine or the inlet side of the engine? The workshop manual makes no mention of this and the con rods appear to be straight with no bias but I have read in another publication that the con rod numbers should be on the exhaust side of the block.

Any guidance or advice? Many thanks.

George

[christopher storey]

The numbers should indeed be on the exhaust side of the block

[gmunro]

Thanks very much for the advice. Much appreciated.

Yours aye, George

[iani]

Yours aye, George

ex RN by any chance George?

[PeterCrespin]

The conrods are symmetrical and it doesn't matter which way round or which cylinder.

If they are not detached from the piston then the piston/rod assembly becomes asymmetrical and cylinder specific and the orientation should be replaced as it came out. But even that only applies *if* the assembly was the right way round to begin with. After 50 years assume nothing, check everything.

I prefer to have all the numbers the same side - the mains are stamped on the intake side so that's where I put the rod numbers too when building from scratch. And yes, to save possible confusion during a later partial rebuild with no rebore, it is obviously good practice to respect cylinder numbering.

Pete

[gmunro]

Hi Iani,

My father was ex-RN but I am a retired Police Superintendent, Royal Hong Kong Police.

[gmunro]

Hi Peter,

Many thanks for your input. I already have mine fitted with all numbers on the intake side - it was just I read that the con rods should be on the exhaust side. To remove them, strip them and turn them around would be a nuisance if not necessary but I thought I'd better seek advice through this forum before I go any further to see what the consensus is. Funny the official workshop manual doesn't address this point!

Your reply is much appreciated - thanks.

Yours aye,

George

[PeterCrespin]

The manual would address the point if it were important. They built the engines with matched sets of pistons assembled to matched sets of rods. Until the pin went through the small end, the stack of six rods could go any way round - there was no right or wrong. Once the pistons are fitted there is only one way because the pin is offset in the piston. That's why there's an F stamp so the offset always relates correctly to the thrust (inlet) side of the bore and piston.

So once they had six rods and pistons assembled, they put them in the block with F facing the timing gear and bolted up the big ends. At that point they stamped the rod and cap and the rod orientation and cylinder match became defined post-hoc. The actual rod, as opposed to piston, was never 'handed' and remains usable in any position. It just makes for fewer mistakes to follow the sequence but there's no absolute requirement. You could have half the numbers inlet side, half the numbers the other way and the cylinders all mixed up, so long as the pistons were right. Because people dont know to look for offset pins and because many overhauls just involve new rings and shells with the piston refitted to each bore, it's much more foolproof to take care with rod numbering.

Pete

[gmunro]

Hi Pete,

Many thanks for the information. I never knew the reasoning for piston direction and I had never heard of the pin being off-set - you live and learn.

Once again, many thanks for your help.

Yours aye,

George

[Allrand]

I would just make sure the bearing shell locating tab recesses are all on the same side. Just my ocd, check the four original ones.

[Allrand]

I like those pistons with 2 oil rings, my Mk VII had them and it used negligible oil.