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Discussion Starter #1
I am installing 748R heads on my 851SP3 and build a spreadsheet to calculate the piston to valve clearance.
Am planning on advancing the cams and wanted to raise the compression by skimming the top of the pistons.

But now that it's done and I am about to install the stuff on my bike I am a little concerned about the validity of my calulations.
Don't get me wrong, I still believe in my calculations, but just in case there's something wrong with my spreadsheet: how do I check the piston to valve clearance with everything mounted on my bike and with the correct timing of the cams without bending a valve or so.

I just want to check if the piston won't hit the valve, don't want to know how much play there's left (so no play doh :)).

Can anybody help me out here?

TIA,
Jacco
 

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Without knowing how much play there it, it is impossible to determine if you will have an interference problem when the motor spins up unless you have followed a precise recipe that has been proven and there is absolutely nothing different from your set up. I'm not saying that math isn't your friend, it is just that it is so easy to make a mistake. That's why most everybody clays the valve to piston clearance with a trial fit. With the Desmo system, not sure exactly how you would degree the cam to the closest point and depress a dial indicated valve to see how much clearance you have post assembly. Maybe remove the closing rocker?? Then there is the issue of squish.

I assume you meant to say you decked the pistons or relieved the valve pockets to allow use of smaller combustion chamber heads because removing material from the piston, by itself, would lower compression, not raise it as your description claims.

The only other alternative would be to rotate the engine sans spark plugs to verify that it has no interference, spin it up and see if it makes noise when it's hot.
 

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Discussion Starter #3
Jahjah,

Thanks for your comment.

I removed 1.3 mm of the top of the piston deck to reduce the depth of the valve pockets. They were up to 8.35 mm deep and caused a significant CR loss.
And then I skimmed the bottom end of the clinder to compensate for this (1.2 mm to get the required squish of 0.9 mm). I measured the volume of the head and piston and added with the squish I will now get 12,2 to 1 (otherwise approx 10:1)

I am not worried about the the remaining depth of 6.05 mm (=8.35 - 1.3) of the valve pocket, I had to enlarge (wider) the pockets because of the bigger valves. And the enlargement is not as deep as the original pocket is. And that's my concern, is the altered pocket deep enough. I calculated that the inlet valve will sink 2.4 mm through the piston deck at 10 degrees after TDC, the wider pocket has a depth of at least 3.7 mm.

Calculating is indeed very theoretical, that's why I want to be sure in real life there's no piston valve contact.
 

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Just measure how much clearance you have. Easy. Take belts of. Put crank to 20; 15, 10 , 5 degrees BTDC. Manually rotate the cams and measure how much the valves can open before they fit the valves. Install the belts snd measure how much the valves actually opens at thouse cam degrees. The difference between the readings are your clearance. Simple.

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With the pistons installed, working and at TDC for the side you are testing place some clay in valve pockets of the piston. You may also use this soft plastic that is sold by engine builder and which is made to a tolerance - I forget what it's called. When the cams or pistons are rotated through a cycle, the plastic squishes to a certain flat dimension and this measurement is taken. It is not the remaining thickness of the plastic that is measured, it is its outside overall dimension that is taken. Then you compare that to a conversion table supplied with this stuff and that gives you the clearance measurement with excellent accuracy. You can rotate either the piston or the valve train to get this measurement and, since it is done by hand, if you have a true clearance issue, you won't break anything doing it this way. You can do the clay thing to start but the plastic is more accurate

I think the stuff is called "plastiguage" BTW Genuine Plastigauge - How It Works

Rob
 

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With the pistons installed, working and at TDC for the side you are testing place some clay in valve pockets of the piston. You may also use this soft plastic that is sold by engine builder and which is made to a tolerance - I forget what it's called. When the cams or pistons are rotated through a cycle, the plastic squishes to a certain flat dimension and this measurement is taken. It is not the remaining thickness of the plastic that is measured, it is its outside overall dimension that is taken. Then you compare that to a conversion table supplied with this stuff and that gives you the clearance measurement with excellent accuracy. You can rotate either the piston or the valve train to get this measurement and, since it is done by hand, if you have a true clearance issue, you won't break anything doing it this way. You can do the clay thing to start but the plastic is more accurate

I think the stuff is called "plastiguage" BTW Genuine Plastigauge - How It Works

Rob
I believe the point is that he has already assembled the engine. Good suggestion otherwise.
 

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Ha ha - I missed that (clearly). Measurement as astutely provided above is therefore the only remaining option....although you could take the heads off again which if it were my bike, that's what I would do. The gaskets are still good and even if they would not be, cheaper than new valves and pistons.
Rob
 

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Discussion Starter #8
Thanks guys for your input. Since the heads are allready installed I am gonna try vij's way!
Hope my calculations are ok :smile2:
 
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