squish or the lack of it

[johnny650]

I have a huge squish measurement with this 185 cylinder kit and I am hoping one of you engine builder Guru's can suggest a solution or give me a range of options .
The edge of the new 185 piston (Indian) is approx 2mm below the face of the cylinder . if I were to compress the cylinder without the head I guess it might reduce to about 1.5mm which would be an ideal squish .However ........


The cylinder head has a machined squish cut into it of approx 1.5mm giving a total squish of 3mm ....but we are not finished ..

If I add the 1mm head gasket it adds up to 4mm and if I use a base gasket that adds up to 5mm !

I guess I could leave out the base gasket and save 1mm.
Could I risk leaving out the head gasket also ?.......that would save a further 1mm
So now I'm back to 3mm squish which sounds huge. I guess it would run but be significantly down on power ?

Last of all I guess I could get the cylinder machined but that would cost more than the cost of a new cylinder kit .

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[bazza3004]

if your using a 58/107 crank you could swap to a 110 rod.

[garry inglis]

We're is the piston at bdc the top edge of the piston should be level with the transfer ports first so you may need a base gasket to set this first then see what squish you end up with

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[johnny650]

if your using a 58/107 crank you could swap to a 110 rod.

thats a good suggestion ...I hadn't thought of that . Unfortunately I only have a TV 3 crank

[johnny650]

We're is the piston at bdc the top edge of the piston should be level with the transfer ports first so you may need a base gasket to set this first then see what squish you end up with

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thanks for that info garry I hadn't even thought of that to be honest I just assumed it would be ok because the 185cc port map I posted last night was identical to the 200cc cylinder kit that I removed and also a 175cc kit I have.

I reckon the discrepancy might be due to the piston rather than anything else as the cylinder measures the standard 128mm height and I'm using the same crank and rod used on the original 150cc and the 200cc kit I just took of . Both of those the piston head was level with the top of the cylinder.

There is one other possibility thats occured to me. the cylinder was difficult to fully seat as if there was something snagging somewhere . I'll find a way to torque the cylinder without the head tomorrow to make sure it is fully seated in the case mouth

[garry inglis]

You don't force the barrel down the spigot or bottom of barrel is only thin it's either the stud holes in the barrel want a drill through them or it's catching the casing as its seated when you say tv 3 crank is this original

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[Minority]

Check for studs (e.g. exhaust studs) protruding into the barrel stud holes. This can make the barrel seem very tight to fit.

[HxPaul]

Before you go any further if I were you,I'd measure the (Indian)piston that your using and compare it to the measurement of the piston you've taken off.

[johnny650]

thanks for the suggestions.
Cylinder studs are all new so no friction or binding there. naturally when i removed the 200cc cylinder yesterday I cleaned everything thoroughly and renewed all gaskets for the 185 cylinder .

I'm not advocating forcing the cylinder with a hammer lol... just using the fixing nuts and spacers to torque the cylinder down to standard torque settings without having the head fitted. That way I can eyeball and measure the squish

i'll check the dimension of the pin to piston top, clearly there can only be two reasons for this height mismatch.

Either the piston is short (ie pin to top edge), or the cylinder is not seating fully on the base gasket . There can be no other explanation.

If you look at my port maps it shows the porting of the 185 is practically identical to 175 and 200 cc cylinders
i'll report on my findings later . thanks guys

[johnny650]

OK an update.
careful inspection in the daylight revealed that the cylinder spigot is slightly oversize at the base and the crankcase mouth had some debris and burrs so I deburred the case mouth at the edge and cleaned everything spotless.

I measured the piston centre of pin to top edge .....39mm exactly the same as the 200cc , 175cc & 150cc pistons I have so thats eliminated the piston being at fault

i then reassembled the cylinder piston and head and torqued it.
Measuring the squish its now down to 0.8mm with no gaskets Yippee thats a lot better.

So what to do? .... I daren't refit the cylinder with no gasket and rely on liquid gasket alone as the amount of area of gasket surface at the base of the cylinder is incredibly small so I'll fit the thinest base gasket I have at 0.45mm and use liquid gasket .
I really don't want to have to completely strip this scoot again for a leaking base gasket !....

Squish now 1.25mm which is probably perfect for reliable performance .

Now...do I fit a head gasket ? Mmmmmm .........thinnest I have is a 0.6mm gasket which would bring the squish up to ...scratches head ...counts fingers ....errr 1.85mm !

Well thats probably perfectly acceptable but I'm greedy. If I'm going to all this trouble and expense I might as well aim for the smallest practicable squish I can achieve so I'll use liquid gasket cement Threebond 1104 in stead of a head gasket.

So anticipated squish will be approx 1.25mm but we'll see in around about half hours time .
185mm squish 08mm no gaskets by John, on Flickr