Crank movement

[Dan Hodges]

VP clearence

Originally posted by trbig


That's what I had been running. Even with the tighter valve lash adjustment of the XJ head, I didn't have any problems with the valve clearance from the 78 motor.

Tod

The valve pockets in the 78-79 pistons are designed for the 78-79 head which has 1 mm smaller exhaust valves and 2 mm smaller intakes than the 80-82 heads.What is your valve to piston clearence? The vp clearence should be at least .060 on the intake and .080 on the exhaust but you can obviously get by with less.The early piston also has a smaller dome volumn than the late one and when used with the 80-82 head will result in a drop in compression.Will it run,sure,can you feel the difference in the seat of your pants,probably not.

[trbig]

The early piston also has a smaller dome volumn than the late one and when used with the 80-82 head will result in a drop in compression

But... using the 78 cams with the bigger valves... alllowing more air/fuel flow... Increase the compression negating anything you lost?

Tod

P.S. How do you measure the valve to piston clearance?

[pggg]

I skimmed out the pocket edges slightly on my 78E pistons just to be doubly safe with an 80 head, luckily the tolerances in these old air-cooled jappos aren't as tight as on modern water-cooled, also my current beasty has wiseco pistons and an 80 head, runs well, I've run both 78 and 80 cams with no probs, and like Dan said, all said and done there's no noticible power difference between the larger valves and the smaller ones.

[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

[TopCatGr58]

Tod,

To provide more piston to valve clearance, you would want to set them to the looser tolerances, that way the cam will have to rotate a few more degrees before it contacts the shims/buckets, and will not depress the valve quite as far down due to the slack of the extra clearance.
T.C.

[trbig]

Do you actually think that there is a danger of the valves hitting the pistons if they are shimmed to the XJ specs? From what they are saying.. the XS cams don't open the valves as far, and there is more dome volume with the newer head??

After thinking... which usually gets me in trouble, is that basically I have an 82 motor without the YICS because of the 78 cylinders (Which internally are the same as the XJ). If the 78 cams actually open the valves LESS than the 82 cams would (Just longer duration) then the closer XJ specs would apply... or in the least, wouldn't hurt.

Please tell me if my logic is faulted!!!

Tod

I'm not trying to argue with you TC, and I do appreciate your comments/ suggestions.

[trbig]

Just an update...

Some good news!! I had the bores measured on my 78 cylinders and they are well within spec!!


I also still am wondering about the question in the previous post...

Tod

[MAXIMAN]

Tod

There's a thousand ways to skin a cat. But the the most straight foward way is Play Dough.

Pull the head and put a little Play Dough in valve pockets of the pistons. Put the head back on and torque it. Then roll the engine through completely at least twice.

Then pull the head ...remove the play dough very carefully and mic it.

The minimum clearance (thickness of the dough) I would give it would be about 25 thous. If you have at least that much you should be ok.

[TopCatGr58]

Tod,

Your logic is faulty! What Dan had said about the piston indents is in the DIAMETER of the head of the valves, not how deep they are protruding. IIRC, the early cams actually have MORE lift....higher lobes, so they will depress the valves farther!

The later cams are lower lift since the valves are larger, letting in same volume with less cam lobe lift.

So, you have the 82 head, larger diameter valves coming down on smaller indents in pistons, and the 78 cams pushing the valves open deeper.....more risk for impact, so looser clearance provides a little more insurance!

Dan, correct me if I'm wrong!
T.C.

[Ken Talbot]

Neat idea, Maximan. If you smeared the the playdough wider than just the valve indent on the piston, you would see exactly where the whole faces of the valves come to. This would answer any concerns about larger valves being used with pistons with indents for smaller valves.

[trbig]

Or... I could just go measure the &^%# things! lol.

On the lobes for the intake, on the 78 cams that I will be using, I got a 1.459" and the 82 intake cam lobe was 1.452. The difference of .007 inches or .178mm.... MORE than enough to warrant shimming to the wider XS specs.
I am still curious to see how much room there actually is in there... so may wipe things down with some light oil and play with a litttle playdough as a science experiment! Thanks Cody.

Tod

[Dan Hodges]

VP clearence again

Originally posted by TopCatGr58
Tod,

Your logic is faulty! What Dan had said about the piston indents is in the DIAMETER of the head of the valves, not how deep they are protruding. IIRC, the early cams actually have MORE lift....higher lobes, so they will depress the valves farther!

The later cams are lower lift since the valves are larger, letting in same volume with less cam lobe lift.

So, you have the 82 head, larger diameter valves coming down on smaller indents in pistons, and the 78 cams pushing the valves open deeper.....more risk for impact, so looser clearance provides a little more insurance!

Dan, correct me if I'm wrong!
T.C.

Class is in session.The depth of the valve reliefs are the same for both early and late pistons but the late ones have a larger area and or diameter to accomodate the larger diameter of the valves in the late head.The valve lift of the early cams measure .348 on the intake and .327 on the exhaust and the late cams have .348 lift on both intake and exhaust.The early cams have 12 degrees more duration on both intake and exhaust than the late ones and have event angles of 101 degrees vs 105 for the late ones.Duration will effect VP clearence more than the lift as the valves remain open longer (later) in relation to the movement of the piston at TDC.Event angles and retarding or advancing the cam will also have an effect on the intake or exhaust depending on whether the cam is advanced or retarded.For every .003 of valve lash you loose or gain 1 degree of duration depending on whether you go tighter or looser.The early cams get .006-.008 of lash on the intakes and .008-.10 on the exhaust and the late gets .002 tighter on both.The clay method can be used to check VP clearence and is used by lots of engine builders but it is not the most accurate.According to both Web cams and Mega Cycle you need at least .060 VP clearence on both the intake and exhaust valves.You can increase the valve lash by .003-.005 and loose one or two degrees of duration but the VP clearence change with this method will be minimal.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 and each point of compression is worth about 4% of hp up to 12 to 1 after which the increase deminishes as the static compression is increased.The duration and event angle will dramatically effect the pumping compression of the motor but the static compression is a result of the cylinder,combustion chamber,head gasket and piston dome volumn with the deck height of the piston figured in.I highly recommend that all those interested in this subject get a Competition Cams catalog as it is filled with all this stuff we are talking about along with a Yamaha shop manual. 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

[trbig]

Ummmmm...



LOL.. That's a lot of information from Dan .... but am I missing the answer?? I HAD the 78 pistons in an 82 head... but not now (Making me a pound foolish.. lol). Is he saying that what I set the shims at now really doesn't make much differencerence?? He says the playdough isn't the best way... then what is?

All I want to know is..... What do I set these shims at?!? lol..

Tod

[Pat Kelly]

According to Tod's question and Dan's dissertation ( ), go with the looser early specs:
Intake .006-.008
Exhaust .008-.010
(inches?)
This is to be safe and have (slightly) less lift yet still keep things from rattling around.

This is just my interpitation of what I'm reading

[trbig]

You can increase the valve lash by .003-.005 and loose one or two degrees of duration but the VP clearence change with this method will be minimal.

That is saying to me that it won't matter much WHAT I set it at?? As far as the numbers you gave Pat... those don't apply to either of the shimming specs... so???

On one hand, I'd like to just play it safe... on the other hand, I'd like to get the performance out of the engine that it's capable of.


Tod