Maximum bore width using OE pistons + ideal ring gap (hybrid build)

[VAG-Slag]

I'm hoping to get my block back from the engineers soon to I can get cracking with this ****** build!

The guy doing the honing phoned me today and said that there's already a lip of AT LEAST (hasn't actually measured it accurately yet) a thou on the bore from wear and even a light hone will take off about 8/10 of a thou... The rings they've ordered me are pre-gapped (mahle), and by the time he's honed the bores he rekons ring gap will be about 27 thou, which he says will lead to leaky bores, high crank case pressures, smokey running and all that lovely stuff

He rekons my best option (and i know he's right) would be to go for an over sized piston and over bore... Which I REALLLLLY don't want to do!!! Mainly cos of the monies $$$'s

So I'm thinking of using a ring that isn't pre-gapped, gapping them my self and still using the AGU pistons I have here...

So as the title suggests, my real question here is what is the maximum acceptable bore size/piston-wall clearance for a hybrid build like this. And what would the ring gap ideally be?

I know I've asked a similar question before but didn't really get the answer I was looking for... I also appreciate that there's loads of information on tolerances on elsawin/internet but as I understand they don't really apply when running high boost and getting the most out of an engine. . .

Brains really in a bit of a pickle over this one, if anyone in the know has some input that'd be very helpful!

thanks in advance guys

 

[superkarl]

Most specs out there are for aftermarket Pistons specifically JE, it does vary dependant on the type of alloy as well. I'm not sure of the specs of oe Pistons.
JE state a 0.004" piston - wall

Ring gap also varies upon application, up to 0.30mm. Mine compression rings are at 0.23, not sure about anyone else's. Might be work searching threads like tufftys or Stacey's.

 

[Broken Byzan]

Most specs out there are for aftermarket Pistons specifically JE, it does vary dependant on the type of alloy as well. I'm not sure of the specs of oe Pistons.
JE state a 0.004" piston - wall

Ring gap also varies upon application, up to 0.30mm. Mine compression rings are at 0.23, not sure about anyone else's. Might be work searching threads like tufftys or Stacey's.

Thats a very small gap JE state .4 ish from memory

 

[superkarl]

That's inches not mm. Stupid Americans, always confuse me, can't skim read these things as you never know what units they're using

 

[Broken Byzan]

Ah ok was just going on you stating 0.30mm so assumed the 0.23 was also mm.

 

[superkarl]

Ah ok was just going on you stating 0.30mm so assumed the 0.23 was also mm.

Yeh what a hypocrite lol

 

[badger5]

measure ring gap on new rings first. then you will know

 

[VAG-Slag]

measure ring gap on new rings first. then you will know

Well he said to me that the ring gap on the pre-gapped mahle rings was already 24 thou (about 0.6mm) and he rekoned that after honing that ring gap would be more like 27 thou...

I suppose I could ask him to work out what the the piston/wall clearance is likely yo be after honing and go from there... If that's within acceptable tolerances then order some ring that aren't pre gapped and then gap them out selves?

 

[Broken Byzan]

That's a huge chasm of a ring gap. What's the bore dia top middle and bottom . That's more important than worrying about gaps currently. If the bores are too worn it'll slap

 

[VAG-Slag]

That's a huge chasm of a ring gap. What's the bore dia top middle and bottom . That's more important than worrying about gaps currently. If the bores are too worn it'll slap

Yea, that's what I was thinking... Hence why I suggested getting him to work out the piston/wall clearance

But you raise my original question again. . . . What actually is maximum acceptable bore size on a build like this when using oe pistons!?

 

[<tuffty/>]

http://workshop-manuals.com/audi/a3...nd_conrods/pistons_and_conrods_exploded_view/

http://workshop-manuals.com/audi/a3...s_and_conrods/piston_and_cylinder_dimensions/

<tuffty/>

 

[Broken Byzan]

I was just gonna say. Gimme half hour ill boot my vm up with elsa on it. Nice one pt

 

[VAG-Slag]

Cheers tuffty, but (and correct me if I'm wrong) I'm certain I recall a conversation on here ages ago between Bill and someone else basically along the lines of - Bill saying that a block was feked cos of it's bore size, someone pointing out that the bore ware was within tolerance, and bill saying those tolerances were fine for a standard engine but no good when trying to contain 300+ bhp. . . .

