TECHNICAL FOOD FOR THOUGHT

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Originally Posted by workingonit

My understanding is that rods/pistons etc stretch a little at higher reves, ie reach higher into the top of the cylinder.

Read some advice years ago to occasionally taking the vehicle to higher speed (higher revs but not red line) to ameliorate the wear ridge, to keep the bore as uniform as possible along its length to cope with rev increases you may experience in normal stress use - this is when my Ford engine had been rebored and I was installing new rings - hope it's a good excuse when the constabulary pull me over for 'slowing time' as I take the D1 Tdi300 to 135kmph.

Piston ring - Wikipedia

When fitting new rings to a used engine, special "ridge dodger" rings are sometimes used for the top compression ring, to improve compression and oil consumption without reboring the cylinder. These have a small step of iron removed from the top section to avoid making contact with any wear ridge at the top of the cylinder, which could break a conventional ring. These are not widely recommended, however, as they are usually not required and may give inferior oil consumption. A more acceptable method is to remove the wear ridge with a "ridge reamer" tool before lightly honing the bore to accept new rings.

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What you are saying makes sense, Maybe this is what the Toyota bloke was trying to say in the first place.

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Originally Posted by Patrick M

What you are saying makes sense, Maybe this is what the Toyota bloke was trying to say in the first place.

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Why not just use a ridge remover or hand scrapers to remove the ridge? Once was standard practice when rebuilding engines to take out the ridge. When done by hand it was an apprentice horror job. " Get your scrapers and knock the ridge out of those bores, young fellow. Should be finished by smoko or else."

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Originally Posted by BathurstTom

The redline is the cruising revs! The pistons will only come out if the piston springs are worn. YouTube

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What a wonderful piece of mechanical engineering, really I can't wait to get some for my 4.6 Rangie engine.
They would have to be nearly as good as "Johnson Rods".

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Originally Posted by rick130

Ash, so I was really lucky when I pulled second instead of fourth in a little Formula Ford several lifetimes ago that the exhaust valves stopped the pistons exiting the bores!!

All four were nicely bent. [emoji15][emoji23]

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So thats what SWMBO must have done to her bike the other week.
One was not bent,it was completely smashed,but i suppose it did stop the piston from exiting the bore[thumbsupbig]

Funny the call i get,its stopped and it won't start,does sound different than it ever has...[bigsad]

The engine,was absolutely &%$#@ed

Is it news to people that steel is elastic?
Not extremely, but still needs to be taken into account.

This gives those engineers something to get their pocket protectors all hot and sweaty over... yield and tensile strengths, modulus of elasticity, etc etc.

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Originally Posted by Bigbjorn

Why not just use a ridge remover or hand scrapers to remove the ridge? Once was standard practice when rebuilding engines to take out the ridge. When done by hand it was an apprentice horror job. " Get your scrapers and knock the ridge out of those bores, young fellow. Should be finished by smoko or else."

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That's a fun job, I did the 6-cylinder in my FFR some years back because the top of the block isn't at 90degrees to the bore (F-head) otherwise I could have used a proper ridge remover......
In the UK years ago I had an Austin Somerset that had the top ring on a piston break out of the piston and the pieces embedded in the piston crown. Found some NOS pistons the correct size but there was a wear ridge in the bores. As the pistons used 3 compression rings I left the top ones out, ran for years like that !

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Originally Posted by Toxic_Avenger

Is it news to people that steel is elastic?
Not extremely, but still needs to be taken into account.

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Originally Posted by workingonit

My understanding is that rods/pistons etc stretch a little at higher revs, ie reach higher into the top of the cylinder.

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Do the parts actually 'stretch' or is it the 'clearance' in the little end, big end & crank bearings that come into play along with any 'whip' in the crank ?

The big end & crank bearings are hydrodynamic so need clearance for a film of oil, at high revs the forces may compress this film so the piston travel is further up the bore by a few thou.

