Deck Height Query

Hi all

Fitting a new 9400T4 AA Piston and liner kit on my stock CU engine. My understanding is this should be like for like replacement, but I'm getting a 0.25mm deck height.

However, given these are dished piston heads, I'm not sure if the guidance in Tom Wilsons rebuild book of getting between 1mm and 2mm is relevant, as I assume that is for flat-headed piston. Basically, how do you measure deck height for dished piston heads, and what are the correct tolerances?

Not sure what deck height the originals were but they are Mahles so the format is identical.

Thanks all

 

0.25mm deck is very tight. Are you using AMC heads as the thickness of the lip in the heads usually about 0.7mm can then be added to the 0.25 as the piston clears the lip.
Otherwise you need to use cylinder base spacers to get your deck to between 0.75-1.5mm depending on your calculated compression ratio.

There are quite a few type 4 engine rebuild threads on here if you search them.
Deefer66 did a comprehensive one a few years back.

 

The guidance in Tom Wilsons book is for a completely standard engine, he doesn’t consider any non-standard parts like AA pistons.

You need to measure the volume of the combustion chamber and the dish in the piston, decide what compression ratio you want, then use one of the online calculators to work out the deck height you need and adjust the height with cylinder base shims.

Or just slap it together and hope for the best but don’t have a deck much less than 1mm or there is a risk of the piston hitting the head.

This is the calculator that I use but there are plenty of others. http://johnmaherracing.com/calculators/engine-calculator/

Incidentally, you need to measure the piston dish volume, don’t believe what AA say.

 

Is that without cylinder base shims and head gaskets?

 

Just as an example this is what I finished up with on my 2.0 build. I also used dished pistons. As 77 Westy says the first calculation is the head chamber volume and piston volume

 

@Shep Woof Woof Is this query for the engine you rebuilt back in March? https://thelatebay.com/index.php?threads/cu-engine-rebuild-ii-aka-its-a-bit-too-sticky.92051/

 

Thanks all

That's just with a standard block-cylinder gasket, no other shims as per instruction in Tom Wilson. I don't have any other shims as the old cylinders were stock with no extras.

 

Yep. Been a family bereavement so all stood still for a long time. Back on it in time for the chilly weather....

..bit the bullet and stripped it back down and started again. Looking better so far. But also bit the bullet in new pistons and cylinders, hence this new query!

 

OK, so that was the conclusion I came to as regards Wilson's suggestion. However, this now opens up a whole new conundrum on what compression and therefore deck head and thus shimps are actually required. Left all my reference books at the van so if anyone has any good guidance or sources, that would help out.

 

If you’re not getting sufficient deck height, then you will need to shim the bottom of the barrels. Easy enough to do if you can find the correct shims.

 

The CR to aim for depends on the cam spec but I don’t think you know that so you’ll probably have to assume that it is more or less standard - but you know what assuming can do.

If the cam is standard around 8.0:1 CR should work, although many will say lower.

A few of things you should know that I don’t remember if Tom covers in his book.

VW deleted the head gasket and replaced it with a cylinder base shim (Technical Bulletin 10-90 T01). A decent quality gasket set will contain both base shims and head gaskets but if you decide to use a head gasket it may leak.

If you have original VW cylinder heads (square port on a CU engine) and if the AA cylinders are exactly the same length as the original VW cylinders and if the AA pistons have exactly the same dish volume and compression height as the original VW pistons then you should be able to use a 1.6mm shim under the cylinders and the CR and deck height will be correct. As you can see there are lots of ifs.

Aftermarket heads may have a step in the combustion chamber and that effectively makes the cylinders longer and the base shim unnecessary – possibly.

Aftermarket cylinders may be a different length compared to VW cylinders.

Aftermarket pistons may have a different dish volume and a different compression height (the distance from pin to crown) compared to VW pistons.

 

Everything is standard including heads, pistons and crankshaft.

From what I can tell, a 1.6mm spacer should put me on 8:1 CR so now on the hunt for that.

Can't find it on Justkampers/VW Heritage/Engine Shop/Schofield, so is anyone able to point me in the right direction to find them, or provide a part number for it??

 

The spacer is p/n 071 101 341 and should be included in the gasket set but if you’re stuck, I have some here and I’m trying to get to the UK for Christmas and could post them if you can wait that long.

You said you were using AA pistons and cylinders; AA never supplied parts to VW, they’re not standard.

PS. I haven’t seen the spacers for sale separately but then I’ve never really looked as they’ve been included with the gaskets.

 

Thanks for that

Sorry, I meant everything is standard apart from the AA Pistons/cylinders but I was of the understanding that they are a faithful copy of the original Mahles.

I don't have and 1.4mm spacers in my engine rebuild gasket kit. The only one in there is the thin (0.14mm??) one.

So yes, I suspect I might need to take you up on that offer unless I can source elsewhere in the meantime. Hi ho, another delay!

 

If you need some sooner give Jim a call at Stateside engineering he will most likely have some.

 

Yep and will probably insult you while charging 3x the price then adding VAT and delivery. Service with a sneer. No, I don't like him.

 

Yeh, been there, seen it, done it. Not one to repeat unless really necessary.

 

Afternoon all - Measured the cc of the dishes on both the old and new pistons and they are pretty close at 54cc(AA) and 59 cc (Mahle) each.

These are my workings for the new piston dish - Inner radius of 52.3mm, outer radius of 62.4mm, depth of 5.7mm. Cone Frustrum calculator gives a CC of 54.09 cm2.

The Compression Radius Calculator gives this. I've remeasured the AA deck height at 0.33mm. The one thing I don't know for sure is the chamber volume which I assume is the cc of the cylinder head chamber and have taken from post elsewhere but I'm not sure of.

I know the AA/Mahles are advertised as low compression but does 5.2:1 seem whack! And surely adding spacers to increase the .33mm deck height will only lessen the CR!!

 

You are a decimal point out on your piston dishes.
5.4cc

 

You do need to measure the volume of the combustion chambers if you’re going this far. Easy enough to do (but messy).