240 Measuring Piston Protrusion for tight squish motor

[bobxyz]

What's the proper procedure for measuring piston protrusion for a tight squish motor, both for determining desired deck machining and for post-machining head gasket selection?

More specifically, what if there's some piston-to-piston height variation and the pistons all rock a bit? Do I measure protrusion on the tallest point on the piston face at max rock and take the result from the tallest piston, or do I do some sort of average? How much of this, if any, is built into the desired squish numbers?

For discussion, say I have:
5 thou of piston-to-piston variation (rod length or piston pin to face variation or deck tilt)
10 thou of min-to-max rocking (does it matter which direction?)
35 thou desired "tight squish"
nothing special 7500rpm 15psi B21FT

Thanks,
Bob

 

[Hoggster]

You have to measure it to determine what head gasket to use on VW TDIs. If you can find some of their service literature it will explain the whole process, including what tools to use.

 

[John V outside agitator]

What's the proper procedure for measuring piston protrusion for a tight squish motor, both for determining desired deck machining and for post-machining head gasket selection?

More specifically, what if there's some piston-to-piston height variation and the pistons all rock a bit? Do I measure protrusion on the tallest point on the piston face at max rock and take the result from the tallest piston, or do I do some sort of average? How much of this, if any, is built into the desired squish numbers?

For discussion, say I have:
5 thou of piston-to-piston variation (rod length or piston pin to face variation or deck tilt)
10 thou of min-to-max rocking (does it matter which direction?)
35 thou desired "tight squish"
nothing special 7500rpm 15psi B21FT

Thanks,
Bob

First, because the piston rocks on the pin, to minimise the effect you have to measure right above the pin. OOOO <-----center-line.

Use a plain ordinary electronic caliper as depth gauge..
Look at piccie 10
http://slideplayer.com/slide/8912815/

Measure 4-5 times..And that after yu have confirmed positively you have actual TDC.
Or this would be spiff-tac-ular:

But centerline! If we say the long way of the block is "North/south axis" and then the cylinder is clock position divided, then you want to be at 12 and 6...

As for variations...better not be .005 variation in rod c-c or comp hgt, that would be in the realm of
Block: how would it be 005 "sloped" unless somebody surfaced it horribly?

Don't know what you do for a living but I am always wary of measurements made by people who only infrequently must do and use precision measuring tools...sorry but the right touch is something that takes practice--and honesty about our potential role in inducing or seeing error..

Be careful, don't get flustered and once you have what you believe is the number--don't get greedy trying to get the last thou squish..
The whole TB Cult-of-the-squish has been carried to absurd lengths....it is a range, not a absolute number.

 

[culberro]

You can also sweep the top of the piston with a dial indicator mounted on a base.

And as John says, measure at the pin centerline.

 

[bobxyz]

Thanks everyone, I'd seen some conflicting techniques and just wanted to make sure I understood what was normally being used for the redblock squish measurements. I have a dial indicator and will run more measurements tomorrow (something still seems wonky).

I did a little more surfing and found a nice brief Elring TDI doc, and a good enginebuilder article:
http://www.wilmink.nl/elring/Elring_koppakking_dikte.pdf
http://www.enginebuildermag.com/2001/08/back-to-basics-preventing-piston-problems/

this would be spiff-tac-ular:

That is a nice tool, but I'd need to drink cheap beer for a year, and besides, who really wants to write down all those extra digits???

Don't know what you do for a living but I am always wary of measurements made by people who only infrequently must do and use precision measuring tools...sorry but the right touch is something that takes practice--and honesty about our potential role in inducing or seeing error..

I'm an electronics engineer and sit in a darkened cube in front of a computer most of the day. The Volvo is nice hands-on hobby. I understand completely about the skill needed to make repeatable and precise measurements. With my dial calipers or dial indicator, I'm probably good to +/- a thou if I'm careful. I tried using a telescoping bore gauge with calipers and just laughed. Felt like I'd be better off with a piece of limp spaghetti and a grade school yardstick.

don't get greedy trying to get the last thou squish.. The whole TB Cult-of-the-squish has been carried to absurd lengths....it is a range, not a absolute number.

