back in the 80s they used a method called the squish method on 2 strokes when they were raced. Solder was placed down the spark plug hole, piston up to TDC, remove solder and measure the squashed solder to give clearance piston to cylinder head. Too small, no racing.
Yep solder wire works really well. I have used this on numerous occasions. It is a good way to confirm what you have before pulling an engine apart. It takes a bit of skill to feed the solder down the plug hole and manoeuvre the solder wire into the right location.
It takes a bit of skill to feed the solder down the plug hole and maneuver the solder wire into the right location It would be almost impossible on the ZXR heads with their deep recess for the spark plugs. I'm going to pull the head off anyway (to deck the barrels), so I might as well do the plasticine method. It'll be a good experience for me. I did the solder squish test on the 2-stroke motor in the Nostalgia drag bike...it was 6mm, so that explains why it was running poorly.
It is not as difficult as you might think. It works fine on GSXR engines and the plugs are in fairly deep on those.
@ruckusman the bigger issue was the (nostalgia) bike's instability. It was shaking from front to back (chain was smashing against the frame and chain guard) and the head was shaking just to complete the picture. The project is parked until further notice as there are zero chances to race a nostalgia bike at the moment.
@Frankster choose some solder wire that is a little bigger than your expected squish clearance. You need to feed it down the plug hole and angle it over towards the main squish area of the combustion chamber. Make sure you are on the compression stroke for that cylinder with the piston just a bit before TDC. maybe 10mm down. This helps direct the solder into the right area. Carefully wind the engine past TDC. You will feel a little resistance as the solder gets compressed. Pull the solder out carefully and measure the thickness. That should be your current squish dimension. Subtracting your compressed head gasket away from this tells you how far below the deck you pistons sit. On older engines, you may get a little piston rock which can complicate things. 2 bits of solder either side of the piston might give a better reading. Some people just like to run zero deck height, leaving only the gasket thickness for clearance. Sometimes this works ok, but depends on your gasket. Single thickness metal shim gaskets will not provide enough clearance.
I use soft solder (i.e. high lead content) to check squish (2t and 4t) and synthetic clay to check valve/piston clearance. Hadn't considered plasticine for piston/valve but I might change to that as it's less likely to deform.
Lift the head again and cut the plasticine to see what your valve to piston clearance is. It's 3.98mm. Does that sound right?
@Frankster did you wind the engine through a full cycle? If yes, then that value should be the your valve to piston clearance. Looking at the images you have posted, you need to measure the minimum clearance value. This is likely to be at the extremities of the valve area. Using the same method, maybe with a bit more plasticine, you should also be able to get the minimum piston to head clearance (squish) also. My recommendations for intake clearance > 1.2mm & exhaust > 1.0mm. Others might say you can run less, but you will need really good valve springs and avoid over revving.
That sounds about right for dead stock. Take a couple of cuts at different angles - you've got enough coverage for a good explore. JMW has his figures wrong, Those figures are for quite large valves - and you should run larger clearance on the exhaust side as that's the one most likely to be tagged by the piston. With very light and small valves as here, I've already said you can come as close as .030in without risk. There's a major weight difference between bucket and shim - and the lever type cam followers used on his GSXR engine. It's apples vs oranges.
Read this, it'll enlighten you http://www.mssholdings.co.uk/documents/1812_1368456939_Performance-Bikes-Article.pdf
Every time I hear a "new" 250/4 buzzing around town @ redline I think of this article , it's easy to get sucked in by the manufacture's claims of the era regarding redlines, inaccurate tach readings & peak HP, but how many of you have pulled a head & found valve sized "shiney" circles of carbon free valve cutouts on the tops of all 4 pistons ?
That CBR250 article is interesting. I wonder if spinning an engine that fast without a head on it would make the rods stretch more since there's nothing (resistance) for the piston to push against. Even with the readings they got at 18,000rpm (23.5 thou) it's only stretching half a millimeter. So, as long as I allow for a small amount of rod stretch, then that should be okay. @gregt thanks for your post. I'll make a few cuts and report back with the figures. I did the squish test on all the cylinders, so I'll sample each one and note the results.