Piston height differs by 10 thou

[Fatmobile]

I'm rebuilding a TD engine.
#4 The most proud piston stands at .032".
#1 The shortest; is .022 above the surface.

What bugs me is .022 is below a 1 notch,.. .026 is minimum height.

and .032" is the shortest allowed for a 2 notch.

Are these different thicknesses of gasket just to keep the pistons from hitting the valves,..
or will compression be noticably lower on one cylinder?

Is 10 thou enough to lower compression enough to notice?

I think I'll swap pistons #1 and #4 and see what happens.
If that doesn't help; maybe even swap pistons on the rods.

[82vdub]

I think the different head gaskets are so the pistons don't hit the head, or valves. If you had the crank machined, it's possible that one is machined slightly off from the others, leading to a piston that's not as high as the other. It would effect compression ratio, but I don't think it would effect it that much. You're talking .010" compared to a stroke of 3.386".

[DanHoug]

my pistons too were varying hts and it had never been rebuilt... you choose the gasket according to the tallest projection to keep the valves from hitting the pistons.

i was in between a 1 and a 2 gasket also. i actually assembled with a 1 and thought i could hear a sharp rapping from interference. as it turned out, that gasket leaked (probably my bad torque technique) so i bought studs and a 2 notch and the engine seemed quieter.

i wanted the absolute best cold starting and so was trying to get the gasket as thin as possible. others always put in a 3 to keep parts availability easy and to REDUCE compression a tad to reduce forces in the engine.

-dan

[Quantum-man]

It would be interesting to measure BDC as well as TDC to compare stroke length/ conn rod length

[surfcam]

I could be wrong but by my calculations .010" will change the compression ratio from 23:1 to 21.5:1. I think a person would notice that on a cold day.

[Fatmobile]

I was hoping someone could calculate this.
Thanks.

Quantum man,.. you lost me, ha.
How far before TDC,.. and what would we learn?

[82vdub]

Quantum's wanting to know what the measurements are when the crank is at bottom dead center and what those measurements when compared to the other cylinders may show. It may show that the crankshaft journals are not ground on the same "stroke" centerline, but off by a bit. Basically, does one cylinder have the rod journal ground so that it's .010" off from the rest?

[Fatmobile]

It's possible.
I used ARP rod bolts and the rods needed resized.
Part of why I thought swapping 1 and 4 would be a good test.

I don't have the tool for checking piston height,.. I take it to the shop.
I suppose I should buy one,.. or see if I can make one using the dial gauge from my timing tool.
Though setting the engine in the back seat is great exercise, ha.

I've never heard of checking BDC and don't know how they'd do it but will mention it on the next trip.
I might have to shorten the tallest rod,
I'd like to be able to use a 1-notch.

How much would that raise compression on the low piston?

[DanHoug]

you probably DO have the tool to measure piston ht.... a straight-edge and a feeler gauge. i have the magnetic base dial indicator and futzed around with it a long time getting all sorts of differing readings. finally, i just laid the straight edge on the top of the piston and measured the gap with a feeler gauge. it worked well and i have confidence in the measurement.

but i wouldn't bother and stress over the differing hts... correcting things so they are all the same will have a marginal return for the effort. yeah, you could maybe get an thinner head gasket but it is too much work unless this is a pursuit of labor and love.

a plug again for having a true straight edge in your tool box.. head warp, deck warp, and piston ht.

-dan

[Fatmobile]

I built a homemade piston height checker,
using the timing tool and dial gauge.
Used a piece of 1" square stock, ground out where the piston sticks up, drilled a 5/8" hole.
I got my timing tool from Jack and it doesn't quite go through the 5/8" hole, sticks there solid and can be removed.

Zeroed on the deck surface, then moved it over a smooth/machined spot on the piston.
One was .027
the other was .025.
I should get the face of my tool sanded/machined flat,.. it's kinda rusty but I ran it on some sandpaper and a file for awhile.


I might have to take it back and have them look again.
Watch close and read the gauge myself.

The machine shop will probably put the new tool face up against their disc sander and clean it up for free,..
just so I quit bothering them with it

I don't have it set up to read bottom dead center,.. but have plans to. The timing tool is long enough to reach so it should work.

[Fatmobile]

Here's a pic of my piston height detector :


I took it to the shop and Dennis put it on the mill.
I told him the center height didn't matter but he wanted to clean it up.
I was kidding but he did the job for free.

I still need to clean the sides up and paint it.

With my new tool, I measured .030" .029" . 026" .026"
I checked a few times and trust my tool,.. but I'll probably check again, ha.
Those number don't bother me.
1 Notch gasket on the way.

[82vdub]

Looks like other piston height/TDC checker tools I've seen. Some are magnetic so they stick to the block, but I don't believe it's necessary. You can accurately determine TDC with that tool and perminantly mark the flywheel for it's correct TDC timing mark.

[rsxsr]

Could you have the machine shop remove .003 from the two taller pistons? There is a ton of meat in the top of those pistons.

edit: you could also have the machine shop then check the balance of the 4 pistons.

[Fatmobile]

I think there is a coating on top of the piston that shouldn't be removed.
and if you have to pull the pistons,.. might as well machine/balance the rods.

[rsxsr]

I did not realize you were on the final assembly. We usually assemble engines twice before the final assembly. I would want to know where the difference is coming from. The diesel rods are very stout, but could two of them have a twist or bend that is not noticeable to the eye, but is measureable? It should be fine.