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yorkshire_spam

Piston sizes vs bore, another newbie question.

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Morning all, something that might be obvious, but I don't know it.... so I have to ask!

When I look in the catalogues for pistons is the specified size the ACTUAL piston diameter or the diameter of the cylinder bore that the piston is designed to fit?

Is there any general rule of thumb for what the difference between the 2 (bore and piston diameter) should be?

 

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Actual piston size, I think.

But always give the pistons to the machine shop for reboring, so they get the size right.

John

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I would always buy the pistons before the re-bore and give them to the machine shop so that they can bore and hone the block to the piston's actual size, using the correct clearances of course. Not the other way round.

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2 hours ago, sparky_spit said:

I would always buy the pistons before the re-bore and give them to the machine shop so that they can bore and hone the block to the piston's actual size, using the correct clearances of course. Not the other way round.

Agree...... but, if you are supplying pistons rather than the machine shop, you need to either have a good measure of your block or have the machine shop measure it, and agree with them how far oversize it will need to go to fully clean up. The “next size up” isn’t always enough. Otherwise you risk buying pistons that are too small.

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3 hours ago, yorkshire_spam said:

So for a specific bore size, what piston size should you buy? 

for example if a re-bore takes you to a bore diameter of 75mm what diameter of piston do you need? 

Pistons (unless you have them made, bespoke) are available in standard oversizes, +10, +20, +30, +40 thou.     If your engine needs an overbore - for wear? - then the machine shop will be able to decide how much the wear is, as they have the precision instruments.     They will take it out to match the next size up of available piston.

I'm not sure what size piston would suit a 75mm overbore, but standard size is 74.7mm, and 0.3mm is just under 12 thou, so a +10thou oversize?     But consult your machine shop.     

John

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10 minutes ago, JohnD said:

Pistons (unless you have them made, bespoke) are available in standard oversizes, +10, +20, +30, +40 thou.     If your engine needs an overbore - for wear? - then the machine shop will be able to decide how much the wear is, as they have the precision instruments.     They will take it out to match the next size up of available piston.

I'm not sure what size piston would suit a 75mm overbore, but standard size is 74.7mm, and 0.3mm is just under 12 thou, so a +10thou oversize?     But consult your machine shop.     

John

Thanks John, I'm a bit off-piste on this one. I get the oversize thing and buying pistons to suit the over-size. What I'm doing as a bit of a thought experiment is trying to identify pistons that are a close match to what would be required as forged pistons don''t seem to be available over-sized for a 1500. It's all totally theoretical and not something I'm ever likely to do in practice, but I'm just sort of wondering. I know there are other factors too like height above the pin for the piston top and the pin size itself. 

Thanks! Sam

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Ah!  Non-Triumph pistons?

Not sure about 1500s but I've often wondered about the claims for 2.7L for sixes, when a +40 over bore really is the max you can get.   Apparently the answer is off-set bores (a recipe for a short life, surely, but these are racers, so who cares?) and ??VW pistons?

John

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1 hour ago, Nick Jones said:

Agree...... but, if you are supplying pistons rather than the machine shop, you need to either have a good measure of your block or have the machine shop measure it, and agree with them how far oversize it will need to go to fully clean up. The “next size up” isn’t always enough. Otherwise you risk buying pistons that are too small.

Exactly, you don't know until the rebore.

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13 minutes ago, JohnD said:

 Apparently the answer is off-set bores 

 

Common practice with 1380 A-Series over bores, mines done a lot of hard miles with no problems.

RR

 

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Good for you!    There must be a way to do it.

J.

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Didn't John Kipping squeeze 1700cc somehow using an unexpected parts combination?

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5 hours ago, JohnD said:

Ah!  Non-Triumph pistons?

Not sure about 1500s but I've often wondered about the claims for 2.7L for sixes, when a +40 over bore really is the max you can get.   Apparently the answer is off-set bores (a recipe for a short life, surely, but these are racers, so who cares?) and ??VW pistons?

John

60 OS easy got for 2500 takes out to 2600.  Max for head std gasket . 2700 is done with a £250 gasket and scraps 2 out of 3 blocks

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8 hours ago, Nick Jones said:

Didn't John Kipping squeeze 1700cc somehow using an unexpected parts combination?

