Author Topic: AAZ bore measurements  (Read 669 times)

January 07, 2023, 05:40:05 pm

oblique

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AAZ bore measurements
« on: January 07, 2023, 05:40:05 pm »
So this was measured with pistons in place so 'bottom' is bore above piston at lowest position.

I used the Mitutoyo telescoping gage set but these numbers are obviously impossible...i used the biggest number, measuring each position about 5 times and taking the biggest numbers so either A. The gage didnt catch the widest point or B. My caliper is ***e.

Measuring pistons (whatever i could reach of the protrusion) they are 78.9 at that point.






Reply #1January 07, 2023, 08:07:48 pm

fatmobile

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Re: AAZ bore measurements
« Reply #1 on: January 07, 2023, 08:07:48 pm »
I don't think pistons are going to be 78.9mm.
I think the biggest piston is 77.5ish.
 Sounds like it's bored as big as a block can get. Unless you go with 1.7TD pistons from a KY block.
What does it say on top of the piston,.. usually has the size stamped on the top.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #2January 08, 2023, 09:56:48 am

oblique

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Re: AAZ bore measurements
« Reply #2 on: January 08, 2023, 09:56:48 am »
Sorry, this is the 1.9 Canadian AAZ TD, the one I have in the classifieds...there's no interest so I guess Im keeping it.
79.5 is the stock bore i believe.

Pistons are typically narrower at the top, correct? To allow for the added thermal expansion the crown sees?

I am looking for a better caliper.
« Last Edit: January 08, 2023, 10:22:33 am by oblique »

Reply #3January 08, 2023, 08:23:42 pm

fatmobile

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Re: AAZ bore measurements
« Reply #3 on: January 08, 2023, 08:23:42 pm »
Oh, right. There is a taper to the pistons.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block

Reply #4January 13, 2023, 05:26:15 pm

libbydiesel

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Re: AAZ bore measurements
« Reply #4 on: January 13, 2023, 05:26:15 pm »
Using excellent condition Starrett telescoping gauges and a good micrometer I still find it challenging to get accurate and repeatable piston and bore measurements.  It is possible but challenging.  A good quality cylinder bore gauge is the tool to use and still takes skill. 

I assume that when you say calipers, you are referring to something like this:



With those, I do not believe there is any hope of achieving a measurement accurate enough to be useful (despite the accuracy the manufacturer might claim).  20ish years ago, before I had decent tools, I tried using digital calipers to measure engine internals and the measurements were not accurate enough to be useful.  Thankfully I had a friend who had decent measuring tools and was kind enough to assist me. 

If you aren't in a position to get (at minimum) a decent micrometer for the distance you are measuring, then take it to a machine shop and pay them to measure it up.  That will actually save you time and money in the long run.

You also can't get useful piston measurements with the pistons installed.  Measuring the diameter of the piston crown as it protrudes from the block is not useful in determining piston wear. 

Yes, the skirts are larger diameter than the crown and the skirt is what you need to measure. 

Reply #5January 13, 2023, 06:37:48 pm

fatmobile

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Re: AAZ bore measurements
« Reply #5 on: January 13, 2023, 06:37:48 pm »
I found that with the old 1.6 engines
(and yes this time I remembered that this is a 1.9 engine, ha)
is the ring gap gets too wide for the rings.
 They got too much side-to-side slop was the reason they needed replaced.

Side note: The AAZ has the same weak crank key as the 1.6.
 It can let loose of the timing sprocket.
 Some say that because the 1.6 alternator used a V belt it didn't get the backlash the 1.9 did with the serpentine belt.
It didn't get driven backwards by the momentum of the alternator when slowing down.
 I still had a 1.6 crank key shear and let go of the sprocket,..
well someone else did and I got a free MK2 jetta out of it.

The cure is to use a sprocket from an early TDI and grind a flat spot on the crank to match it.
Tornado red, '91 Golf 4 door,
with a re-ringed, '84 quantum, turbo diesel, MD block