Piston Questions

[Nigel Incubator-Jones]

After far too long doing nothing with my 100E's dismantled engine, I did manage to get around to replacing the piston rings tonight. So far so good, but my curiosity was piqued by a couple of things.

1) I couldn't help but get a few scratches on the pistons as I was putting the new rings on. Is this going to be a problem? They're too shallow to be perceptible by touch, and the old side-valve isn't exactly what you'd call a precision engine, so I'm thinking not but thought I ought to check. When do a few scratches cross the line from acceptable to unacceptable?

2) The tops of the pistons have what seems to me like some fairly heavy pitting on them, which only became apparent once the oily layer had been scrubbed off. I was wondering, for one thing, how that would happen and, for another thing, if this is also a cause for concern?

I appreciate pictures might help so if anyone should want to see some then I'll go and take them.

[stilltrying]

Pictures would help BUT, a few scratches on the pistons as you put the rings on is not uncommon and will not be a problem. Gouges on the piston would be a bit more of an issue. Remember the pistons don't do the sealing the rings do so make sure you gapped the rings first (you did do that didn't you?)

Pitting on the top of the piston is not a major issue on the engine you have but would be on something more revy and "precision" could have been caused by detonation in the past or just water damage from standing unused for some years.

I once had a hillman Minx 1500 and the top of the pistons were well pitted but the engine ran fine for many many thousands of miles, mine was pitted through water as the head gasket had gone and had been still running for weeks like it till I got round to replacing it

[roscobbc]

Used to get pitting on piston crowns of 2 stroke scooters. That was from broken piston rings when they used to seize up from lack of oil in petroil mixture. I blamed Bardharl oil. Advertising sail to use half the amount.......

[Nigel Incubator-Jones]

Pictures would help BUT, a few scratches on the pistons as you put the rings on is not uncommon and will not be a problem. Gouges on the piston would be a bit more of an issue. Remember the pistons don't do the sealing the rings do so make sure you gapped the rings first (you did do that didn't you?)

Pitting on the top of the piston is not a major issue on the engine you have but would be on something more revy and "precision" could have been caused by detonation in the past or just water damage from standing unused for some years.

I once had a hillman Minx 1500 and the top of the pistons were well pitted but the engine ran fine for many many thousands of miles, mine was pitted through water as the head gasket had gone and had been still running for weeks like it till I got round to replacing it

Thanks for that, sounds like I haven't got anything to worry about, then. Gapping the piston rings, though - I've not done that. I would have thought that they're all ready-made correctly gapped?

[Plumpcars]

They should be but the key with an engine build is 'never take anything for granted'.

[stilltrying]

They should be correct and as you are not to my knowledge fitting a blower or nitrous you will be fine.

A lot of people don't check as they go along just blindly fit what they have been given, however use this build as a learning curve, you should be able to basically throw it together and it will run, this will get you the basic's, next step is towards blue printing, manufacturers make the first engines to very tight tolerances but then when mass produced the tolerances get stretched and the engines are less than perfect, you then build the engine back to manufacturers original specifications, mainly for high HP or racing but done on a daily driver would ensure a bit more longevity.

One thing you will need to gap is the plugs, people buy a set of plugs for their car and blindly fit them because "they are already gapped correctly" this is far from correct, various vehicles/engines will use the same plugs but will need a different gap between say a Ford and a Nissan or a Land Rover etc. etc.

Keep up the good work.