Piston Height (Protrusion) B6 1.8 2005

[JDBeck]

Hi:
From my previous post - lost teeth on timing belt - bent valves.
Purchased Audi - Bentley "Official" service manual on DVD -no info on Piston Height - (Protrusion).
Want to be sure I don't have a bent rod and so want to measure piston heights.
Can't find anything in the Audi - Bentley "Official" DVD.
Any help as to where to find info would be great .
Thanks....

[Denio24]

I don't think VAG gives a max spec for protrusion interference with the head. But what you're asking to do is just basic engine blueprinting knowledge. Just measure the piston in four corners while a buddy (or you) spins the crank and you measure with a dial gauge.

[JDBeck]

I don't think VAG gives a max spec for protrusion interference with the head. But what you're asking to do is just basic engine blueprinting knowledge. Just measure the piston in four corners while a buddy (or you) spins the crank and you measure with a a dial gauge.

Yes, I think your suggestion will tell me if the piston is square in the bore. But if the piston has been squarely hit and the rod bent squarely, I still have no reference number for maximum linear displacement referenced to the deck. Specs on DVD as you suggest don't include the data.

[Denio24]

For any case, VAG would still not specify these specifications. I have checked specifications for the block assembly, piston and crankshaft, and cylinder head categories, but no luck in finding.

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[JDBeck]

For any case, VAG would still not specify these specifications. I have checked specifications for the block assembly, piston and crankshaft, and cylinder head categories, but no luck in finding.

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Thanks for your reply. I have found the same (or lack of) info in the service manual on DVD. I did come across a blog on VW TDI where they have info on protrusion on similar engines. Like AJH or some such engine series and state something like FSM says 0.98 - 1.01 mm protrusion. I guess they have better data than Audi owners. They also make reference to "number of holes in head gasket" ie. 1, 2 or 3 indicating head gasket thickness. All of this is great info if you have to cut the head. But I suppose Audi owners can not have access to this.

[Denio24]

Yes EXACTLY, usually this data goes hand in hand with picking the right headgasket thickness. Will let you via PM if I ever come across this data.

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[diagnosticator]

The piston crowns on 1.8Ts don't extend above the block deck surface at TDC, unlike pistons in a TDI, so there is no piston projection spec to worry about.

[JDBeck]

This is info from Tdiclub-dotcom - "Don't know about ALH, but AHU piston protrusion specs are as follows:

0.91 to 1.00 mm—use 1 hole head gasket
1.01 to 1.10 mm—use 2 hole head gasket
1.11 to 1.20 mm—use 3 hole head gasket

ALH could be similar, but check to be certain. "

[JDBeck]

The piston crowns on 1.8Ts don't extend above the block deck surface at TDC, unlike pistons in a TDI, so there is no piston projection spec to worry about.

Thanks for your reply. However, the info I want relates to determining if a connecting rod is bent.

[JDBeck]

Yes EXACTLY, usually this data goes hand in hand with picking the right headgasket thickness. Will let you via PM if I ever come across this data.

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This is info from Tdiclub-dotcom - "Don't know about ALH, but AHU piston protrusion specs are as follows:

0.91 to 1.00 mm—use 1 hole head gasket
1.01 to 1.10 mm—use 2 hole head gasket
1.11 to 1.20 mm—use 3 hole head gasket

ALH could be similar, but check to be certain. "

[JDBeck]

I understand piston protrusion is commonly associated with diesels. If there is an easier way to determine if I have a bent rod I would love to learn about it - short of direct measurement of the con rod length/parallelism etc...

[diagnosticator]

Thanks for your reply. However, the info I want relates to determining if a connecting rod is bent.

Use a depth micrometer to measure the distance from the block deck to the piston crown surface at TDC. Compare the measurements of pistons to deck of cyl 1/4 and 2/3.

[diagnosticator]

I understand piston protrusion is commonly associated with diesels. If there is an easier way to determine if I have a bent rod I would love to learn about it - short of direct measurement of the con rod length/parallelism etc...

See my edited reply above. You would measure piston to deck distance for pistons 1 and 4 then rotate the crank and measure pistons 2 and 3. The piston pairs should both have the same distance to the deck. Measuring 1/4 and 2/3 does not have to be at exactly the same crank position since you are comparing the two pistons that are together at the same position, not between the two pairs. Use a piston stop across one of the cylinders of the pair not being measured, to place the crank in the same positon for each measurement of the two piston pairs.