Oh I'm not trying for the last little bit, I just want to understand enough of where the recommendations come from so that I'm comfortable picking a head gasket thickness for a block that came back with larger than expected piston-to-wall clearance.

-Bob

 

[bobxyz]

I measured the piston protrusion on my machined block and am still confused on what I'm getting.

There's a fair amount of front-back (or North-South) rock, I assume due to the piston-to-wall clearance (estimated at ~3.8 thou). When using firm thumb pressure to rock the pistons, the rock is ~4 to ~7 thou on pistons #1,#2,#4 (total max-min range, not +/-). Piston #3 is installed without rings and rocks ~10 thou.

The rock makes repeatable measurements difficult. The most repeatable procedure I found was to: adjust to TDC, measure ~1/8" in from front or back edge of piston, rock back and forth 4 or 5 times and record min/max depth gauge readings. After seeing the initial results, I also added in a depth measurement in the center of each piston.

Here's my measurements (in inches):

Measurement min max
Deck 0.2690
#1 Front 0.2860 0.2930
#1 Center 0.0510 0.0510
#1 Back 0.2895 0.2970
#2 Front 0.2930 0.2970
#2 Center 0.0530 0.0530
#2 Back 0.2880 0.2920
#3 Front 0.2890 0.2985
#3 Center 0.0510 0.0510
#3 Back 0.2840 0.2935
#4 Front 0.2840 0.2910
#4 Center 0.0465 0.0465
#4 Back 0.2825 0.2890

I stuck the measurements into a spreadsheet, normalized them to the deck height, calculated min/max average for front&back of each piston, then plotted the results:

Here's what my measurement setup looks like, showing a #1 and #4 dish center difference of ~5 thou (this was from a different measurement set, and is slightly different than above).

What's going on, both with the differences in measured dish center depth and with the seeming front-back tilt of each piston???

Based on the max averaged protrusion of 26 thou, and a target of 35 thou tight squish, it looks like I need a ~61 thou head gasket.

Thanks,
Bob

 

[John V outside agitator]

I measured the piston protrusion on my machined block and am still confused on what I'm getting.

There's a fair amount of front-back (or North-South) rock, I assume due to the piston-to-wall clearance (estimated at ~3.8 thou). When using firm thumb pressure to rock the pistons, the rock is ~4 to ~7 thou on pistons #1,#2,#4 (total max-min range, not +/-). Piston #3 is installed without rings and rocks ~10 thou.

The rock makes repeatable measurements difficult. The most repeatable procedure I found was to: adjust to TDC, measure ~1/8" in from front or back edge of piston, rock back and forth 4 or 5 times and record min/max depth gauge readings. After seeing the initial results, I also added in a depth measurement in the center of each piston.

Here's my measurements (in inches):

Measurement min max
Deck 0.2690
#1 Front 0.2860 0.2930
#1 Center 0.0510 0.0510
#1 Back 0.2895 0.2970
#2 Front 0.2930 0.2970
#2 Center 0.0530 0.0530
#2 Back 0.2880 0.2920
#3 Front 0.2890 0.2985
#3 Center 0.0510 0.0510
#3 Back 0.2840 0.2935
#4 Front 0.2840 0.2910
#4 Center 0.0465 0.0465
#4 Back 0.2825 0.2890

I stuck the measurements into a spreadsheet, normalized them to the deck height, calculated min/max average for front&back of each piston, then plotted the results:

Here's what my measurement setup looks like, showing a #1 and #4 dish center difference of ~5 thou (this was from a different measurement set, and is slightly different than above).

What's going on, both with the differences in measured dish center depth and with the seeming front-back tilt of each piston???

Based on the max averaged protrusion of 26 thou, and a target of 35 thou tight squish, it looks like I need a ~61 thou head gasket.

Thanks,
Bob

Bob, stop. relax. BREATHE...

