Deck Height variation tolerance

[Gnasha]

Newbie to VWAC. Stock 2.0 type4, refurbishment

New bearings on Crank
New top end bushes
Case decked
Cylinder faces equal.
Piston heights, pin to crown equal.
Deck heights measured with micro depth gauge.

Measure above the pin front and back

#1 0.002" High at back
#2 0.004" High at front
#3 0.007" High at back
#4 0.007" High at back

Does anybody know if there are tolerances in this area between front and back of each piston?

Thanks for reading this.

[Clatter]

With variation, Less is Best..

You are bound to find some differences in parts, they will always have some variation, question is how much.

Variation that causes the heads to not lay flat across the cylinder tops are the worst IMHO,
As you want the heads to seal tight.
This would be variations in deck height, head fly-cut, and -especially- differences in cylinder length.

Piston pin height, or distance from pin to crown, is typically pretty tight tolerance-wise.
most piston manufacturers have this right.
The variation usually comes when you are measuring and the piston rocks in the bore.
Because the piston needs clearance to the cylinder wall, this rock can amplify the number when deck height is measured.

Rods having some variation in overall length is common,
Especially if they have been rebuilt.

Cranks have been known to have differences in stroke between rod journals as well.

Best practice is to move the parts from hole-to-hole during assembly,
And follow which trends follow which parts.

If you can, swap parts around to get the variations to stack up and minimize differences.
Ideally, you can switch things around to get deck height the same at TDC on all four..

Good luck!

[Gnasha]

Can I assume 1&2 are good to go at 0.002" and 0.004"?
Measurements taken at 90Degs to the pin on all pistons are within 0.001-0.003 of each other.
I have swopped cylinders, pistons, and tried new pistons and cylinders and the results are the same in positions 3 & 4. Con rods removed and checked for twisting and bending both ok.


Thanks again.....................

[GARRICK.CLARK1]

I had this with my AA piston kit. You will have to trim the longer cylinders to match the shorter cylinders, you want them to be as exact as possible, then trim the piston crowns to get an equal deck height on all 4 pistons. Takes ages

[Jesper_P]

I have exactly the same issues with at set of 96x66 biral p&c set from AA pistons ..deck height varying from 174 til 195. It really is annoying.

[sideshow]

I'm curious have you measured the cylinders off motor like on granite table and a tool (https://en.wikipedia.org/wiki/Height_gauge) combustion side down to the case seating cut?

Also the pistons top to pin bore, you could probably do this by pushing a wrist pin 50% into another one of the set and try to lay both flat.

If that explains nothing, taking off the rods and use one wrist pin in two or more rods to quickly hold them together and compare distances to crank journal edge.

[raygreenwood]

This is actually common. Working on one like this now. If you measure with a dial depth gauge....the main bores in each case half.....you will most likely find that the crank bore was offset towards the 1/2 side.
How much difference this makes will depend on everything else. I used a diamond lapping plate to work the cylinder seating areas down .003". Yes....I could have sent it in to have it decked....but after a few similar issues a few years back......I dont trust most shops I have access to. The front to rear tilt was 0. They were within .001"-.002" front to back.....the crank is just off-center.

Ray

[rwk2]

Here are some pics of checking a case as I do it, you need a surface plate parallel blocks and some indicators, basic toolmaker,machinist stuff.
Pics (last) show flat decks checked to case split, (most important) parallelism of crank bore to case split, and parallelism of crank bore to decks, pics are of T1 case and an aftermarket case (.004 out) but principle is the same with T4

IMG_4525.JPG

IMG_4524.JPG

IMG_4518.JPG

IMG_4516.JPG

IMG_3802.JPG