yeah, another gapping ?

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acadian

acadian

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Read through the litany of differing opinions on end gap, and I'm a bit confused. Manual gives broader gap range for middle ring yet many here suggest reversing that? Also, can find nothing on oil ring gap. Hastings suggests measuring for gap at bottom of bore, while "experienced" members say top? I'm not going to shoot for min gap range, but middle of the road. I've had my +20 bore honed with +30 rings on the way, cylinder wear bottom to top is .004 so I'm still in good shape.
 
Shooting for the median when rebuilding is like purposely building an engine HALF WORN OUT.
 
concours said:
Shooting for the median when rebuilding is like purposely building an engine HALF WORN OUT.
Click to expand...

Really? So you're suggesting I aim for min. tolerances then?
 
Shooting for the median when rebuilding is like purposely building an engine HALF WORN OUT.
Click to expand...

Amen Concours [again]. Top Ring gap should start out close to .120" and second ring a couple .00x's wider. Can't do much about the oil ring with inter locking spring and twin thin rings pretty much set if gaps just skewed 120' or more. Only over heated racers, special fuels and boosted applications benefit by opening gaps over factory initial gap. As gaps wear into .20+ range, blow by and smoking can get so bad you are ticketed for pollution or miss-firing fouling. Run a good hi end oiled K/N type filter to hasten this process faster.
 
acadian said:
concours said:
Shooting for the median when rebuilding is like purposely building an engine HALF WORN OUT.
Click to expand...

Really? So you're suggesting I aim for min. tolerances then?
Click to expand...
Yes, for a "like new" engine.
 
Always set the ring gap at the tightest part of the bore.
You don't want them getting hot, and meeting and having nowhere to expand to...

BTW, hobots 0.120 gap is ten times bigger than it should be !!?
Its more like 10 or 12 thou (0.012"), depending on the ring maker....

Most rings will have the same gaps - unless the ring maker says otherwise.
Bear in mind - Too tight is a recipe for piston failure.
 
Rohan said:
Always set the ring gap at the tightest part of the bore.
You don't want them getting hot, and meeting and having nowhere to expand to...

BTW, hobots 0.120 gap is ten times bigger than it should be !!?
Its more like 10 or 12 thou (0.012"), depending on the ring maker....

Most rings will have the same gaps - unless the ring maker says otherwise.
Bear in mind - Too tight is a recipe for piston failure.
Click to expand...


Bear in mind... the MINIMUM ring end gap is designed for things work as engineered (no seizure). If you were to set up to say.004" ring end gap, then, yes, you could be sure of a seizure. There is no reason to add MORE clearance ON TOP of already engineered minimums.
 
acadian said:
Read through the litany of differing opinions on end gap, and I'm a bit confused. Manual gives broader gap range for middle ring yet many here suggest reversing that? Also, can find nothing on oil ring gap. Hastings suggests measuring for gap at bottom of bore, while "experienced" members say top? I'm not going to shoot for min gap range, but middle of the road. I've had my +20 bore honed with +30 rings on the way, cylinder wear bottom to top is .004 so I'm still in good shape.
Click to expand...

Are you saying you "honed" out .010"?
 
What's the rationale behind the broader range for the middle ring? Just out of curiosity...
 
acadian said:
What's the rationale behind the broader range for the middle ring? Just out of curiosity...
Click to expand...


The range is broader on the LOW end of the middle ring, most likely the reasoning being that it's further from the combustion chamber, exposed to less heat, therefore the amount of expansion would be less. The engineer likely reasoned that it will be acceptable to allow .002" less gap on the low end of the tolerance.
 
concours said:
acadian said:
What's the rationale behind the broader range for the middle ring? Just out of curiosity...
Click to expand...


The range is broader on the LOW end of the middle ring, most likely the reasoning being that it's further from the combustion chamber, exposed to less heat, therefore the amount of expansion would be less. The engineer likely reasoned that it will be acceptable to allow .002" less gap on the low end of the tolerance.
Click to expand...

Got it, so how does one reconcile the manufacturers .035 per bore inch recommendation which may contradict the manual's range? For example, based on my bore and using hasting's formula, I get a min gap recommendation of 10 thou. In this case would it be safe to assume I should gap the top ring at 12 thou and the middle at 10?
 
acadian said:
concours said:
acadian said:
What's the rationale behind the broader range for the middle ring? Just out of curiosity...
Click to expand...


The range is broader on the LOW end of the middle ring, most likely the reasoning being that it's further from the combustion chamber, exposed to less heat, therefore the amount of expansion would be less. The engineer likely reasoned that it will be acceptable to allow .002" less gap on the low end of the tolerance.
Click to expand...

Got it, so how does one reconcile the manufacturers .035 per bore inch recommendation which may contradict the manual's range? For example, based on my bore and using hasting's formula, I get a min gap recommendation of 10 thou. In this case would it be safe to assume I should gap the top ring at 12 thou and the middle at 10?
Click to expand...

Min gap is stated as .010" in the workshop manual. From what I understand of your first post, the cylinder bores have already been machined? Modern piston ring manufacturing produces rings that do not require filing to fit. That is, if your bores size is correct, then the ring end gap falls right in place. It's best to have the new piston and rings available for the machinist to work to when doing the job.

Just re-read it... .020" over existing bore honed (just to break the glaze?) and you'll file to fit the .030" over rings. Got it. What is your piston clearance?
 
concours said:
Min gap is stated as .010" in the workshop manual. From what I understand of your first post, the cylinder bores have already been machined? Modern piston ring manufacturing produces rings that do not require filing to fit. That is, if your bores size is correct, then the ring end gap falls right in place. It's best to have the new piston and rings available for the machinist to work to when doing the job.

Just re-read it... .020" over existing bore honed (just to break the glaze?) and you'll file to fit the .030" over rings. Got it. What is your piston clearance?
Click to expand...

My barrels were honed, not over bored. Still within specs as per machinist. I have two sets of rings, +20 and +30. The +20 rings give me gaps in the 14 - 16 thou range, the +30 rings would allow me to file down to whatever gap I want.
 
Interesting reading.

I still set my gaps at the absolute minimum (or slightly below...!) and yes, I stagger the gaps. Mind the gaps!
 
mikeinidaho said:
Piston rings manufactured for a 0.30" oversize bore are likely to exhibit some distortion(and less than ideal sealing) when filed down and shoved into a 0.20" over cylinder. 0.20" over parts will be a better fit , even with a few thousandths more end gap. Some interesting reading on ring gaps: http://www.diagnosticengineers.org/jour ... 20Gaps.php
Click to expand...

Interesting reading indeed, and another opinion to throw on the pile. Listening to engineers debate the importance of rings gaps is as entertaining as watching lawyers argue over a contract... cylinders are just under +20 service limit, +30 rings will not cause worlds to collide as some purport.
 
I thought it was interesting that they took the time to assemble an engine with various different ring gaps and test it each time for blowby and other factors. Most of us wouldn't have that much time on our hands. You might want to square up one of those, filed to fit, 0.30 over rings in the cylinder and shine a light behind it to see how much of it is actually touching the cylinder wall.
 
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