Crank movement

[TopCatGr58]

Can anyone play Reveille for my '78 motor? I tore it open today and split the cases, then took the crank out to get checked. A crank bearing on the right side had spun, and it effectively RUINED my crank AND cases.

Hey Tod,

I think you mean "Taps", the funeral song! Revelle is to wake up!

As for your #1 cylinder, can you get it measured/micrometered to see if it's more worn or out of round vs. the other cylinders? If it's too worn, not sure if the Xj rings will do much good? A big bore might be in your future? Good Luck!
T.C.

[trbig]

LOL.. yup TC you have it correct... maybe I was hoping you could play it and wake the dead!

I just got home from John's house, and the way my luck has been lately...I was really surprised. This XJ motor appears to be in very good shape... very low miles. I am going to use these pistons.. but they aren't XJ pistons, and they are higher domed than the XS ones I had. Someone has been in to this motor... the cylinder cooling fins had already taken a beating before me... and there was "50" marked on all the pistons, but they measure up for a standard piston... part number on top is 3H5.
We split the cases and looked around... did the 2nd gear fix... and 1st... and 4th... lol. Then we installed the extra steel plate in the clutch basket and installed new springs.
Since I have it down this far, I am going to clean it up a bit before I stick it all back in the bike, but I hope to have it running again within a few days. Of course my timing is impeccable... it's getting nice and cold now after a couple days in the mid 80's this week. I'll let you know more when I do...

Tod

[trbig]

Well... never mind. As usual, I was wrong again. The part number I listed for the piston seems to be the right one for the XJ, as well as the '80 and '81 models.

I thought the pistons from the earlier models made higher compression... I also thought if that two pistons were on the same connecting rod, the one with the higher dome would create more compression?







The piston on the right is the 2H7 out of the '78 motor, the one on the right is the 3H5 out of this XJ motor. You can see the difference. I put a rod pin through both so they would be equal for you to see. The XJ piston measures @ 2.37 inches from the bottom of the skirt to the top of the dome, while the XS piston measures @2.31 inches. Did the tops of the XS pistons get bent down slightly over time? Can someone enlighten me?

Tod

[mro]

3H5 piston is for 80 and newer, other for 78/9

Even tho 3H5 looks "taller" compression is 9 to 1, little less than 78/9 pistons. Crank/rods on XS'es are same, not sure about XJ.
___________

Will be starting on my 80LG engine soon. Putting 79 cylinders and pistons with 80 head, maybe the 79 cams too.



mro

[trbig]

According to the Yammer-hammer parts list, they are all the same rods. That's why I'm not understanding the lower compression?? Not that it really matters, they are the ones I'm using... it's just a curiosity thing.. (I know... it killed the KAT!!) lol

Tod

[Douglas Money]

Piston height is determined from the center line of the piston pin hole to the top of the piston dome. Amount of dome also changes compression. Good Luck

[trbig]

I understand where the height is measured from, but I didn't have a sure way to measure precisely. Since the pistons are identical below that pin, it gave me an equal on both to measure from.

I also knew (Or thought I did) that the amount of dome will change compression. That's why I was wondering what would make the earlier motors have a higher compression. Has anyone ever measured the actual cylinder length or head thickness? I guess with the cylinder, what really matters is the amount of air space left at the top of the compression stroke. All things being equal, that would the only way to affect (Reduce) the compression really isn't it?

Which leaves me to question and wonder if the higher domed pistons paired with my '78 jugs (Possibly shorter cylinder/ tighter head space?) with the XJ's larger valves will leave room for the valves. It is shimmed to the closer XJ intake clearances(.11~.15mm) versus the .16~.20mm but has the '78 cams?? But.... these pistons were made for the motors with the bigger valves... just a different cylinder part number.

Any suggestions??

Tod

[TopCatGr58]

Hey Tod,

The cranks are the same, the strokes are the same. The increased compression of the earlier models is probably due more to the cams than the pistons. The later model pistons, even though apparently more domed probably have more recess allowance for the LARGER sized valves you mentioned that are on the 81 and XJ head which you have on now. Having the closer shimming is probably good, and with the earlier cams, you may actually get more compression!?!?

You never answered my previous question, have you micrometered the cylinders to verify if they are within spec or worn, out of round, etc.??

Also, why did you split the cases on the XJ engine just to fix the gears, or were you plastigauging the bearings? Good luck getting it back together and running!
T.C.

[GNEPIG]

.. have you checked the difference in the depth and shape of the combustion chambers on the two heads[78/79 versus the 80 and newer]?

i dont think that they changed the cylinder height. or the one size fits all, big bore kits, might not work or raise compression.
but i could be wrong.

compression could also change with the cam difference, less duration means less air/fuel mixture being sucked into the cylinder before it closes the valves. less mixture means less compression. the newer cams have less duration than the older ones. the newer exhaust cam lobes are .020 taller than the older exhaust cam lobes.

[chevy45412001]

compression

compress. can also be altered with different thickness gasket on the head, or pocket porting the head also...or i don't have a stinkin clue. ahyhoo let us know your problem may be my problem, recently developed a not quite right vibration in motor.also like stated above, the measure for pistons is from the rod c/l to top of piston. HE DID WHAT IN HIS CUP ...had too doo it )

[John]

Chevy

Funny you should say that...I'm watching Cars now. Good little flick.

[trbig]

TC

I don't have a bore gauge, so I can't answer that about the cylinders. All I could check with my gauge would be the very top or the very bottom of the cylinder, and neither get any wear.

We split the cases at John's because he thought it would be easier to do the gear fix, and so we could check all the bearings out. We didn't use any plastigage since everything looks dang near new.

I just threw some thinner shims in the intakes to get it in XS specs... If the newer cams have a .020 TALLER lobe on the cam, then I shouldn't have any problems with keeping the 78 cams at the closer XJ shim distance then correct? It would only take a minute to throw the other shims back in. I'd like a little more performance if I can get it, but since this is the last of my heads... I really do worry about bending more valves.

Tod

[Dan Hodges]

Tall Domes & Short Domes

Originally posted by trbig
TC

I don't have a bore gauge, so I can't answer that about the cylinders. All I could check with my gauge would be the very top or the very bottom of the cylinder, and neither get any wear.

We split the cases at John's because he thought it would be easier to do the gear fix, and so we could check all the bearings out. We didn't use any plastigage since everything looks dang near new.

I just threw some thinner shims in the intakes to get it in XS specs... If the newer cams have a .020 TALLER lobe on the cam, then I shouldn't have any problems with keeping the 78 cams at the closer XJ shim distance then correct? It would only take a minute to throw the other shims back in. I'd like a little more performance if I can get it, but since this is the last of my heads... I really do worry about bending more valves.

Tod

The 78-79 head had smaller/tighter combustion chambers and smaller valves than the 80-82 head,hence the reason for the difference in piston dome volumns.The stock valve reliefs in the 78-79 piston will not provide suffcient clearence for the larger valves in the 80-82 head.

[pggg]

The ex and in valves recesses are different sizes that's all, my bike has an 80 head on a 78 engine with valves 1mm larger than the 78 valves, it runs fine, the domed piston has more cut out than the lower piston.

[trbig]

The stock valve reliefs in the 78-79 piston will not provide suffcient clearence for the larger valves in the 80-82 head.

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