Fitting new pistons

[BobinAustria]

I believe I have at last found the cause of the knocking sound which my 1958 Minior produces under load, not on the overrun. I have removed the pistons and found

20201111_102413.jpg (3.47 MiB) Viewed 1388 times

scoring marks on nos. 3 and 4 together with distortion on the piston skirt, which is able to slop about 0.5 mm from side to side on bottom dead centre, pushing from beneath. ( nos. 1 and 2 don't). I presume that at some time in the past they must have overheated.

I have ordered replacements.

In past threads several commentators here have recommended glaze busting before fitting the new pistons, but as the engine is in the car with of course all the rest of its innards I'm fearful of getting debris in the crankcase. How important is it? Has anyone fitted new pistons without glaze busting? There are fortunately no score marks in the bores and no trace of a ridge near the top. According to it's number the engine is a factory recon and I have no idea how many miles it's done, the crankshaft is still in good order.

[oliver90owner]

I would recommend careful measurement of the engine components before taking any further action. Something caused those damaged pistons and there may be other parts out of spec.

[Murrayminor]

As above, check for crankshaft throw fore and aft movement, also check the bores, they may need to be re-bored to remove damage.
Yes it does look like that piston has certainly been flopping about in the bore.

What is the condition of the big end bearings and are any of the other pistons damaged?

Looks like the engine needs to be gone through and rebuilt.

Nice little project, it will be lovely when done.

[Murrayminor]

I have linked this thread to your original thread so people can see you found a solution to your original query.

[kevin s]

De-glazing the bore is important, if you don't the rings won't bed in and it will drink oil. I had something very similar on a ford engine (1600GT X flow), it overheated and knocked afterwards, piston looked almost identical, de-glaze and new piston fixed it..

The stones are usually used with oil so there won't be dust flying around but gritty oil will drip down the bores, to keep the grit out of the crank all you can do is use polythene and tape to mask as much of the crank and crankcase off as possible.

[BobinAustria]

As above, check for crankshaft throw fore and aft movement, also check the bores, they may need to be re-bored to remove damage.
Yes it does look like that piston has certainly been flopping about in the bore.

What is the condition of the big end bearings and are any of the other pistons damaged?

Looks like the engine needs to be gone through and rebuilt.

Nice little project, it will be lovely when done.

Thanks for the swift reply!

I replaced the mains and big ends (together with oil pump, timing chain etc. when I got the car a couple of years (but only a few 100 miles) ago as part of work to stop an oil leak. (I think the real culprit was a crack in the oil pump cover). The crankshaft was and still is in good condition, I've checked again no. 4 big end with plastigauge. The crankshaft has no discernable axial play, the conrods can move about 0,5 mm sideways . The previous owner had done the cylinder head and the engine had perfect compression and ran well apart from the rattle - which I didn't notice until I got the car on the road again.

Pistons 1 and 2 look perfect. There are by the way 3 such score marks on each of pistons 3 and 4 at 45° to each other (the photo is of the worst). The bores show no scoring at all. I can feel no ridge at the top of the bore with my fingernail, but do measure an approx. 0.010 mm wider gap in an inserted old piston ring at the critical depth in the bore, which should correspond to a ridge of the order of 0.015 mm or less than 0.001".

[BobinAustria]

Thanks, I thoght it would be logical to have some connection but had no idea how to do it.

[kevin s]

As it's 3 an 4 One other thing to check would be the block and head water passages, if they are all silted up it will do the same again (best way is to knock a couple of core plugs out).

[BobinAustria]

Following the advice given here I did "deglaze" the bores (using strips of emery cloth in a drill) and discovered more marks in the bore than I saw at first, specifically some radial marks, which I think must have come from corrosion between piston rings and bore at some time in the past when the engine was stationary for a long time. These I managed to almost erase with 180 grit paper. I thought a rebore might hence be necessary but as I already had the new pistons and rings (5-Ring pistons from ESM) I carefully installed them - they all fitted well with piston ring gaps within tolerance as delivered - and after putting everything else back together I went for a short test drive today. Result: no rattle any more and no blue smoke from the exhaust (which I had feared with new pistons, old bore).

It'll be a while till it's run-in (I don't drive the car in the winter due to the generous salting of the roads here) but at the moment I'm very pleased with the result.

Incidentally the service at ESM was excellent: parts ordered on a Tuesday evening and delivered Friday afternoon.

[philthehill]

Well done.
If you do not use the car on a regular basis - put a good egg cup of engine oil down each bore and turn the engine over every now and then. Go over an odd number of compressions every time so that the same valve springs do not end up being compressed every time and engine components do not reside in the same place either.