On modern engines with modern oils you don't seem to get bore wear like you used to so maybe this is less of a problem nowadays ?


Colin

They stretch, or more correctly elastically deform, although other stuff can happen as well. Think about this, the force applied by the rod to stop the piston at the top of its stroke equals the mass of the piston multiplied by its acceleration. Although the piston is fairly light the acceleration required is significant. This force is transmitted from the crankshaft through the length of the rod to the piston.

All steel is elastic, up to the point it isn't (it's yield stress). How elastic is defined by Young's Modulus (or modulus of elasticity) and is simply defined as stress divided by strain (the amount it stretches). This value is derived through testing and for steel is in the order of 200GPa. Stress is simply force divided by area (so a smaller rod cross sectional area will have a higher stress).

So by applying the force to a cross sectional area, you can derive a stress. From this stress you can then calculate the strain. Simple so far. (Although the actual loads are a bit more complex than that).

As engine speed increases, the force applied becomes higher because the acceleration increases. (Acceleration is the rate of change of velocity). As a result, the stress on the rod increases and therefore the strain increases (ie, how much it stretches).

Cheers,
Jon

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Originally Posted by gromit

Do the parts actually 'stretch' or is it the 'clearance' in the little end, big end & crank bearings that come into play along with any 'whip' in the crank ?

The big end & crank bearings are hydrodynamic so need clearance for a film of oil, at high revs the forces may compress this film so the piston travel is further up the bore by a few thou.

On modern engines with modern oils you don't seem to get bore wear like you used to so maybe this is less of a problem nowadays ?


Colin

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Head bolts stretch when fastened- this is how they apply clamp load to the surfaces. Torque them too much and they yield, and your head is held on with very minimal force.

Engine oil aside, the rotating assembly of the engine has mass and inertia, which would come into play when it comes to the piston's acceleration and deceleration up and down the bore... albeit to a very minimal degree.
Force = mass x acceleration ... there is probably a whole heap of other calculations engineers use, but without an extra few zeros on my payheck, that will remain a mystery.

Don't get me wrong, the OP's statement was complete fabricated BS, but there is a whiff of science in there that should not be discounted.

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Originally Posted by Toxic_Avenger

Head bolts stretch when fastened- this is how they apply clamp load to the surfaces. Torque them too much and they yield, and your head is held on with very minimal force.
Different situation to the engine rotating. I agree that the parts may elastically stretch but you also need to take into account play in bearings, crankshaft whip etc.

Engine oil aside, the rotating assembly of the engine has mass and inertia, which would come into play when it comes to the piston's acceleration and deceleration up and down the bore... albeit to a very minimal degree.
Agree (as above), in post #4 I mentioned a test I remember in the UK years ago that demonstrated the extra travel caused by running at higher revs.

Don't get me wrong, the OP's statement was complete fabricated BS, but there is a whiff of science in there that should not be discounted.
Not sure it was bull dust just badly worded by a non-engineer

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Colin

it seems well recognised by those who have seem or heard about examples of it that it is a bad idea to run a worn engine that may have a lip near the top of the bores at the normal limit of travel of the top ring at significantly higher revs than normal if it has not had regular runs at high revs. The extra inertia of the pistons can cause enough stretch etc in components to result in the top rings hitting the lip created by themselves at the top of the bore very hard. Spmetimes breaking them. Resulting in suddenly sick engine.

In 1990 when doing an agricultural contracting job for a farmer, for it we had to go and collect another machine about 100 km away. I went off first in his farm truck. He left some time later in Holden ute in which motor had probably done mainly lots of short runs around farm - mainly at lower revs than for normal road speeds and also probably had significant bore wear from a lot of running cold with short runs. Anyway, this vehicle was given a burst of probably over 50 km at high road speeds as he wanted to catch up with his truck, which I was driving. Was beginning of end for motor. Remember it then started doing things including blowing smoke and burning oil due to broken top rings.