Pistons

[Chris Edgar]

I have bought an old 950cc engine which I plan to re-build as a nice project.
This will eventually replace my existing one.

Spent about 5 hours cleaning the crud off the outside prior to doing any dismantling, then the first thing off was the head.
Looking at the piston crowns, it seems that the engine has never been rebored...
the piston are marked:
M1710/4 MOWOG and either G48 and 3 (in a diamond) or G16 and (4 in the diamond ).
No oversize in a oval stamp.
I wonder if an expert can cornfirm that they are original size bores.

Oddly there are 3 of the pistons which have small chips out of the edges...see photos, and evidence of damage as if something metallic has been bashed around inside...? pieces of broken ring? There is no scoring of the bores.

I have yet to mesure the slight lip at the tops & remove the pistons to check the rings, but would be very grateful for any comments on what can be seen so far...

Thanks,
Chris
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[bmcecosse]

Could be detonation causing the marks - although the one with the 'hole' at the side does rather suggest a ring has escaped.....

[Chris Edgar]

Thank you Roy!

"detonation" as in ign. too far advanced & pinking?

Am I correct in assuming that the block has not been re-bored?

[bmcecosse]

Detonation is severe pinking!! Can't really tell from pics - but oversize pistons are usually marked 010/020/030 etc etc.....

[Chris Edgar]

Many thanks...there are no oversize markings, so I am assuming no re-bore

[bmcecosse]

Since you will be buying pistons - you may as well bore it and be back to 'as new'.......

[Chris Edgar]

Indeed!!
many thanks

[Tom and Maria]

Looking in the workshop manual the number in the diamond definitely denotes the bore size, but I don't know what. Surely they would be different sizes?

Tom

[bmcecosse]

Bore GRADE size........they matched the pistons to the actual measured bore size. I guess as the tool wore - the bore size reduced very slightly. Pistons available today are NOT graded....

[MarkyB]

Unusual to see one different, maybe there was a slight casting fault that got bored out when blocks were in short supply?

[bmcecosse]

Or more likely - one piston has been renewed some time in the past.

[Chris Edgar]

Two are graded "3" and two graded "4"

[hanvyj]

Is it a trick of the light or is one of them different to the others? More dished/with a slightly more pronounced ridge?

[bmcecosse]

I was just thinking that too -= looks like a low comp piston in there. Although - maybe just the light......

[Chris Edgar]

Just seen the comments regarding a ? low comp piston, I will go & have a look again.
Meanwhile, a photo of the damage....

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it is the same damage on all 4 pistons. The top rings seem to have jammed in the grooves, broken up & the bits have made their way through the top of the pistons at the edges.

The engine number is 9MUH 366806, which appears to be much earlier than the one already in the car (1962), which is ....622275.
Yet, it has not been re-bored & all the main & big end bearings are standard size & in very good condition, showing extremely light wear. Camshaft bearings are good too.
To me this suggests quite a low mileage for its age.

Is there a way to determine the year of maunufacture from the engine number?

I am taking the block & c/shaft to the local engineers tomorrow for his opinion but I do not think that the c/shaft will need a grind. I have measured the journals to 1 thou accuracy using a "proper" tool lent by a friend

[bmcecosse]

Camshaft bearing - it only has the one....

[MarkyB]

With that damage they are all low compression!