Topic: Squish-how tight?

[bwaller]

CB550 uses a 15 degree squish band in the combustion chamber. I've been running 0.029-0.032" clearance. I'm now setting up with new pistons and am trying to determine a minimum clearance. This engine includes Carrillo's plus metal head & base gaskets.

How tight is too tight?

[Old Scrambler]

Our need for a squish band resulted from using 1mm longer rods. We decided to taper the squish band because of the high-dome on our 12.5 to 1 J&E pistons that were meant to be run in a K-head. Dave Murre milled my F2 762 motor at 0.000 at the outer edge but tapered the band at 35-degrees to promote swirl and avoid detonation. The milled area in the chamber is 0.045 larger than original and about 0.010 larger than the piston diameter.The edge of the pistons were measured at 0.008 short of being flush with the top of the copper head gasket. After considering some compressed amount for the torqued head gasket, and piston growth from heat, we estimated the edge of the piston would be flush with the top of the gasket and the distance to the head would be 0.005 opening to 0.020 or better. The outer edges of the piston domes are estimated to be within 0.035 of the chamber but the large valve-reliefs dispel any concern about detonation. 

That was the plan before Dave did the milling. How the actual measurements ended are thought to be close. What we do no for sure is we have NO detonation and no piston contact with the head or valves.   

[simon#42]

it needs to be tight to work correctly , on two stroke engines i run usually run 0.2mm or about 0.008inch
on the 125s i go a little tighter but you need to be very strict on your crank replacement milage , when they come apart i can usually see a slight witness mark on the piston .
the clearance does increase when the engine is up to temperature .

[turboguzzi]

i run 0.035 and that is regarded already tight for four storkes at least. some race setup manuals, say of air-oil gsxr quoted 0.040, even more if it's an endurance motor.

i can say it is working as in the squish areas i dont see any black deposits due to the gas speed wiping them off... fits john robinson's book says it's a good sign...

maybe the big difference between 2T and 4T is because on every other turn there is no pressure on the piston/rod in 4Ts ?



 

[bwaller]

Thanks Dennis, I was hoping you'd see this. So if I read correctly, untorqued & unheated, your minimum clearance is 0.008" (piston in the hole) plus 0.005" OR 0.013" to 0.020" total? This is really tight so maybe I'm not following?

Under torque, with one ring on the piston, I use six pieces of solder around the piston crown to check clearances. Mr. Rieck & I have discussed this before and Mike has set up more modern 600's tighter than I went previously, so I plan to reduce my clearances . I'm not going to bother our friend Mike at this time so was hoping for some here to share what they've done.

EDIT, just saw your post Simon. These new pistons are also forged but will not grow as greatly as the previous ones. That said it was evident the squish works by seeing the clean sections of piston crown.

[bwaller]

Thanks TG, 4strokes are different beasts and yes I agree the swept areas of the crown indicate how well that squish band works. As you know the 550 squish area is not symmetrical so there are areas tighter than others. Maybe I should just continue to use 0.029" as a minimum number.

[FunJimmy]

Always an interesting topic. Keep in mind that aluminum cylinders expand at a greater relate than steel connecting rods so squish clearance will grow slightly as engine comes up to temp.

[bwaller]

Right Jim, but there is a steel sleeve to consider, and forged pistons!

[calj737]

Brent - for what it's worth, my machinist used 0.08 on my 500 with forged Wisecos. 0.60 is his recommended absolute minimum.

[FunJimmy]

Right Jim, but there is a steel sleeve to consider, and forged pistons!

The steel sleeve will not counter the growth of the aluminum. It might reduce it to some degree but not prevent it entirely.
This is a significant consideration when building HP Buell (HD Sportster) engines.
The growth factor will be significantly less with the shorter 550 cylinders compared to Buell cylinders but it is still a contributing factor.
Forged piston growth is greater across the bore than in the vertical plane and that is typically only a couple thou at most.
You might want to consult the piston manufacturer.

[FunJimmy]

Here's an interesting article.

http://www.hammerperf.com/ttcheckingsquish.shtml

[flatlander]

i have about 0.040 in squish in my 550 with forged wiseco pistons. this after consultation with mike who did the headwork.
if you're now around 0.030 it sounds pretty tight already.

[simon#42]

i run 0.035 and that is regarded already tight for four storkes at least. some race setup manuals, say of air-oil gsxr quoted 0.040, even more if it's an endurance motor.

i can say it is working as in the squish areas i dont see any black deposits due to the gas speed wiping them off... fits john robinson's book says it's a good sign...

maybe the big difference between 2T and 4T is because on every other turn there is no pressure on the piston/rod in 4Ts ?



