Crank movement

[MAXIMAN]

Re: VP clearence again

Originally posted by Dan Hodges
Class is in session.

The domes on the early piston measures approximately 5.5 cc's and the domes on the late ones measure approximately 8.5 cc's and in this application every cc is worth a quarter point of static compression and so the use of a stock 78 piston (uncut) in a stock 80-82 head will result in only about 8 to 1 static compression ....

There are scads of 80-82 XS stock pistons available and because of this it is penny wise and pound foolish to stick 78 pistons in an 82 head but of course you can but it is simply not kosher.Good luck in your endeavor.Dan

Sensei

The real question that isw bugging me is the resultant static compression from coupling late pistons with n early head...

However that may be a recipe for "bent valve soup".

TOD

Like I said there are many ways to skin a cat. The clay method ...like Dan said ... isn't the most accurate but it is the most straightforward. If for instance you use playdough and determine the clearance is 60 or 70 then I would not worry about it. If the clay indicates it's 25 or 30 then you'd better use a different method (ie solder, direct measurement, etc) to insur accuracy. It would be a shame to bend a valve after all that work.

[Dan Hodges]

Only A Geologist

[QUOTE]Originally posted by MAXIMAN


Sensei

The real question that isw bugging me is the resultant static compression from coupling late pistons with n early head...

However that may be a recipe for "bent valve soup".

TOD

Like I said there are many ways to skin a cat. The clay method ...like Dan said ... isn't the most accurate but it is the most straightforward. If for instance you use playdough and determine the clearance is 60 or 70 then I would not worry about it. If the clay indicates it's 25 or 30 then you'd better use a different method (ie solder, direct measurement, etc) to insur accuracy. It would be a shame to bend a valve after all that work. [/QUOTE I was wondering when someone would see the inherent possibilities of using the late pistons in the early head and it was the resident geologist who saw it.You can do the math and look at the pistons side by side in the photograph of the early and late pistons that were posted and see that this combination would result in a substantial increase of static compression.There would be no problem with VP clearence with this combo as the valve reliefs in the late pistons are larger than the early ones however the domes on the late pistons are larger than the domes on the early pistons and the chambers in the early heads are slightly smaller than the late ones and so you would need to measure the piston to head clearence with this combination and make sure you have at least .060 clearence.The clay method is a good method for measureing piston to head clearence.Cut it (clay) in thin strips and coat it with oil so it doesnt stick to the head and lay the strips on the piston top away from the valves,bolt on the head with a junk gasket,torque it down to 15-20 ft. pounds of torque,it doesn't have to be much for this and simply rotate the motor over very slowly by hand a couple of times and then remove the head and measure the strips of clay you placed on the piston tops and if you have .060 or more you are good to go.You can get by with .040 piston to head clearence but it's close.You always want to be on the safe side and leave some room for a margin of error.Using stock parts,a torquer motor would consist of late pistons with early heads used in conjunction with the late cams. A killer motor designed for a Special to beat up on V-Twin metric bikes would consist of the late heads cut .015-.020 for added compression and the early cams and a bag full of money.All of this might be a moot point as new ring sets for these stock pistons are getting hard to come by as they are no longer available from Yamaha.71.5 mm rings can be had,you just have to think and go outside the box.Cody,are you listening?

[trbig]

Not a geologist, but I was actually wondering the same thing earlier. It doesn't matter really to me since my earlier 78 head is full of bent valves. But some day it won't be.

The only thing that concerns me with trying the playdough, is after my first head mess up, I am leary every time when installing cams and chains. I am always so relieved when I get it all back together... Then I would be turning around and pulling them right back out again.

The problem I see with buying a set of rings.. even the 71.5 then grinding the ends to fit... is for a little more, you can get a whole piston kit! You'd then have some machine work needing done.. but whichever way you go I guess is your decision.

Anybody got a kid that doesn't use his playdough any more.. (That isn't all hard and crispy in the can!) lol..

Tod

[TopCatGr58]

Dan,

Thank you Sir for the fine cam/valve lesson!

Originally posted by trbig
I'm starting assembly today.... So which way do I shim this thing? To the closer XJ specs .11~.15 or the XS with .16~.20??

Just in recap... 82 pistons, cases, and head, with 78 cylinders and cams.

Thanks.

Tod

Tod,

To get back on track, what Dan said was that the earlier cams have 12 degrees longer duration than the later ones, and actually less lift for the exhaust, but same for the intake. You will have the matching heads/pistons with matching piston valve recesses and matching piston/head domes, but the longer duration means the valve will be sticking down longer before it starts to retract as the piston comes up.

So....for the safe side for the intakes, set the valve lash to the looser range, for the exhaust you can set them to the closer specs since there is less lift. JMHO!
T.C.

[trbig]

The exhaust setting is basically the same for both motors. I have the looser spec shims in the intake now, and I am thinking of doing the playdough thing just because I am curious. Can anyone kind of explain the process to me a little more? I din't really understand what Dan was saying about laying the strips on top of the pistons away from the valves... Huh?? Why put it away from the valves if the gap between those two is what you're trying to measure when the pistons come up?? And... how much is a strip? I will leave the spark plugs out just in case I use too much, but have never done this. Also.. is the .060 mm or inches?

