Pistons

["Slack Alice" Simon]

I am re-building a rebuilt engine.   It was rebuilt by someone with even less A7 knowledge than I have.  I am having to do up those things that were left undone and re-bodging bodges to my own (better quality bodging) standard.

I am at the point where, having fitted block and pistons to crankcase, I am checking it is back together ok so far.


With the little end bolt heads on the offside, the "front" marking on the pistons is at the back.

1)  Does this matter?   I am loath to undo the little ends if I don't have to.


2)  The block has been sleeved back to standard, the engine has been run for a very short while (not by me).   There is some scuffing on the piston sides, which I would expect, I think.

I suppose I ought to check the bore diameter is correct, before going further.   If the pistons are standard and the bores are new, what difference should I expect between one and the other?

Standard engine, standard (non sporty) pistons.

Cheers

Simon

[Robert Leigh]

Yes, it does matter which way round the pistons are fitted. The front marking exists in particular on split skirt pistons; they should be fitted so that on the power stroke the plain side is the opposite side of the block from the crank throw; this keeps the side thrust away from the split. I understand that the split is to reduce piston slap in the interest of quiet running (a relative term!). Long ago I did fit some pistons the wrong way round; it did not seem to make a big difference, but it was pointed out to me by the next person to dismantle the engine.
Robert Leigh

["Slack Alice" Simon]

There, that makes the point not to be afraid of asking stupid questions - of course, there is the split to think of.

I'm doing the engine in between cycling and massive amounts of gardening, and not really giving it my full attention.

Thanks

Simon

[Chris KC]

Indeed. Although not all pistons are split skirt types, I'd hazard that if the maker marked the front with 'front' then it should be oriented.

Clearances I dare say will attract a variety of opinions and stubble-stroking. I have a note in my notebook that says 1.5 thou between a (new) piston and bore, measured at the BOTTOM of a split skirt piston (4 thou for plain) with a feeler gauge. Heaven knows where I noted that down from. I know for a fact that in the past I've run 6 thou at the bottom and 10 thou at the top - at that point I splashed out on a new set. For what it's worth my old Dad might have invested in new rings now and then but he very rarely bought new pistons to go with them!

[Reckless Rat]

While you've got the pistons out check the ring gaps before reassembly. That is as important as the difference between the bore and piston diameter. There's a video on you tube that shows you how and where to check, and how to adjust. Make sure you keep the rings in the right order and the right way up...

[Robert Foreman]

Beware, I have had a new set of split skirt pistons marked Front and they were incorrect with the split on the opposite side. So always check first.

[Steve Bryant]

You may not see it but on pistons the wrist pin is usually offset to one side to avoid direct inline impulses to the crank which is why they are marked front. This insures that the power impulse happens slightly after top dead center.
Stephen

[Zetomagneto]

Not sure that's the case with Austin Seven Pistons.The solid skirt Slipper Pistons are extremely good, and as far as I know are not marked for direction which means the pin must be central.

[Steve Jones]

Yes, slipper pistons not marked. I 'covered' a set up 10 minutes ago, hopefully for a long time!

Steve

["Slack Alice" Simon]

Just to show you lot I do listen ..... sometimes.

Your replies made me go and check in the book about ring gap sizes.   Which in turn, reminded me about "belling"the white metal in the big ends.

Which meant the block off and the rods out, again.

So I might as well take a ring or two off to check, though that is a job I really, really hate - I know I will break at least one.

In the mean time, I looked at a You Tube video or two (Thanks, Zeto) which showed that too small a gap can cause scores in the bores.

Light bulb moment.   So that is where it came from - in an engine that has hardly been run.

The top rings checked so far are fine 6 - 7 thou, as per Woodrow.

The oil rings - almost no gap at all.

Thinking about it - I don't think I have ever possessed an engine with new pistons, so have never had to do this before.   I have seen it done, by my Dad 60 years ago, but I have always had

hand-me-down bits, and not had to bother.

Cheers

Simon