Years ago the TSSC were selling a 1700 engine and stupidly called it a supersport? Think they had a lot of warranty claims on them, soon stopped production. Think they stroked a 1500 or something,  Maybe used 2.5 pistons?

 

I remember an article years ago, some chap had built a "hot" engine using 2.5 pistons as he reckoned they are stronger/lighter than the 1500 type. Then decked the block to suit. I didn't understand why it was such a good idea, as it still had the long 1500 stroke. Guess it meant the engine was lighter....

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3 hours ago, zetecspit said:

I remember an article years ago, some chap had built a "hot" engine using 2.5 pistons as he reckoned they are stronger/lighter than the 1500 type. Then decked the block to suit. I didn't understand why it was such a good idea, as it still had the long 1500 stroke. Guess it meant the engine was lighter....

I suppose if they were significantly lighter you'd have a lot less reciprocating mass, so the crank wouldn't get as stressed, so you could rev it a bit harder. But probably not much in reality!

17 hours ago, RedRooster said:

Exactly, you don't know until the rebore.

This is something I was thinking about yesterday. My FWD engine needs a rebore after the exhaust valve breakage, but I don't really know how much so will have to take it to the machinist before buying any pistons, then he'll have to wait for the pistons to arrive before doing anything with the block. 

On 1/12/2020 at 10:29 AM, yorkshire_spam said:

When I look in the catalogues for pistons is the specified size the ACTUAL piston diameter or the diameter of the cylinder bore that the piston is designed to fit?

Is there any general rule of thumb for what the difference between the 2 (bore and piston diameter) should be?

Sam I just checked my 'glovebox' workshop manual for the 1296 and 1500 Spitfires and it says:

    "Piston clearance in bore --------- 0.0009 to 0.0024 at bottom of skirt" [inches]

So for a rule of thumb, I'd call it 1 or 2 thou!

Interestingly the piston diameters are quoted at top and bottom, and they're slightly smaller at the top - didn't realise that. And the tolerance is tighter at the bottom.

For example, a Hepworth grade F piston is 2.8752" to 2.8799" at the top, and 2.8976" to 2.8981" at the bottom

Pete

 

Edit: the dimensions in mm are given as follows:

Cylinder bore: 73.66 to 73.64

Piston top dia. 73.15 to 73.03 

Piston bottom dia. 73.61 t0 73.59

Edited by PeteStupps
Add dimensions in mm

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For the 1700 I have a feeling it was something along the lines of offset grinding a 1500 crank (for 2.5L 95mm stroke) by using small-crank big end diameter with small-crank rods. Not a recipe for longevity in my mind given the 1500's less than sterling reputation.

Not sure of an alternative piston for the 1500, but for the 1300 I've read that 2.0L pistons may be slightly beneficial due to less gudgeon pin offset although I think the compression rings are thicker (5/64" vs 1/16") which would increase friction?

Edited by JumpingFrog

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1 hour ago, PeteStupps said:
On 1/12/2020 at 10:29 AM, yorkshire_spam said:

When I look in the catalogues for pistons is the specified size the ACTUAL piston diameter or the diameter of the cylinder bore that the piston is designed to fit?

Is there any general rule of thumb for what the difference between the 2 (bore and piston diameter) should be?

Sam I just checked my 'glovebox' workshop manual for the 1296 and 1500 Spitfires and it says:

    "Piston clearance in bore --------- 0.0009 to 0.0024 at bottom of skirt" [inches]

So for a rule of thumb, I'd call it 1 or 2 thou!

Interestingly the piston diameters are quoted at top and bottom, and they're slightly smaller at the top - didn't realise that. And the tolerance is tighter at the bottom.

For example, a Hepworth grade F piston is 2.8752" to 2.8799" at the top, and 2.8976" to 2.8981" at the bottom

Pete

 

Edit: the dimensions in mm are given as follows:

Cylinder bore: 73.66 to 73.64

Piston top dia. 73.15 to 73.03 

Piston bottom dia. 73.61 t0 73.59

Fantastic, thanks Pete!

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