Not only measuring that needs a feel, but you need a feel for numbers..
Lookie:
Measurement min max
Deck 0.2690
#1 Front 0.2860 0.2930
#1 Center 0.0510 0.0510
#1 Back 0.2895 0.2970
#2 Front 0.2930 0.2970
#2 Center 0.0530 0.0530
#2 Back 0.2880 0.2920
#3 Front 0.2890 0.2985
#3 Center 0.0510 0.0510
#3 Back 0.2840 0.2935
#4 Front 0.2840 0.2910
#4 Center 0.0465 0.0465
#4 Back 0.2825 0.2890

You're saying stuff is more than 1/4" indeed more than 7mm what?

.028"? you mean..

Your mag base looks waaaaaaaaaaay unstable.
Scoot it over between cylinders.
And you're waaaaaaaaaaaaaaaaaaaaaay waaaaaaaaaaaaaaaaaaay over injur-nearin it with chartys and sh1t.
Mag base on stable base...swing the clock over to the centerline of block..zero, and note in book, then move the pointy bit over to the piston like 6 or 12--at the edge-- note, record.

Zero on block, measure piston..

Trust me it ain't .289" and better not be .028 either or somebody fawked up good.

Where all the numbers come from for what is "ideal" is: pure 100% assertion for the great mass of TBers, somebody said xx" is good, therefore more must be better.

Me, its from a bit over 30 years doing zero deck or .004" aka 0,1mm motors..Not doing nearly as many as say 20 years ago...just 24-27 engines since '02

 

[4x4_Welder]

Don't worry about where the dish lands at. Measure the rim of the piston. CC the dish to make sure they are all even, otherwise the actual depth of that dish means nothing.

 

[bobxyz]

Sorry, it's a really cheap gauge with no way to zero it out. The deck measures 0.2690" on the gauge so you need to subtract that from every other measurement. That's what I meant when I "normalized" the measurements in the spreadsheet. The numbers in the graph should be true zero-based protrusion. + for protrusion, - for below-deck.

Unfortunately, the block wasn't machined as I expected. I'm estimating the side-wall clearance at ~0.0038" - a 0.003" feeler gauge (full length of skirt) goes all the way around, a 0.004" feeler gauge gets snug just before perpendicular to the wrist pin. I can post a photo if that helps. The protrusion, using these measurements, is ~0.026" instead of the requested 0.012". The shop has offered to get a custom head gasket to take care of the excess decking, and new oversize rings to take care of the slightly out of spec ring gaps and sidewall protrusion. I'm trying to understand enough about other's tight squish results and recommendations to know where to go with this block.

Edit: here's the normalized measurements.
Measurement min max
#1 Front 17.0 24.0
#1 Center -218.0 -218.0
#1 Back 20.5 28.0
#2 Front 24.0 28.0
#2 Center -216.0 -216.0
#2 Back 19.0 23.0
#3 Front 20.0 29.5
#3 Center -218.0 -218.0
#3 Back 15.0 24.5
#4 Front 15.0 22.0
#4 Center -222.5 -222.5
#4 Back 13.5 20.0

The gauge is steady enough -- I hang it off the side so that it can reach all the pistons without needing to re-adjust the holder arms at all and so I can see it from my stool. I'm ordering a better gauge that can be zeroed as soon as I can find a bridge to go with it.

For the dish measurement, I'm wondering if I have rod length differences.

 

[John V outside agitator]

Zero by moving that pointer around the edge...
Measure like I said. block then immediately adjacent on the flat part of the piston ..6 and 12 O'clock not in the middle of the damn thing...I am not a computer-boy or I'd draw you a picture.

Your rings are fine...
Cylinder to wall clearence of 003-004 will do unless you're thinking really crazy amounts of heat aka loadsa boost--like 20psi up.

And your measurements STILL do not make sense.. don't post columns of doo-doo post the stick-out like I said..I am an engine builder ya know, follow my nice free advice on this..When I need electrical engineering advice then I'll call you...

 

[John V outside agitator]

Don't worry about where the dish lands at. Measure the rim of the piston. CC the dish to make sure they are all even, otherwise the actual depth of that dish means nothing.

They were done CNC, the whole pistons were, they're fine enough that the error is less than the ability to measure...

 

[culberro]

Zero the dial indicator by rotating the grooved bezel around the outside. If it's difficult to rotate, then loosed the screw clamp. I once too had a HF dial indicator and base.

 

[OttoB]

I've been using "piston deck bridge". Made own when times were hard. Seems to cost raaund thirty now.

