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post #1 of 7 (permalink) Old 08-02-2019, 01:36 PM Thread Starter
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Question Bent rods?

466 stock crank/rods. H612CP 30 piston, press fit wrist pin.

I had local machine shop 'recondition' the rods. After installing and measuring deck, I see that 2 rods may be bent causing the piston crown to be tilted compared to block deck.

I am measuring the piston to deck with a depth gauge at the front and back of the piston, where the wrist pin is. 6 of the 8 pistons measure a difference of 0.001 or so. The other 2 measure 0.008. As a sanity check, I also measured the top and bottom but the piston can rock on the pin.

Cylinder 3;
front = 0.001
back = 0.009

top = 0.005
bottom = 0.005
average = 0.005

Cylinder 8;
front = 0.006
back = +0.002
(above deck)
top = 0.003
bottom = 0.001
average = 0.002

0.008 over 4.390 piston diameter is an error of tan-1(0.008/4.390) = 0.1 degree

Is this an indication of a bent rod?
Is this a major issue that must be corrected, or can I run it as is?
What is the method a machine shop uses to determining if a rod is straight?

2 trips to machine shop is 5+ hrs of my time. Can I straiten them myself with a press without removing the piston? I get that this may put correction bend in different location and offset little end from big. There is a lot of axial piston to rod clearance. This is a low $, low rpm engine that will see minimal miles.

I disassembled the engine myself and didn't find any water in cylinders or other damage to anything. Had not be rebuilt previously.
Thanks

Justin
72 F250
14 F250 6.7, CCLB, 37s
04 Mustang Cobra, 14psi + N20

Last edited by rattle_snake; 08-02-2019 at 01:41 PM.
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post #2 of 7 (permalink) Old 08-02-2019, 05:46 PM Thread Starter
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I called machine shop, they didn't seem concerned with 0.008 difference as long as rods on crank journal had a few thou of clearance. I did not check so will look into it. They did offer to check the rods and said the pistons will have to come off to do so.

Justin
72 F250
14 F250 6.7, CCLB, 37s
04 Mustang Cobra, 14psi + N20
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post #3 of 7 (permalink) Old 08-05-2019, 11:35 AM Thread Starter
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Measured end play clearance on the rod journals.
1,5 = 0.011
2,6 = 0.010
3,7 = 0.012
4,8 = 0.018
Spec I found was 0.010-0.020.
The gaps are consistent around the rod so doesn't look like the rod if bent has an effect, at lease in a no load/pressure condition.

So the piston is crooked in the bore due to the rod, which is possible as it is tapered on the top.
Seems pretty far off to me.

Thoughts, advice?

Any concern on having the piston pins pressed off and back on again?

Justin
72 F250
14 F250 6.7, CCLB, 37s
04 Mustang Cobra, 14psi + N20

Last edited by rattle_snake; 08-08-2019 at 01:49 PM.
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post #4 of 7 (permalink) Old 08-06-2019, 01:47 PM
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Your piston/wall clearance is going to be pretty tight with a cast piston like that, so you're right to be concerned.

When a machine shop assembles a press fit piston/rod, they just put the rod/piston in a jig to check/straighten while the rod is still red hot from heating it up to install the pin, and straighten the rod as needed.....or they should. If you want to do it yourself, you might be able to just heat it up with a torch and bend it, but it's a fairly tight tolerance to do in a home shop, and might take you more time/money than just making the drive.




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post #5 of 7 (permalink) Old 08-06-2019, 06:35 PM Thread Starter
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Quote:
Originally Posted by CarsByCarl View Post
Your piston/wall clearance is going to be pretty tight with a cast piston like that, so you're right to be concerned.

When a machine shop assembles a press fit piston/rod, they just put the rod/piston in a jig to check/straighten while the rod is still red hot from heating it up to install the pin, and straighten the rod as needed.....or they should. If you want to do it yourself, you might be able to just heat it up with a torch and bend it, but it's a fairly tight tolerance to do in a home shop, and might take you more time/money than just making the drive.

.
Thanks for the feedback Carl.
Since I don't have real experience I use math to create this to worry about. The piston-to-wall should be 0.0015 to 0.002. The bottom of the skirt should be off by a similar amount as the crown (0.008) which in theory will drag in the bore and/or transfer the angle/error to the big end/crank.

My local machine shop said they should have been checked before assembly, but they don't straighten rods. After more research it's not something I would want to try.

My current plan is to ship the 2 offending rods/pistons to the machine shop where my heads are being done, to be replaced.
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Justin
72 F250
14 F250 6.7, CCLB, 37s
04 Mustang Cobra, 14psi + N20
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post #6 of 7 (permalink) Old 08-08-2019, 01:30 AM
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Have to had the block decked/ squared? They can be off quite a bit sometimes..
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post #7 of 7 (permalink) Old 08-08-2019, 01:49 PM Thread Starter
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The block was decked and ended up an average deck height of 0.005 on right and 0.002 on the left. Since the pistons within one bank are close to one another the deck appears to be OK.

Before I removed the 2 pistons, I figured I would recheck them... The measurements aren't repeatable enough given the target. I very carefully remeasured all 8 two more times turning crank only in one direction, and got different answers.
So I decided to complicate things and tried to identify why.
1) I tapped on the front of the piston to take up any slack and measured the front and back depths.
2) I then tapped the back side of the piston and measured the front and back depths again.
The pistons can rock about 0.012 along wrist pin. This slack/uncertainty is beyond pass/fail limit.

For example cylinder #1 measured a front back difference of 0.000 in condition 1, and then a difference of 0.013 in condition 2.

Taking average/center of the two of measurements, I get the following;
1) -6.5
2) -8
3) -8
4) -6.5
5) +4.5
6) +3
7) +2.5
8) +7.5
where a positive number is front of piston lower than rear. Note that all right bank are tilted in same direction (back) and left bank all the other way.

So it appears I either need to replace all the rods or run it as is. The assembly was balanced so a new rod set would need to be lightened accordingly. I'm concerned that a different set of stock rods may not get me to 0.002 or better.

I pulled #2 out and verified that the pin-to-crown measurement was the same on front and back of piston, it was right on.

What is the correct procedure for this measurement? Who actually measures this?

EDIT: I wanted to see the effect of the big-end end play at the piston crown. Moving the big end of the rod back and forth on the crank journal the end play amount (about 0.010) moves the piston tilt about 0.006-8. This indicates that most of the slop is on the big end. Not useful, just an observation.

Justin
72 F250
14 F250 6.7, CCLB, 37s
04 Mustang Cobra, 14psi + N20

Last edited by rattle_snake; 08-12-2019 at 07:17 PM.
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