A lesson in pistons

[AlanBartlett]

I'm current trying to figure out which size piston i require for my engine rebuild. I'm currently getting lost in number at the moment and was wondering if any one could shed some light on deciphering the piston code so to speak. My current measurements I have are...

The bore size on my cylinders is 2.998 inch, and the size of the old pistons are 2.982, the reason i cannot use the old pistons is because of the excessive ware on piston top, and missing half the top rung of the ring gutter. I'd like to know whether these pistons would be std or plus ??. As ive just recently ordered a set of std sized piston which are 2.954 inch, but this appears to be very sloppy in the cylinder bore.

[ian Howell]

Alan: -

I cannot be sure that this was ALWAYS the case - especially if non-Jowett pistons were installed at some time in the past - but on my Long Four the pistons are clearly (if faintly) marked with their size - in my case 'STD', on the rim around the 'dish' on the top face of the piston.

75.4 mm equates to 2 31/32" or 2.968"

So 2.998" seems to be 0.030" or 30 thou' oversize.

Hope this helps!?

(Watch this space!).

[AlanBartlett]

hello Ian,

Thanks for the information, I've noticed that most pistons i have, have their size marking on the top, though I'm lucky enough for the original ones to have been so worn that not even that is present any more. But judging by your conclusion if they are 30 thou then I think that is the maximum the bores can be bored out by. Correct me if I am wrong. If they are what is the possibility of using liners to go back down to standard?

Does anyone know the local Jowett agent, maybe I should have a word with them haha.

[Forumadmin]

Bore diameter minus piston diameter should obviously be positive; but by how much? Well that depends on the materials of both and the expected difference between cold and maximum working temperatures.

It probably also depends on surface finish of the bores and the type of compression rings.

For a pre-war engines, piston speeds will be much lower than for modern engines and tolerances will be much sloppier. So I guess the question is first what is allowable. I would measure the fitted pistons in many places and see what variation there is on the diameter. The largest diameter is likely to be what they were originally, unless they have been sat upon! Do the same with the bores to find the smallest diameter, both at different angles and at different depths. Bores will be worn at top and bottom where the rings change direction. Pistons will wear in the same plane as the motion of the con rod. The widest part should be across the gudgeon pin centreline.

For the Jupiter fitted with Jowett pistons the skirt clearance is .0015in or .0381mm, the ring gap in cylinder is .007 to .015 in (.178 to .381 mm)

I do not have the same data for the pre-war; but I would do extensive checks if the bore diameter minus piston diameter was greater than .004in. You had .016in on your old pistons/bore.

http://www.mgcars.org.uk/mgcc/sf/000304.htm
http://www.vintageaustinservices.co.uk/rebore.html

When no specs are available I have used a Rule of Thumb of .003" ring gap per inch of bore diameter.

To check more accurately remove the rings from the piston that has been working in that bore and fit it into the cylinder, the right way up and the right way round. Using feeler gauges inserted between the piston and the cylinder wall, determine the clearance between the piston and the cylinder at the bottom, middle and top of the bore (avoiding the unworn ridge at the top). The normal clearance for split pistons is about 0.0015 inch. Up to about 0.0045 inch is probably acceptable, possibly with new piston rings, whereas a gap approaching 0.010 inch indicates a re-bore is needed.

It’s also worth inserting a piston ring into the top of the bore, above the wear ridge, level with the top of the block. Check the gap in the piston ring. It should be between 0.005 and 0.006 inch. Using the piston to keep the ring square in the bore, push the ring down the bore so that it is below the ridge and recheck the gap. An increase in gap of more than 0.003 inch indicated that a re-bore will be necessary.

[Alastair Gregg]

Alan, you are the Jowett agent in Zomerzet!!!

[Tony Fearn]

Can't help on this one Alan. Don't know much engineering practice, but I'm wondering if George is hibernating know.

Perhaps next Spring he'll join in.

Glad the package arrived safely.

Tony.

AND A VERY HAPPY NEW YEAR TO YOU AND YOUR MUM, AND TO EVERYONE ON OUR WORLDWIDE FORUM, FROM HILARY AND ME.