[Denio24]

See my edited reply above. You would measure piston to deck distance for pistons 1 and 4 then rotate the crank and measure pistons 2 and 3. The piston pairs should both have the same distance to the deck. Measuring 1/4 and 2/3 does not have to be at exactly the same crank position since you are comparing the two pistons that are together at the same position, not between the two pairs. Use a piston stop across one of the cylinders of the pair not being measured, to place the crank in the same positon for each measurement of the two piston pairs.

And this measurement determines exactly what? In my opinion there are too many variable which could account for such as rod bore distortion, crank wear, etc for this block deck to piston crown measurement to be suffice.

[diagnosticator]

And this measurement determines exactly what? In my opinion there are too many variable which could account for such as rod bore distortion, crank wear, etc for this block deck to piston crown measurement to be suffice.

A difference between the measured distance between the block deck comparing pistons 1 and 4 and 2 and 3. The actual measurement value is irrelevant. The only important aspect is a difference between the two pistons. As a practical matter, any bending of a rod would have to cause a significant enough difference in rod length that would exceed the stack up of the tolerances or clearances in the rod bearings. Otherwise, the rod could not be considered to be bent.

The tone of your comment suggests some defensiveness, are you feeling insecure here? If that may be the case, no worries, everyone's contributions are worthwhile.

[Denio24]

No, I'm trying to gain knowledge, all emotions aside. So this measurement would provide just a general offset value of the wear is between pistons 1-4 and 2-3. Correct? I remember vaguely different techniques of trying to figure out rod bend, but the pistons were disassembled. If you say I'm insecure, I'm certainly not lacking thereof -- in some aspects of engine building. :) so please. Educate me.

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[Seerlah]

Haynes has more technical specs than Bentley. For example. technical data when building an engine can not be found in a Bentley. But can in a Haynes.

[diagnosticator]

If the measured distance between the block surface and the piston crowns comparing pistons 1/4 and 2/3, are different, exceed the clearance stack up in the rod bearings, then the piston with the larger measurement it's con rod can be considered likely bent. Since the wear in the rod bearings would be the same, within the error of the measurements, the wear in the bearings would not be relevant to the measured comparison between the pistons 1/4 and 2/3. The only situation when a large difference in the bearing clearance could be involved is if the force that cause a rod to bend deformed the bearing shell also. In addition, any force large enough to exceed the strength of the rod against bending could also result in distortion of the piston and or the piston pin as well, effecting the measured distance between the piston crown and the block deck surface, compared to the other piston of the pair, crown to deck measurement.

[JDBeck]

If the measured distance between the block surface and the piston crowns comparing pistons 1/4 and 2/3, are different, exceed the clearance stack up in the rod bearings, then the piston with the larger measurement it's con rod can be considered likely bent. Since the wear in the rod bearings would be the same, within the error of the measurements, the wear in the bearings would not be relevant to the measured comparison between the pistons 1/4 and 2/3. The only situation when a large difference in the bearing clearance could be involved is if the force that cause a rod to bend deformed the bearing shell also. In addition, any force large enough to exceed the strength of the rod against bending could also result in distortion of the piston and or the piston pin as well, effecting the measured distance between the piston crown and the block deck surface, compared to the other piston of the pair, crown to deck measurement.

Thank you for your input. I have not researched the cumulative clearances for piston pin, rod to bearing or bearing to crank journal. My understanding from your explanation is that the sum of clearances would most likely be be eliminated or reduced with a bent rod and further the bent rod, measured in pairs for comparison would have less vertical displacement than the unbent rod.
My method to measure is to use a deck bridge with a digital dial indicator than stores maximum piston displacement through a rotation cycle. By this method I believe I would see if either in a pair has less displacement or vertical travel up the bore measured against the value for the deck surface.
I'm not sure what a piston stop would be though.
Thanks again.

[AudiA4_20T]

Dude you don't have a bent rod. There have been probably 100+ users on here that have bent valves.

