The Triumph Dolomite Club - Discussion Forum

It occurred to me today that I'm agonizing over a few grams weight difference on the pistons. We all agree that 7g is way too far out and that needs to be addressed but what should weight variation should I be aiming for given that I'm not building a race engine.

The closer to Zero your tolerances are, ie the closer to "blueprint" you get the engine, the better it will work, it will be more powerful, more reliable, last longer and get better fuel economy. From this point of view, it doesn't matter whether it's a full house 220 bhp racer or bog standard, the gains are the same.
In a perfect world, engines would come out of the factory like this, most modern engines ARE much better in this respect, but Leyland in the 70s were, i'm afraid, far from perfect.
I know I am invoking your OCD at full tilt, but the best you can get it, balance wise, is where you want to be!

Steve

If you rev your engine to 7000 rpm, a 4 stroke piston will travel up/down 28000 times per minute. (Have I got this right?).
Your lightest piston is 1.1% lighter than it's nearest in weight terms.
The other pistons are within 0.4% of each other.
I know my maths is supect, but you get the drift.
Given that three of the pistons are reasonably close, I would not accept the lightest piston. I have no idea waht those differences would mean at full tilt, but I would guess it's significant.
Another thought - given the build quality and tolerances of the originals, the difference wouldn't matter. But if you're building a decent engine the way you are, it could be that the 7g could be very detrimental.

The manufacturer/supplier may well argue that the piston is within manufacturing tolerances and specifications - ask them to have a rummage and find another piston that is withing the range you want.
3 out of 4 pistons can't be wrong.
I'm no motor engineer, maybe Hans or Jeroen will see this thread?

Mahle std sprint or ae are within 1gram and that's okay. I do not have experience with jp, only je but these jp look like some cheap boogie woogie pistons. Maybe the seller can select a matching set for your engine if he has more in stock. Even our beloved county seem to produce pistons of a better quality. These were never an issue.

Jeroen

If they have more in stock, take you scales with you.

Being pedantic but 4 cylinder 4 stroke is, starting at tdc no1 firing 1 down and 1 up for exhaust on no1 piston, so 2 strokes per crank revolution, if you get my drift, so 14000, not 28

Jonners

Jon Tilson wrote:Being pedantic but 4 cylinder 4 stroke is, starting at tdc no1 firing 1 down and 1 up for exhaust on no1 piston, so 2 strokes per crank revolution, if you get my drift, so 14000, not 28

Jonners

Thats still a lot when you think a piston starts at rest, accelerates to several hundred mph then returns to rest in the space of a few inches and it does this twice in every engine revolution. The loads and G forces involved are staggering!
"The amazing thing about an amazing dancing bear, is not how well it dances, but that it dances at all" (Mark Twain) The same thought applies to the internal combustion engine, when you look at it in purely analytical terms, it beggars belief that such an unlikely, inefficient thing could have been invented.......or worked at all........let alone being refined into what we have today!

Steve

If it hadnt been we might have better batteries by now and decent airships...

Or small efficient gas turbines....

Jonners

Jon Tilson wrote:If it hadnt been we might have better batteries by now and decent airships...

Or small efficient gas turbines....

Jonners

More likely we'd all still be riding horses, with a smattering of steam power, a much neglected technology IMO!

Steve

Anyway we are off topic, but car folks always miss that up til the 40's, internal combustion engine development was driven by the
need to increase aircraft performance. Sleeve valves are the one thing that never carried over into road engines.
Electric propulsion, and as you say steam too, was as strong contender for road use in the early days.

Jonners

Indeed, the first land speed record was set with an electric car in 1898

Tony

Quick update, I am speaking to Rimmers as they supplied them as part of their engine rebuild kit, just waiting for a return call from them. I have had good customer services from them in the past, they may have a bit more clout than me when it comes to dealing with JP.

Three of them look like skirts have been filed down which is strange as the box was sealed properly when I received them. This explains why those three are about 2-3mm lower in height to piston number 3.

Probably nothing but this is what number 3 (the lightest) looks like



Numbers, 1, 2 and 4 look like this







There's something odd going on here!!

That looks like a bit of fettling from the factory to get them "somewhere" near right. maybe just to remove casting flash without care for the weight but either way not a good ad for the manufacturer

Tony

And you'll want to remove the filings they've left before you fit the pistons in your engine.

Jeroen