yorkshire_spam Posted January 12, 2020 Posted January 12, 2020 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?
JohnD Posted January 12, 2020 Posted January 12, 2020 Actual piston size, I think. But always give the pistons to the machine shop for reboring, so they get the size right. John
yorkshire_spam Posted January 12, 2020 Author Posted January 12, 2020 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?
sparky_spit Posted January 12, 2020 Posted January 12, 2020 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.
Nick Jones Posted January 12, 2020 Posted January 12, 2020 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. 1
JohnD Posted January 12, 2020 Posted January 12, 2020 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
yorkshire_spam Posted January 12, 2020 Author Posted January 12, 2020 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
JohnD Posted January 12, 2020 Posted January 12, 2020 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
RedRooster Posted January 12, 2020 Posted January 12, 2020 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.
RedRooster Posted January 12, 2020 Posted January 12, 2020 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
Nick Jones Posted January 12, 2020 Posted January 12, 2020 Didn't John Kipping squeeze 1700cc somehow using an unexpected parts combination?
Gt6s Posted January 13, 2020 Posted January 13, 2020 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
toofast2race Posted January 13, 2020 Posted January 13, 2020 I've never scrapped a block going to 77mm or 77.5mm. I must be lucky 1
zetecspit Posted January 13, 2020 Posted January 13, 2020 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....
PeteStupps Posted January 13, 2020 Posted January 13, 2020 (edited) 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 January 13, 2020 by PeteStupps Add dimensions in mm
JumpingFrog Posted January 13, 2020 Posted January 13, 2020 (edited) 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 January 13, 2020 by JumpingFrog
yorkshire_spam Posted January 13, 2020 Author Posted January 13, 2020 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!
TRier Posted November 20 Posted November 20 On 1/13/2020 at 3:23 AM, toofast2race said: I've never scrapped a block going to 77mm or 77.5mm. I must be lucky Hi, I have to do a couple of 6 cyl Triumph engines over the next couple of months, for my TR6 and TR5. I'm going to go with the VW pistons as they are readily available over here at the right price (and I need 12!). I'll give them to the machine shop to bore to suit but I am curious, what size did you go out to for the 77mm units you have done? I looked at the Mazda pistons as well @77.5, they arent as readily available and seem perhaps a little too marginal for road cars. Thanks. Niall
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