Tod

[pggg]

My 78E motor with an 80SG cylinder head ran good, 120mph easy, probably the designers wouldn't let the 80 head slam on straight interchangeable if there were piston/valve clearance problems, I still played it safe and dremelled the pocket edges wider before bolting down the head, I doesn't matter how far the valves open coz the piston's miles away at that point, and at TDC the inlet valves fully shut, but you're still hoping like hell they don't stick down as low as in this pic.

[Dan Hodges]

VP clearence 3

Originally posted by pggg
My 78E motor with an 80SG cylinder head ran good, 120mph easy, probably the designers wouldn't let the 80 head slam on straight interchangeable if there were piston/valve clearance problems, I still played it safe and dremelled the pocket edges wider before bolting down the head, I doesn't matter how far the valves open coz the piston's miles away at that point, and at TDC the inlet valves fully shut, but you're still hoping like hell they don't stick down as low as in this pic.

In a 4 cycle engine the intake valve is closest to the piston at 10 degrees ATDC and the exhaust valve is closest to the piston at 10 degrees BTDC.When checking your valve to piston clearence these are the positions the pistons must be to get an accurate measurement when checking with a light spring and a dial indicator.The clay method is simpler as you are simply measureing the depth of the depression the valves made in the clay when you turned the motor over.

[TopCatGr58]

Have to read carefully!

Originally posted by trbig
The exhaust setting is basically the same for both motors. I have the looser spec shims in the intake now, and I am thinking of doing the playdough thing just because I am curious. Can anyone kind of explain the process to me a little more? I din't really understand what Dan was saying about laying the strips on top of the pistons away from the valves... Huh?? Why put it away from the valves if the gap between those two is what you're trying to measure when the pistons come up?? And... how much is a strip? I will leave the spark plugs out just in case I use too much, but have never done this. Also.. is the .060 mm or inches?

Tod

Dan Said: measure the strips of clay you placed on the piston tops and if you have .060 or more you are good to go.You can get by with .040 piston to head clearence but it's close.

Tod, that's 0.060"= 1.5mm, but the important thing Dan said was that you were going to be measuring Piston to HEAD clearance, not Piston to Valve clearance! SO...you put the clay strips down the middle of the piston between where the valve depressions are, and it's the space between the dome of the piston and the inside dome of the combustion chamber in the head that is going to be measured with the playdough! As long as the Piston to Head clearance is as stated, then you should have more than enough clearance for the valves to pistons. It's when folks go shaving the heads, that they start playing with the closer P2H clearance values, and start flirting with the valves into the pistons syndrome!
T.C.

[pggg]

There's around 18mm or so of clearance between piston crown and chamber, the stock head gasket keeps the piston to chamber EDGE within specs.

[MAXIMAN]

Re: Only A Geologist

Originally posted by Dan Hodges
[BAll of this might be a moot point as new ring sets for these stock pistons are getting hard to come by as they are no longer available from Yamaha.71.5 mm rings can be had,you just have to think and go outside the box.Cody,are you listening? [/B]

I have a hard time thinking outside the box....unlike engine parts rocks don't come in boxes.

Except for the rocks my wife likes. And those rocks ain't cheap.

[trbig]

Man...I really feel like an idiot.. but I am cornfused. Like Pggg said.. there is going to be way more than that to the top of the chamber. Is it like he said.. to the very EDGE of the head just inside the combustion chamber?

Tod

[pggg]

XS has closed chambers, the 'squish' areas are the closest to the piston, they 'squish' the air inwards.

[trbig]

This just doesn't make any sense to me. Why am I worried about what the head to piston clearance is, when my concern is what a domed piston to VALVE clearance is? Even if the head was shaved... bringing the valves even closer to the piston, the piston to head clearance would essentially remain the same due to the head gasket wouldn't it? If your head gasket measures .060... then your head will ALWAYS be that far from the piston. You could essentially slice the head in half... and you still couldn't get it any closer to the top of the piston. A shaved case... yes. But not the head.
I need to get all this figured out so I can get it together. I was wanting to get it done today. The only way I can think of checking it, would be to put the strips of playdough across the piston in the area of the valves...put the cams back in, and rotate it that way.. checking to see how much room there is between the pistons and VALVES.

Tod

[trbig]

OK.. I just got the head back off after getting it all together, timed, and rotated a couple times. According to my "Smoosh" test with the playdough, I have at least .132 inches of clearance (3.3528mm) between the valves (Intake) and the pistons at their closest point. This is shimmed to the wider XS specs. Can this safely be shimmed in another .005 to the quieter XJ specs? Someone who knows.. please tell me?!? I am wanting to get this thing put back together and this is what's holding me up. Thanks alot, and hope to hear from you soon.

Tod

[Goin Commando]

Since the magic numbers are .060", as stated by the people who know, in the earlier posts, it looks like you are good to go.
Happy riding