1. Clean off pistons
2. Get new head or fix bent valves
3. Install head, set timing
4. Enjoy driving car

[diagnosticator]

Thank you for your input. I have not researched the cumulative clearances for piston pin, rod to bearing or bearing to crank journal. My understanding from your explanation is that the sum of clearances would most likely be be eliminated or reduced with a bent rod and further the bent rod, measured in pairs for comparison would have less vertical displacement than the unbent rod.
My method to measure is to use a deck bridge with a digital dial indicator than stores maximum piston displacement through a rotation cycle. By this method I believe I would see if either in a pair has less displacement or vertical travel up the bore measured against the value for the deck surface.
I'm not sure what a piston stop would be though.
Thanks again.

Your method reveals nothing. The displacement is a function of the bore diameter and the stroke. A bent rod would have the same stroke as an straight conn rod. The clearance in the rod bushing/bearing is irrelevant. A piston stop is just a device rigged to stop the piston at a specific location so that the opposite pair can be measured for crown to block deck distance.
A means of locking the crank by a blocking insert in the ring gear teeth of the flywheel would serve the same purpose.

[diagnosticator]

Dude you don't have a bent rod. There have been probably 100+ users on here that have bent valves.

1. Clean off pistons
2. Get new head or fix bent valves
3. Install head, set timing
4. Enjoy driving car

Yeah, as a practical matter, I agree with you. If the OP is seriously worried about a bent rod, it's unlikely he will be satisfied unless he confirms that there are no bent rod/s.

[JDBeck]

Your method reveals nothing. The displacement is a function of the bore diameter and the stroke. A bent rod would have the same stroke as an straight conn rod. The clearance in the rod bushing/bearing is irrelevant. A piston stop is just a device rigged to stop the piston at a specific location so that the opposite pair can be measured for crown to block deck distance.
A means of locking the crank by a blocking insert in the ring gear teeth of the flywheel would serve the same purpose.

My method of measuring is identical to what you entered in this thread in that it is the inverse of your recommendation. I apologize that you can not follow the logic of the methodology I am using.
The displacement I refer to is linear not volumetric. A bent rod can not have the same maximum linear travel with respect to the deck as a straight rod does.
The method I use will indeed reflect a piston that has less travel with respect to the block deck or does not have a linear elevation equivalent to a given pair or comparable piston and thus a bent rod is evident.
A bent rod will always have a smaller linear displacement and as well cause the piston crown to be much less perpendicular to the cylinder wall. This is as simple as comparing a straight line that is converted to an arc.
Thank you for your comments.

[diagnosticator]

At what point do you begin and end the stroke measurement? The only relevant measurement would be the full stroke, and that is the same for a bent rod as well as a straight rod. I don't understand why comparing the two pistons for distance to the block deck doesn't satisfy your need to detect a bent rod. If that is what you want to do, why are you asking for advise here?

[old guy]

A bent rod can not have the same maximum linear travel with respect to the deck as a straight rod does.

As Diagnosticator has tried to point out, this statement is simply not true. The total piston travel, regardless of the rod length, will always be controlled by the throw of the crankshaft. Think about it this way. For sake of argument consider the crankshaft has a five inch throw. One full revolution moves the rod journal through five inches of total travel. It doesn't matter if you attach a four inch or a five inch long connecting rod to the journal. It's still going to move the connecting rod and attached piston five inches. If the five inch rod places the piston flush with the top of the block the bottom travel of the piston will be five inches from the top. If the four inch rod places the piston one inch below the top of the block the bottom travel will place it six inches from the top since it will travel down five inches because of the crank throw.
.

A bent rod will always have a smaller linear displacement and as well cause the piston crown to be much less perpendicular to the cylinder wall. This is as simple as comparing a straight line that is converted to an arc.
.

Also not true. It doesn't matter if the rod is straight, bent or zig-zag. The piston will rotate to center on the wrist pin and the crown will remain perpendicular to the bore.

Just do what Diagnosticator has suggested and check the relative distance to the deck. Done.

[Seerlah]

Dude you don't have a bent rod. There have been probably 100+ users on here that have bent valves.

1. Clean off pistons
2. Get new head or fix bent valves
3. Install head, set timing
4. Enjoy driving car

Some people don't like to accept the simplest answers. Been in that boat before, and I'm sure you have too. Learning experience. With that being said, I bent valves before too