 

could be yossef , also a two stroke piston is much lighter and that could have an effect .

brent even with an iron sleeve the cylinder will grow more than the piston ,  as long as the rods dont stretch you wont have any problems .

[simon#42]

oh and i dont use solder it is to tough to squash and gives a false reading . i use clay

[Captain]

There are many factors to be considered when addressing the issue, combustion chamber shape, bore size, piston to bore clearance, piston type, piston alloy, piston speed, Max engine RPM, rod material, weight of piston, head gasket fitment, gasket type and so on..........

 Factoring all of the above and assuming everything is conventional then with the 750 and up SOHC & DOHC Honda 4's  the figure that gives the best results is .038" thru  .041"  Over .041" and the benefits start to decrease rapidly. Below .038" also can have a detrimental effect if the squish "zone" created is not parallel. If for any reason it is tapered and smaller towards the centre than the cylinder wall (piston rock "can" create this) the effectiveness is reduced.

 Captain
 
 
 

[Old Scrambler]

When we built the TRIUMPH CUB motor were had to radius the factory squish-band to fit the significant overbore. We then made custom copper head-gaskets to get the clearance down to 0.030. That Hepolite piston is in a Yamaha steel sleeve fit to a cast-iron barrel. To our eyes, the factory squish-band was very slightly tapered. Its still running and hard to kick-start for such a small motor.......243cc from a stock 199cc.

On the HONDA motor with the F2 head............I think the valve areas give the squish-band an egg-shape and significantly reduce the chances of detonation. Our 35-degree sloped squish band is only 'tight' at the very outer edge of the piston.  If we had to do it over, we might reduce the slope to about 20-degrees. Following the dyno-runs we looked inside the motor through the exhaust ports and the plug-holes and saw very clean pistons. Hopefully we will have a better look and some pics if we get to impound for a tear-down to certify a record

[simon#42]

There are many factors to be considered when addressing the issue, combustion chamber shape, bore size, piston to bore clearance, piston type, piston alloy, piston speed, Max engine RPM, rod material, weight of piston, head gasket fitment, gasket type and so on..........

 Factoring all of the above and assuming everything is conventional then with the 750 and up SOHC & DOHC Honda 4's  the figure that gives the best results is .038" thru  .041"  Over .041" and the benefits start to decrease rapidly. Below .038" also can have a detrimental effect if the squish "zone" created is not parallel. If for any reason it is tapered and smaller towards the centre than the cylinder wall (piston rock "can" create this) the effectiveness is reduced.

 Captain
 
 
 

i would never make a squish band parallel it should always taper by about 2 degrees [ smaller towards the cylinder wall ]  to help push the gas towards the centre of the cylinder
as you say though anything bigger than 0.04 is not actually a squish band as fuel will burn there . dont think that just because that part of the piston is clean its ok

[Old Scrambler]

The 1st pic shows the original TRIUMPH squish-band after larger valves were installed. This was our first build. We later built the current motor with a bore to the center of the oil-return holes. The squish area was enlarged and slightly angled to provide room for the larger piston taken from an early T110 motor.

The second pic shows the HONDA F2 head after it was milled.  As I commented earlier, we severely pitched the squish to avoid detonation.  Maybe we went too far and could have saved a slight amount of compression.

[bwaller]

The last race of the 2012 season I increased the rev limit (to 12K) to compensate short gearing. The pistons just kissed the head, although I didn't suspect anything, and there was no damage. They had never showed contact previously. This was with +-0.032" clearance and stock rods. Mike agreed that the cause had to be rod stretch. I reduced the clearance slightly and with new Carrillo's at the same rpm's there is no issue. I think it would be safe to be a couple thou tighter, but obviously better safe than sorry.

I have worked to be sure the squish angle is correct and tight. It shows on the dome, plus the turbulence created helps and allows for less ignition advance.

Simon, each to his own. I find clay tougher to measure precisely, I like thin solder as it squishes nicely and is positive to measure.

Thanks all, guess I knew the answer all along!

[simon#42]

its an interesting subject  and it is always good to hear other opinions

[PeWe]

Interesting thread
OK, I'll cover the piston with more clay next time and measure the distance between piston and combustion chamber.
Head and cyl block need to be milled to ensure flatness. A little bit more compression will not harm either.

Old photos of piston when I checked it before restore.

[turboguzzi]

I'm with simon.... I use plasticine

[bwaller]

I've used plasticine but find it changes shape slightly when removing. Soft solder is the most positive to measure but likely does rock the piston more. There is a neat looking wax product made for the job I've seen. (pieces in different thicknesses)

[turboguzzi]

brent, spray the piston and head with some WD40 so plasticine doesnt stick....

[bear]

I use modlers clay, good stuff.