Just wondering...What is acceptable in Squish Variance?

Another quick question... Is there a desired distance from bottom of plug to top of piston. I guess what I am asking is how close is to close?

It depends on the motor. I have always been told the squish should be anywhere between .055 and .065. As far as distance between piston and bottom of spark plug that I couldn't tell you. Riley

This topic might be better served in the High Tech discussion area.

First off, you need not measure the squish on the intake and exhaust sides of the cylinder. Due to the natural rocking of the piston on its bearing(s), squish measurements are not accurate perpendicular to the wrist pin. Always measure squish parallel AND inline with the wrist pin for accurate measurements. Intake and exhaust side measurements CAN vary and still be normal, nearly as much as the piston to cylinder clearance measurement, hence the "rocking" principal on the wrist pin.

Wrist pin parallel measurements can and do vary, typically just a few thousandths. Greater variations than that are TYPICALLY due to improper measurement as it is very likely to get multiple varying numbers each time you measure. For example, typical mistakes people make when measuring squish include material being compressed is not thick enough to get a proper impression, the material is too thick which provides some spring back memory, or not cutting the ends of the measuring material to a precise perpendicular cut (which ensures that the measuring material is allowed to travel entirely to the outer squish band area).

If a high degree of side-to-side squish variation is not due to human error, mechanical aspects of the engine should be checked. Items such as worn wrist pin bearings, worn rod bearings, damaged crankshaft bearings, bent rod, "tilted squish" machining, etc., should be investigated. Again though, this is not as common as one would think. Unless the engine has experience some large mechanical failure in the past, it is very unlikely that these conditions would exist.

Although you didn't specifically ask for minimum squish clearance, this is also an important factor. As Riley stated, minimum squish clearance does depend greatly on type of motor, displacement, targeted max RPM range, and intended purpose.

A rule of thumb though is that the minimum squish measurement should never be lower than .040 - .045 of an inch. It is very important to note that this stated minimum is more of a physical limitation rather than a performance or thermal limit. Engine application may dictate that .060 of an inch may be your absolute limit before detonation issues arise or flat RPM operation, but it is also important to understand that beyond runability, there does exist some physical properties that also limit this number. Squish readings below ~.040 of an inch can lead to actual "piston bounce", where the piston is actually striking the head. Under high RPMS and heat, pistons and connecting rods "stretch", possibly allowing the piston to come in contact with the head, which obviously could be a very catastrophic situation for the engine.

For mild racing applications on small bore triples, squish measurements are typically in the .060 to .070 range for safe operation. Race engines, like my 1100, have slightly lower numbers to increase maximum squish velocity (MSV) and to improve gas scavenging.

As far as a minimum spark plug tip to piston crown measurement, I have never heard of one. An obvious limitation would be at the point where the piston strikes the plug, but again, this is surely obvious. I personally know individuals that have miscalculated their plug reach and have had issues with the piston "tapping" the electrode closed, but even this did not produce any major ill effects to the engine, other than poor operation. While maintaining the appropriate distance should be considered to prevent the two from colliding (remember that rods stretch under high RPM and heat - hard collision could severely damage the engine), the importance here is simply to have a rapid, but controlled burn in the combustion chamber.

As long as the plug reach is enough not to obscure the tip, the actual position is not nearly as important as some believe. Even a plug manufacturer will admit that plug indexing, at best, gives an increase of less than 1% of maximum engine output HP, on your best day with a dyno (that's about 1 HP for most of us). That's provided you even know what direction to point the gap since facing it towards the intake is not ALWAYS the best location.

I realize that plug reach and plug indexing are two different subjects, but I wanted to demonstrate positioning and make a comparison...

Originally posted by SWWebmaster@Jul 16 2005, 10:36 AM
This topic might be better served in the High Tech discussion area.

First off, you need not measure the squish on the intake and exhaust sides of the cylinder. Due to the natural rocking of the piston on its bearing(s), squish measurements are not accurate perpendicular to the wrist pin. Always measure squish parallel AND inline with the wrist pin for accurate measurements. Intake and exhaust side measurements CAN vary and still be normal, nearly as much as the piston to cylinder clearance measurement, hence the "rocking" principal on the wrist pin.

Wrist pin parallel measurements can and do vary, typically just a few thousandths. Greater variations than that are TYPICALLY due to improper measurement as it is very likely to get multiple varying numbers each time you measure. For example, typical mistakes people make when measuring squish include material being compressed is not thick enough to get a proper impression, the material is too thick which provides some spring back memory, or not cutting the ends of the measuring material to a precise perpendicular cut (which ensures that the measuring material is allowed to travel entirely to the outer squish band area).

If a high degree of side-to-side squish variation is not due to human error, mechanical aspects of the engine should be checked. Items such as worn wrist pin bearings, worn rod bearings, damaged crankshaft bearings, bent rod, "tilted squish" machining, etc., should be investigated. Again though, this is not as common as one would think. Unless the engine has experience some large mechanical failure in the past, it is very unlikely that these conditions would exist.

Although you didn't specifically ask for minimum squish clearance, this is also an important factor. As Riley stated, minimum squish clearance does depend greatly on type of motor, displacement, targeted max RPM range, and intended purpose.

A rule of thumb though is that the minimum squish measurement should never be lower than .040 - .045 of an inch. It is very important to note that this stated minimum is more of a physical limitation rather than a performance or thermal limit. Engine application may dictate that .060 of an inch may be your absolute limit before detonation issues arise or flat RPM operation, but it is also important to understand that beyond runability, there does exist some physical properties that also limit this number. Squish readings below ~.040 of an inch can lead to actual "piston bounce", where the piston is actually striking the head. Under high RPMS and heat, pistons and connecting rods "stretch", possibly allowing the piston to come in contact with the head, which obviously could be a very catastrophic situation for the engine.

For mild racing applications on small bore triples, squish measurements are typically in the .060 to .070 range for safe operation. Race engines, like my 1100, have slightly lower numbers to increase maximum squish velocity (MSV) and to improve gas scavenging.

As far as a minimum spark plug tip to piston crown measurement, I have never heard of one. An obvious limitation would be at the point where the piston strikes the plug, but again, this is surely obvious. I personally know individuals that have miscalculated their plug reach and have had issues with the piston "tapping" the electrode closed, but even this did not produce any major ill effects to the engine, other than poor operation. While maintaining the appropriate distance should be considered to prevent the two from colliding (remember that rods stretch under high RPM and heat - hard collision could severely damage the engine), the importance here is simply to have a rapid, but controlled burn in the combustion chamber.

As long as the plug reach is enough not to obscure the tip, the actual position is not nearly as important as some believe. Even a plug manufacturer will admit that plug indexing, at best, gives an increase of less than 1% of maximum engine output HP, on your best day with a dyno (that's about 1 HP for most of us). That's provided you even know what direction to point the gap since facing it towards the intake is not ALWAYS the best location.

I realize that plug reach and plug indexing are two different subjects, but I wanted to demonstrate positioning and make a comparison...
424735

wow==> :blush: