Type 4 2.0l Deck Height and Compression Ratio

So after a very long day looking down a barrel. The difference in measurments in deck height of #4 piston above the gudgeon pin is due to movementtt back and forth (piston tilt) in line with the pin, eg flywheel to pulley direction. I had a straight edge across #3 and #4 and its perfect. ALL pistons have this movement, I tried a feeler gauge down the sides (90deg to pin) of each piston and a 0.004" was a snug fit. I did measure during the build using plasti-gauge the botom end clearances, ranging from 0.002"o.oo25". I swapped pistons, slight change in results measured but still movement from flywheel to pulley direction.

Both halves were decked.

I have a set of new AA P&C's im going to try one of those in #4 position, I also have some used pistons I'll try those.

Currently fittted are second hand pistons none VW, VW barrels have been honed and measured ok

I'm wondering if the top of the pistons have excessive clearance allowing a "tilt" of 0.006"

If I was to take account of the tilt, meaning I take 2 measurments above the pin at either end rather than one I get the following results after averaging.

#1 0.021" #2 0.015" #3 0.012" #4 0.016


Results taken @ 90degs to the pin and allowing for a "normal" tilt and taking two measurements on each side are as follows after averaging

#1 0.02" #2 0.015" #3 0.011" #4 0.015"

 

It’s not possible for the piston to tilt along the length of the pin – the pin is a sliding fit with near zero clearance – so what you are saying is the piston and rod are tilting on the big end journal.

Rotate the crank a couple of times pushing the crank towards the flywheel (thrust face touching the bearing) the rods should then be at right angles to the crank and the piston at right angles to the rod and the deck should be the same, or very close, from one end of the pin to the other.

 

Ill try that. good thinking.

So yesterday I measured across the top of 3 &4 cylinder face's, perfect, full contact across all 4 faces.

I removed 3 & 4 pistons and cylinders and used a straight edge across the decks, perfect full contact across the faces and no rocking

I fitted two old Mahle pistons in 3 & 4 cylinders, similiar movement across the top of the pin and surprisingly still good clearance between P&C. 0.003"

I then fitted a new AA piston and cylinder in position No3 clearance to P&C 0.002". However, it still "rocked"/"tilted" the same as the Pistons I'm intending to use, to within +/- 0.001/2" above the pin.

Cylinder heights, #1 91.3mm #2 91.4mm #3 91.3mm #4 91.4mm

Pin fit to the pistons and rod bushes are too tight and will need to be opened up slightly.

thank you to all who are still reading this epic tale.

 

That sounds good, but you said - Cylinder heights, #3 91.3mm #4 91.4mm

I really don’t see how that is possible; tilt of the rod on the crank is minimal, maybe a couple of tenths, at most, measured along the pin.

My AA Biral cylinders were all exactly 91.79mm.

Not too much, they should be a push fit and shouldn’t drop through.

I’m still reading but I think I’ve lost the plot. Is this an engine with new AA pistons and barrels and a Scat C35 cam? Or are you just building an engine with some random parts?

 

lmao thanks for hanging in there. This is the stock engine, nothing fancy basic 2.0 T4 rebuild with decent parts.

 

That sounds good, but you said - Cylinder heights, #3 91.3mm #4 91.4mm , Yes I need measure again, I only have a vernier, i used the Chinese one.......these were VW used cylinders



I really don’t see how that is possible; tilt of the rod on the crank is minimal, maybe a couple of tenths, at most, measured along the pin. Could it be the distance from the rod on the crank ?


My AA Biral cylinders were all exactly 91.79mm.

Not too much, they should be a push fit and shouldn’t drop through. yes i agree, mine are currently a firm push fit, certainly not a floating clearance.



I’m still reading but I think I’ve lost the plot. Is this an engine with new AA pistons and barrels and a Scat C35 cam? Or are you just building an engine with some random parts?

lmao thanks for hanging in there. This is the stock engine, nothing fancy basic 2.0 T4 rebuild with decent parts.

 

If the rods are a different length there will obviously be a difference in deck between cylinders but not along the length of the pin.

Just looking at the numbers there are a couple of problems, deck differences between cylinders is one but the biggest problem is the difference between one end of the pin to the other. One cause of this is cylinders out of true, another is bent rods. You have eliminated the cylinders, so...

Have you used a matched set of rods or ones from a couple of engines? And did you have them checked for twists and bends?

Are the pistons a matched set with the same compression height (pin to top of piston)?

Swap cylinder #2 with #3 so you have cylinders the same length on each side of the engine. And if the decks are uneven one side to the other you may have to swap the cylinders over – but you haven’t got that far yet.

 

back,

work done,

Measured cylinder heights....properly... as follows:

#1 91.73mm
#2 91.74mm
#3 91.73mm
#4 91.73mm

I dont see any gain moving these, except to eliminate other problems.

Measured from piston crown to the TOP of the gudgeon pin as follows:

Two results for each piston, either end of each pin. mm

#1 23.754, 23.752
#2 23.758, 23.755
#3 23,754, 23.755
#4 23.753, 23.754

I dont see any problems

Took another set of deck height readings using your method with crankshaft hard up to flywheel end as follows: (using depth gauge)

#1 0.019" - 0.022" delta 0.003"
#2 0.018" - 0.017" delta 0.001"
#3 0.021" 0.016" delta 0.005"
#4 0.021" - 0.015" delta 0.006"

These are much better, and I think allowing for minor discrepancies in taking measurements are near to acceptable?

Would it be lightly two con rods 3+4 could be out of aligment?

would you eventually machine piston tops to correct the deck height?

I have inspected the other crankshaft with rods attached, the numbers all match and are facing up. I also tried a gudgeon pin through two adjacent con rod bushings and they both are a push fit.

So the set I'm working on is a complete set.

Gasps...........................

 

Cylinder lengths are OK. Although #2 is slightly long and you could lap the top face on a flat plate. I assume you haven’t (or don’t intend to) lap the cylinders to the heads?

Piston compression height is OK.

The deck height is all over the place, particularly #3 and #4 and my guess is the rods are bent or the small end bushes have been badly replaced. Bear in mind that if the deck isn’t equal from one end of the pin to the other the piston must be tilted in the barrel.

Taking a mean of the deck dimensions I’d leave each piston/barrel where they are and, if necessary, adjust the barrel shim thickness to equalise the deck from one side of the engine to the other.

 

Sounds to me to be poor measuring equipment and technique. Digital callipers (not vernier) are +- 0.1 mm
Lapping heads to barrels is an engineering impossibility

 

But better than not doing it at all and lapping one component to another has been a standard engineering practice for 150 years, or more.

 

I should have added the

Thank you.

For peace of mind I will remove one rod 3 or 4. I will fit it to an already assembled crankshaft (removing one rod) and insert a pin through the bush into the adjacent rod. Allowing for the bottom end rod clearance this will prove if the rod is twisted, bent and prove small end bushes are true, but I'm not sure about the length??? Do you agree?

What would be the best method for a man in a garage to check length? Would measuring distance from bottom internal face of small bush, to the top of bottom end bearing with the bearing in place, prove anything at all?

Eventually when the time comes I will lap the cylinder to the head, not primarily to seal it, but just to clean the faces as a precaution. I'm thinking the head being aluminium will "mould" itself to the shape of the cast cylinder under torque if the heads are true to both each other and the head.

So if I understand you correctly I'd shim up 1, 3 & 4 to suit No2 at 0.001"? I'm asssuming this would this be taken into account when the desired CR and the final deck height clearance were calculated? However, would this not misalign the cylinder heights to the head?
#1 0.019" - 0.022" delta 0.003"
#2 0.018" - 0.017" delta 0.001"
#3 0.021" 0.016" delta 0.005"
#4 0.021" - 0.015" delta 0.006"

Thanks again


PS
I think during the deck machining 3&4 had less taken off than 1&2, in hindsight I should have asked for the dimensions before and after

 

That's not true .. lapping two non flat faces does not make them flat or square ….

you would lap them both separately using a lap table or failing that flat glass …

One being aluminium and the other cast its never going to work and you would never get both barrels and heads square

 

Moving on.....

I removed #4 con rod and attached it to a fully built up crankshaft (after removing one rod) I pushed a gudgeon pin through#4 and straight through to the adjacent small end bush. I think this will prove the #4 is not twisted or bent.

I laid the rod on a marble flat plate with no rocking on either face.

Refitted the rod inside the casing and decided to take aditional dimensions.

I measured each piston at its center in both the TDC and BDC positions.