 

[4x4_Welder]

They were done CNC, the whole pistons were, they're fine enough that the error is less than the ability to measure...

What was done to them after? Shot peening? The surface didn't look very machined.

 

[John V outside agitator]

What was done to them after? Shot peening? The surface didn't look very machined.

Yeah, all rods get shot peened--production, aftermarket, ancient, new.. they figured that out god knows when but I've poked inside of engines from the 1920s and they were shot peened..Ancient process.

 

[bobxyz]

I picked up another HF dial gauge today and the zero works fine on this one. Soooo much easier.

Here's a set of pictures showing an example of how I measured:
- zero on gauge
- measuring front of #1 at max and min rock
- measuring back of #1 at max and min rock

middle of rock is ~0.021"

middle of rock is ~0.024"

The protrusion measurements, in inches and at middle of rock, are:
#1 front 0.021"
#1 back 0.024"
#2 front 0.025"
#2 back 0.020"
#3 front 0.025"
#3 back 0.019"
#4 front 0.020"
#4 back 0.015"

I'm still confused by these. Based on previous center of piston measurements, #4 rod may be ~0.005" shorter than the others? But what's going on with the front-to-back slope across the pistons? Is this just normal machining and measurement variation?

Here a zoomed in shot of the block and piston machining (Mahle 2nd OS B21FT).

 

[John V outside agitator]

First: oh sh1t.
Clearance on those pistons being OEM cast should be 0.0012 to 0.0015. I don't have to look it up. That's just normal cast Mahle pistons and I've used B21E in literally several dozen 1.5L Ford V4 builds

Second..now we can fer-sure say Oh dear....roughly 020 out the bore...
Normally B21 and B23 are approx 0,7mm or roughly .028 down the bore on an uncut clock with stock M rods....or aftermarket H profile rods done to normal 145mm length..
How in the world did they cut things THAT far off?
I didn't see what rods but M rods are for all the horrible weight pretty spot on at 145mm and the aftermarket ones are also very consistent. If using M rods we can forget about the possibility of a bent rod--they're so over the top strong they ain't gonna bend ever..

next; with that surface finish you really can't consider a MLS gasket like Cometic because they want a virtually perfectly smooth RA whatever the hell finish..

You're in a situation...
I know you don't want to think of it but the easiest and probably best solution is to scare up another block---they pay--and bore and deck that correctly....

Or surface again to get the right finish and compensate with thicker gasket...what can you do?

 

[John V outside agitator]

Hell I'm eating diner so not thinking..The crowns can be lopped off say 016-018..All those 40-50 V4s I built using B21E (flat tops) were all cut .016 to get the desired 004 out the bore I shoot for.. The dish volume reduction would be minimal, probably less that 0.15 to 0.2 points in compression.
With the right jig to hold the pistons--by the pin---its a very easy job

 

[brian smith]

Hell I'm eating diner so not thinking..The crowns can be lopped off say 016-018..All those 40-50 V4s I built using B21E (flat tops) were all cut .016 to get the desired 004 out the bore I shoot for.. The dish volume reduction would be minimal, probably less that 0.15 to 0.2 points in compression.
With the right jig to hold the pistons--by the pin---its a very easy job

Say John -
Do you aim for that 4 thou out of the hole in just the V4s, or do you like that for a b230 as well?
I've got 5, and was thinking that I wanted to take more off the deck not the head.

 

[John V outside agitator]

Say John -
Do you aim for that 4 thou out of the hole in just the V4s, or do you like that for a b230 as well?
I've got 5, and was thinking that I wanted to take more off the deck not the head.

Just in general...Its safe...cranks do flex. Rods (and bolts, especially weenie little skinny ass rod bolts) do stretch...How much? who can say? I know the zero to 004-005 out drastically reduced tendency to ping on the first motor i did it...went from a 8:1 motor with piston 2mm down the bore which pinged if given any more than about 6 degrees advance to a 10.87:1 thing that didn't ping even on Regular with 12 degrees advance...

So that works...more work better-er-er? maybe...but no tests support that, only claims that "I like it"..

Don't worry about 005 out.