Pistons and Bearings Early Mark 1

[aussieal]

My early P5 3 Litre Mk 1 has smoked up excessively and consumed lots of oil.

Body number is 631900045 engine No 631900048 making it a RHD export model (was it common for cars to be delivered without matching numbers?)

So I have removed the pistons to find broken compression rings on Cylinders 1,2, 5 and 6 and severe damage on piston 5

I then went to my donor motor 6319000187, only to find the pistons are completely different

The original piston has the following inscribed on the crown: TOP 002 TB RA2553 STD. The Donor piston has no such numbers just B and a peculiar symbol plus the cylinder number stamped on the top.

      

I presume that the original piston is factory fitted and that the donor motor piston is an after factory installation. Obviously it is not a suitable replacement. So I need to source a compatible replacement : any ideas?

The cylinder bores are in good condition with no scoring and an almost undetectable lip, so I see no need for a rebore. The big end bearings and the crankshaft also are in good condition. The big end bearings are stamped STD 2881 18AU82 R770 which again I presume are original factory fitted. However I thought that it would be a good idea to replace these whilst the motor is apart. Any comments from the experts please?

Having seen the condition of the bores and crankshaft and bearings, and coupling this with the minimal wear on the brake and accelerator pedals, it would appear that the 49270 miles on the odometer may be correct.

I have no history of the car but understood that it was shed stored for at least 16 years before I purchased and recommissioned it about 10 years ago. Being a bit impatient and a novice at these things I severely overheated after about 10 miles of driving on my first run. This was probably the cause of the initial damage which has now come home to haunt me!

[Phil Nottingham]

F W Thornton in the UK has pistons for everything

[djm16]

It must be pretty unusual for the pistons to be that badly damaged and the bore to still be serviceable. As a full rebuild is going to be expensive and time-consuming, can I suggest that you take the block to a machine shop to see if the bores really are OK?

BTW, what are your camshaft bearings like?

You can check the big ends and journals with plastigauge. Better than just eye-balling.

[aussieal]

I have had two of my brothers, both professional mechanics with over 30 years experience, take a look and they both expressed surprise at the lack of damage to the bores and suggested that a simple reaming would be OK. One reckoned that replacing big end shells not necessary, the other suggested I should replace them whilst the motor is apart, so I will probably take the latter's advice. I have not removed the motor from the car so camshaft action is not possible at present. Having had the motor out once before to replace the rear welch plugs, and found it a real pain, I was reluctant to do it again 

[enigmas]

If the bores are truly as good as your brothers have indicated, use an appropriate ball hone (not a stone hone) to produce the correct cyl surface finish. Ball hones due to their design require little to no skill and will reproduce the required cross hatch finish in the cyls. A stone hone will remove material.

NB. If you need to remove a top cyl lip with a cyl ridge reamer...the depth of the ridge indicates the actual cyl wear. An unworn cyl will not have a ridge.

To do an easy cylinder wear check, invert one of your best pistons. Lower the piston into the cyl (inverted) and position one of the thrust faces (that's 90° to the gudgeon pin) just below (10mm) the ridge line. Hold the piston using the gudgeon pin. Now slide a .002" - .004" feeler gauge between the thrust face of the piston skirt and just below the ridge.

You'll probably find the factory clearance is in the order of .0015" ...but check the specs.

You can probably run .003" - .004" clearances without issue...you're now in the racing clearance category...less friction.
A bell like noise during warm-up will be most likely occur and then dissipate when the engine is warm. It'll be fine like this as long as you warm it before working it hard. And you certainly won't seize a piston again. The Rover engine is part side valve (exhaust side)...this heats the block and can cause cyl distortion and seizure...as you've encountered.

As a matter of example the twin cam MGA engine I had as a young man ran better with looser piston clearances than it did after I rebuilt it with tighter clearances. Mechanical quietness is not a prerequisite for a performance engine.

HPC coatings in Leongatha Victoria, can coat the piston skirts for you and build up to .003" in skirt thickness at about $35 per piston. I've had this process done a number of years ago for an engine (non Rover) that I was building.

If the cyls are as good as you say they are...there should be no issue.

[djm16]

I recall seeing a while back a video showing Rover sorting nomimal STD pistons into ABCD and E according to their "nearest" tenth thou size. They were then stamped on the crown with the letters ABCD or E and then matched to individual bores.

[aussieal]

I have been offline for a while due to health issues. I have been unable to source a replacement piston which is compatable with the remaining 5 so have had to go the whole hog and obtain a full set of pistons and rings from JP Pistons in Adelaide, so that part of the fix is on the way. I am now having difficulty in sourcing standard big end bearings. Wadhams UK, Scotts Old Rubber and Rover Parts, and Autosurplus have not been able to help. Any ideas?

[djm16]

Have you tried John Wearing?

I am surprised about the difficulty finding parts. The block is common to late P4s and 6c Land Rovers. Mind you, it is 5 years since I put a new set of big ends and journal bearings in.

Now the camshaft bearings! That is another story. My advice there if they need doing is to have them re-metalled.

[Mike’S-a-loon]

Have you asked Mark Richmond?

0417 537 452.

He’s in Ballarat and has a lot of P5 stuff.