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Author Topic: Piston height. What would you do?  (Read 783 times)

September 06, 2018, 12:04:51 am

fatmobile

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Piston height. What would you do?
« on: September 06, 2018, 12:04:51 am »
The #1 piston is .021".
  #2 is .019"
 #3  is .017"
 The 4th is .011".
 Those are under a 1 notch gasket (.0264-.0315)  some by quite a bit.
 and  there is only 4 or 5 thou between notches.
 The surface isn't great but would probably stick with a little hylomar.
 I'd like to get the block surfaced.

 Should I have them take 10 thou more off one end?
 Something like
  .021  + .002  = 23
  .019  + .0053  =  .0243
  .017  + .0086  = .0256
  .011  + .012  = .023

It would still use a 1 notch but compression would be better and all cylinders would be closer.


Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #1September 06, 2018, 01:44:26 am

libbydiesel

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Re: Piston height. What would you do?
« Reply #1 on: September 06, 2018, 01:44:26 am »
Have you tried swapping rods from one piston to another to see if you can even them out any? 
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Reply #2September 06, 2018, 02:10:21 am

fatmobile

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Re: Piston height. What would you do?
« Reply #2 on: September 06, 2018, 02:10:21 am »
Yeah, 3 and 1 got swapped.
 Didn't change much, except make them all graduate from one side to the other.
 Instead of high in the middle.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #3September 06, 2018, 02:57:59 am

libbydiesel

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Re: Piston height. What would you do?
« Reply #3 on: September 06, 2018, 02:57:59 am »
I think your idea of shaving more off one side that the other has merit if the machine shop will do it.
Use, abuse or ignorance of any information is at your own risk, confirmation or refutation is your own responsibility, no further 'proof' will be given, take it or leave it, YMMV,terms subject to change without notice, avoid contact with eyes or skin, contents known to cause cancer in California

Reply #4September 07, 2018, 01:38:34 am

fatmobile

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Re: Piston height. What would you do?
« Reply #4 on: September 07, 2018, 01:38:34 am »
I read your question wrong.
 I didn't swap rods I moved pistons to different holes.

 Actually swapping 1 and three showed 3 was .023 and became .021 in hole 1.  Went down 2.

 1 went from .016 to .0175 in hole 3.  Went up 2ish.
 So it seems like the block has a slight lean now.
 
I've had this done on another block. Only once.

 The pistons are out of the old VNT-15 Rabbit block that cracked on my trip last fall.
It always had something different about how one cylinder sounded.
  Maybe this was it. I don't think I wrote down the piston height numbers like I did this time,..
just took the highest piston and picked a gasket.

Sounds like it's easy for the shop to do it,. $40.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #5September 07, 2018, 11:23:57 am

libbydiesel

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Re: Piston height. What would you do?
« Reply #5 on: September 07, 2018, 11:23:57 am »
Sometimes slight differences in pistons/rods add up.  I would try swapping the pistons or rods between #1 and #4 (so the #1 rod is with the #4 piston) and see if you can even those out a little. 
Use, abuse or ignorance of any information is at your own risk, confirmation or refutation is your own responsibility, no further 'proof' will be given, take it or leave it, YMMV,terms subject to change without notice, avoid contact with eyes or skin, contents known to cause cancer in California

Reply #6September 08, 2018, 10:43:56 pm

fatmobile

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Re: Piston height. What would you do?
« Reply #6 on: September 08, 2018, 10:43:56 pm »
I took it in to get surfaced. He took 3 off one side and 8 off the other.
 Unfortunately he took it off the wrong side.
 Went from 21 to 29 and 11 to 14. 10 thou difference changed to 15.
 I figured he might not have it right in his head when he said he was going to take 21 down to 11 and I said you need to take 11 up to 21.

 I was wrong he needed to get he pistons to stand above .027 for good compression with a 1 notch gasket.

 I swapped 1 and 4 it went to 19 and 24.
 Still not above the lowest .027 for the 1 notch.
 He's going to have to do it again,.. I'm sure he'll be glad to hear he doesn't have to buy me a new block, it can be saved.

 I will try a rod swap between 1 and 4 just to see what happens. Good idea.
 But it will still need to be surfaced again to get the pistons to stick up higher and improve compression.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #7September 09, 2018, 12:21:38 pm

libbydiesel

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Re: Piston height. What would you do?
« Reply #7 on: September 09, 2018, 12:21:38 pm »
[facepalm]

Ah well, it happens to everyone from time to time.  :)
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Reply #8September 10, 2018, 12:16:13 am

ORCoaster

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Re: Piston height. What would you do?
« Reply #8 on: September 10, 2018, 12:16:13 am »
I would have the machinist mark it prior to set up.  That way if they turn the block around, which is what it sounds like they did, they would know not to turn on the machine just yet. 

Looking to add fender trim, LED lights and a bumper push to the truck soon.

Reply #9September 10, 2018, 12:44:04 am

RunninWild

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Re: Piston height. What would you do?
« Reply #9 on: September 10, 2018, 12:44:04 am »
Doesnt look like its been asked. Have the rods been inspected for straightness?

Reply #10September 11, 2018, 03:08:09 am

fatmobile

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Re: Piston height. What would you do?
« Reply #10 on: September 11, 2018, 03:08:09 am »
 I did mark the block with piston height in magic marker on the surface.
 So he just got it turned around in his mind.
 I think he registered it as that side of the block was too high,... instead of the piston being too high.

 Luckily it didn't ruin the block,.. and he did take off as much as I told him to.
 But like I posted earlier I was thinking the goal was .027
 Thats the minimum, we should be aiming more towards .031.
 So he should get it right the second time,.. and get us above .026.

 Moving the pistons and seeing the height change shows the deck is slightly sloped.
 At first near .0035 higher on one end,  now .005 toward the other.

 The block didn't suffer a tragic death either. From an '84 Rabbit.
 The previous owner did put another head on it but there were no new machine marks on the deck surface.
 There were big ridges at the top of the cylinders, with stock pistons that had no valve marks.
 It died because they didn't adjust the valve right and it was staying open enough to fill the exhaust with carbon:
4bluecarbon4clyexhaust by vwfatmobile, on Flickr
It was worse but some carbon fell off.


Runningwild: I'm trying to think which engine these rods came from.
 They were in my VNT Rabbit until the block cracked.
 I think they were from an '85 Jetta TD that never suffered a catastrophic failure,just wore out.
 I haven't checked them for straightness but my old shop had some great guys.
  Damn, I wish they hadn't sold the place and retired.
 They balanced the rods, pressed the bushings in and reamed them.
 They would have noticed.  They checked them for round and they were fine.
 They were the first ones to suggest checking the height and making the deck match the pistons, on a different engine.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #11September 11, 2018, 10:21:32 pm

fatmobile

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Re: Piston height. What would you do?
« Reply #11 on: September 11, 2018, 10:21:32 pm »
 I swapped rods on the 2 end pistons and the height stayed with the pistons.
 So it isn't the rods.
 Kolbenshmidt.

 Took it back today and we marked how much to take each end down to, right on the deck.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

 

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