TDC BDC Stroke" MM
#1 0.245 3.037 2.792 70.92
#2 0.242 3.036 2.794 70.97
#3 0.238 3.031 2.793 70.94
#4 0.242 3.031 2.789 70.84

I did measure, for what is worth, the dimension from bottom of #4 bush to top of the bottom end bearing (removed).

The result was 92.40 and 92.41 either side.

I'll take the same measurements from a con rod on the other built up crankshaft and compare.

 

Except that there is the big end bearing clearance.

If the rods aren’t bent, the case deck is flat, the cylinders are parallel top face to bottom face and the pin is at a right angle to the piston top why is the deck height so uneven? #4 has 0.006” difference one side of the piston to the other and if the top is tilted the piston must be tilted in the cylinder – tight at the crown on one side and tight on the skirt on the other.

 

@Gnasha I’ve followed your other posts on The Samba and STF; it’s interesting that there are several interpretations on what deck height is and where and how it should be measured.

IMO measuring deck height at right angles to the pin is somewhat meaningless because the piston is free to tilt within the cylinder one way or the other and measuring just in the centre of the piston doesn’t indicate if the piston is tilted. I’m not convinced that everyone is reading or understands what you’re asking but bent rods is mentioned a few times – and you read it here first. It’s a good idea to reverse a piston on the rod to see if the tilt changes from one end to the other.

 

I put more faith in the just above the pin readings. The others are more for a check on clearance. A Westy says spinning the rod around will give a good indication of a bent rod..

Some of our friends over the water are a bit more OCD but I'm a bit rougher than some and if it's not out of spec on where Bentley measures I'd go with it..

it might make less total mileage but most likely only needs 50k max miles before it'll be out to change something else! Or even swap for electric

Sent from my SM-T580 using Tapatalk

 

The biggest problem is the 0.006” tilt on the pin/piston. Turning the piston on the rod would indicate a fault with the piston if the tilt goes from one direction to the other.

 

Thank you both for your tenacity, much appreciated.

Deck heights: Pistons and con rods in correct postion, (piston arrow facing the flywheel, and rod numbers facing up when fitted)

#1 0.019" - 0.022" delta 0.003"
#2 0.018" - 0.017" delta 0.001"
#3 0.021" 0.016" delta 0.005"
#4 0.021" - 0.015" delta 0.006"


Deck heights: #1&2 Pistons and con rods in correct postion, (piston arrow facing the flywheel, rod numbers facing up when fitted) #3&4 rods reversed.
Two sets of readings for each piston, first reading of each set is taken closest to flywheel, Crank pushed towards flywheel and rotated several times to its natural alignment

1st set 2nd set
#1 0.019" - 0.022" 0.020" - 0.022"
#2 0.018" - 0.019" 0.019" - 0.020"
#3 0.014" -0.013" 0.013" - 0.012
#4 0.021" - 0.021" 0.017" -0.017"

#4 first reading appears to be not at TDC


All piston-pin to crown heights are good, brand-new cylinder & piston tried in #4 position, little or no change in deck height, used cylinder and piston in #4 position, little or no change in deck height.

So apart from the issue of #3&4 rods reversed, these look good to go. What damage if any would this change con rod position cause when running?

I'm going to re-check the case deck heights before I make a call.

Thanks again

 

I’m not sure what tolerance or dimension you’re looking for. Is it the difference between cylinders or the difference between one end of the pin and the other, or the difference from one side of the engine to the other? Differences between cylinders could make a difference in CR, but only if the head combustion chamber volumes are exactly the same. Work out the final CR with the cylinder spacers you’ll use and you might find the deck difference is insignificant. And the spacers you need might be thicker on one side of the engine (I have 2.26mm spacers on 1&2 and 2.31mm on 3&4 to equalise the deck and CR).

The #3 and #4 delta seems to have disappeared by swapping rod positions (if that’s what you’ve done), most odd but if it works leave them where they are. Just as a test turn #1 piston and rod so the arrow faces away from the flywheel – what happens to the deck delta?

Type 4 rods are symmetrical and could be fitted either way up and on any big end journal. Incidentally, if the pistons have direction arrows the pin is offset to the piston c/l and they must be fitted with the arrow towards the flywheel – most aftermarket pistons do not have offset pins and can usually be fitted in either direction.