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Explain this please

4

496 BB

New member
How do I go from .040" in the hole with a dished piston buy a +.020" longer billet rod with flat top hemi piston and end up at ~.050" in the hole?!

Heres where I am now:

T - .042"
B - .0505"
L - .053"
R - .0435"

Also whats up with left and right differences. This is on new never been fired motor.
 
Was the dished piston that much thicker/longer? Did you measure before assembly?
We found our Honda Flat tops to be a bit shorter/not as thick as a clone or Honda dished piston...
Mind you we only seen a .002-.003 difference...
 
No I didnt measure. I figured it was a flat top designed for this engine from the factory that it would be the same. Maybe the Hemi flat tops are shorter. This is getting irritating...lol.
 
Has to be the block but haven't measured the differences between the dished piston and flat top. Put a +.020 rod in my hemi block with flat top and its .005 in the hole.
 
ALL blocks, even the yellows and reds especially the blues are not decked level. what you are seeing is typical. If you want it level all the way around you will need to deck it.
 
But what I dont understand is it was within .003" BEFORE I installed the new flat top and rod. I understand I will need to mill the block now but I dont understand why it is rocked and why I am in the hole even more now.

Is the piston rock form the rings not seated yet?
 
I suspect a piston with a different compression height, poorly machined compounded with an out of square deck. No the rings would not play a role in this. You have your side cover seated and torqued properly, the side cover mates with the dowel pins tightly for a positive register.
 
No. I am not sure how to do so since its from center to center. The new one is .020 over per NR. When I say piston rock I am talking about side to side measurement. Its is a .010 difference. I did not have that before. Is that from the rings pushing piston to one side? Since they have not fired up yet? I didnt try to move the piston to see.
 
OK just messed with it again as its bothering me. I "centered" the piston in the bore. Checked clearances to bore down to first ring with feeler gauge and am at .003-.004" sidewall. Thats good I assume. Now with it being centered I checked again in the hole amount and they are both side to side within .001" being .043". I feel better with that but is it supposed to move like that? I never noticed it on my other motor. Maybe it is. Either way its still in the hole the same amount as before so I would assume the flat top hemi piston has a smaller compression height. Has anyone else seen this or noticed this? I didnt take it out to measure but that has to be it unless I was given the wrong rod and I dont know how to measure those.
 
First off, you need to measure everything, trust no-one, then piston C/L should be done with narrow feeler guage before rings are installed.
 
The other thing you can do is put the dish piston on the .020 rod and check.hard to say with out looking at it.
or measuring it myself.if it brings it up then you know.then you have to either get the right piston or just
tossing stuff out weld up the dish and then turn it.
 
496 BB said:
OK just messed with it again as its bothering me. I "centered" the piston in the bore. Checked clearances to bore down to first ring with feeler gauge and am at .003-.004" sidewall. Thats good I assume. Now with it being centered I checked again in the hole amount and they are both side to side within .001" being .043". I feel better with that but is it supposed to move like that? I never noticed it on my other motor. Maybe it is. Either way its still in the hole the same amount as before so I would assume the flat top hemi piston has a smaller compression height. Has anyone else seen this or noticed this? I didnt take it out to measure but that has to be it unless I was given the wrong rod and I dont know how to measure those.
Click to expand...

Are you checking the clearance from the top of the piston down to the first ring and finding .003 to .004"?
 
The piston to Cyl clearance is checked at the skirt. the typical way is remove the piston rings insert the piston upside down wrist pin parallel to the crankshaft. With your thumb hold piston against cyl wall and measure clearance at opposite side with feeler guages.

Apiston is usually smaller in diameter parallel to the pin, (cam ground) the piston will also be substantially smaller in diameter at the head, above the rings, than it is at the skirt 90* to the pin. This allows the piston to expand with heat and the thicker portions expand more thus more clearance is required cold.

That brings us to another problem you should be finding more clearance at the point you are measuring. I think you better check clearance at the skirt before you go further.
 
496 BB said:
No. I am not sure how to do so since its from center to center. The new one is .020 over per NR. When I say piston rock I am talking about side to side measurement. Its is a .010 difference. I did not have that before. Is that from the rings pushing piston to one side? Since they have not fired up yet? I didnt try to move the piston to see.
Click to expand...
For a fairly accurate 'read' on rod length, use your calipers and measure outside-to-outside and inside-to-inside of the crank journal (big) hole and the wrist pin (small) hole. Then 'subtract' 1/2 the diameter both holes from the 'o/s-o/s' and 'add' 1/2 the diameter of both to the 'i/s-i/s' measurment. From what you have stated, 'both' totals Should be very close too '3.328+.020'....(maybe) JMO
 
I would think the piston pin to top of the piston on the flat top had to have been shorter then the dish piston... that would be my thought... but I would look at the rod too...
 
496 BB said:
OK just messed with it again as its bothering me. I "centered" the piston in the bore. Checked clearances to bore down to first ring with feeler gauge and am at .003-.004" sidewall. Thats good I assume. Now with it being centered I checked again in the hole amount and they are both side to side within .001" being .043". I feel better with that but is it supposed to move like that? I never noticed it on my other motor. Maybe it is. Either way its still in the hole the same amount as before so I would assume the flat top hemi piston has a smaller compression height. Has anyone else seen this or noticed this? I didnt take it out to measure but that has to be it unless I was given the wrong rod and I dont know how to measure those.
Click to expand...

Pull the piston and rod back out, measure from the top of the rod journal to the bottom of the wrist pin journal, that will give you your rod length, at least that is how i measure it and have seen everyone else measure rod length to compare rods. basically your measuring the distance between the 2 journals on the rod, center of one to the center of the other one. I believe your issue here is that the flat top is a shorter piston than the dished piston that came in these engines. The wiseco pistons iv seen for these engines look quite a bit shorter than the stock dish piston, which i believe is why most people go with a longer rod. I have the same engines as you, old style 69730 predators. Also, put the dish and flat top pistons both side by side and compare, then measure the distance from the top center of the wrist pin bore, to the top of the piston on both and compare, this should give you an idea which has the taller compression height if it isnt clearly visible by looking at them. You can also measure from the top edge of the top ringland to the top of the piston on both and compare, but i like to measure it from the wrist pin journal to the top of the piston. Several ways to skin a cat i guess you can say.

Once you get the clearances where they need to be, if your still .040 in the hole or still in the hole, you can have the block decked to zero, so you dont have to go throwing parts and more money at it trying to figure out how to get the piston up in the hole more. Cheap, simple and easy way to do it.
 
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