No?

 

[badger5]

no - indeed

context sensitive statement

 

[VAG-Slag]

no - indeed

context sensitive statement

Ok, thanks for clearing that up for me Bill... So it's safe to go of manufacturer/elsa tolerances then!

In which case I'm gonna go with my earlier plan: get him to work out what piston/wall gap is likely to be after honing (top and bottom), and if it's within tolerance order some rings that aren't pre-gapped thanks fellas, will let you know how I get on

 

[badger5]

elsa vag tolerances are sound yea
a light hone/deglaze would'nt remove 2 thou off the wt tho..

 

[VAG-Slag]

Ahhhhhhrghhhhhh!

This is doing my swede in now... So they're saying that block is feked and it's either a re-bore and over sized pistons or a new block. . . And I'm certainly not spending 400 quid on pistons that's for sure!

It's ok though, I think I've found another block fairly locally... Back to the rings though, am I right in saying that most rings come "pre-gapped"? Seems mental if that is the case, what rings have/do other people use?

 

[superkarl]

I used Goetze rings that Phil Miller got me. I don't know where from tho

 

[VAG-Slag]

I used Goetze rings that Phil Miller got me. I don't know where from tho

Yea, that's what I was initially planning on using... Can get em pretty cheap from Germany:
http://www.ebay.co.uk/itm/281307427539?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Did they not come pre-gapped then? . . Also, are you running OE pistons as well?

 

[Broken Byzan]

They provided you any dimensions of said unusable bores!?

 

[VAG-Slag]

They provided you any dimensions of said unusable bores!?

No, he basically said once they take any more metal off they'll be piston slap and they won't even be able to hone properly with the lip that's there... I did ask him to give me some more accurate measurements but he said it wasn't worth me paying for the time to do so

I know these guys know what they're doing but I get the feeling they're perhaps being a bit pedantic/OCD if anything. . . As i said I've found another block, might take it else where for honing though

 

[superkarl]

Yea, that's what I was initially planning on using... Can get em pretty cheap from Germany:
http://www.ebay.co.uk/itm/281307427539?_trksid=p2060353.m1438.l2649&ssPageName=STRK:MEBIDX:IT

Did they not come pre-gapped then? . . Also, are you running OE pistons as well?

I honestly don't know lol. The gaps were fairly large and within tolerance. I didn't need to remove very much at all, I can only assume they were.

 

[<tuffty/>]

Ahhhhhhrghhhhhh!

This is doing my swede in now... So they're saying that block is feked and it's either a re-bore and over sized pistons or a new block. . . And I'm certainly not spending 400 quid on pistons that's for sure!

It's ok though, I think I've found another block fairly locally... Back to the rings though, am I right in saying that most rings come "pre-gapped"? Seems mental if that is the case, what rings have/do other people use?

For std rings in std setups then pre gapped makes perfect sense... they are supplied assuming std bore and wear characteristics....... bores don't wear evenly... if you have a tighter gap at the top to account for wear chances are they would bind further down the bore... you need to check ring gap at points all the way down the bore and gap to the tightest point (typically at the bottom)

<tuffty/>

 

[VAG-Slag]

I honestly don't know lol. The gaps were fairly large and within tolerance. I didn't need to remove very much at all, I can only assume they were.

Mmmmmm, makes me think as they said the gap with the new mahle rings was already 24 thou (bout 0.6mm) as it is... Maybe pick up this other block and see how it goes

 

[VAG-Slag]

For std rings in std setups then pre gapped makes perfect sense... they are supplied assuming std bore and wear characteristics....... bores don't wear evenly... if you have a tighter gap at the top to account for wear chances are they would bind further down the bore... you need to check ring gap at points all the way down the bore and gap to the tightest point (typically at the bottom)

<tuffty/>

Well based on that then the ring gap of 24 though before they've taken any metal off the bores would make about sense that the bores are FOOOOKED then...

 

[badger5]

engines do wear out

 

[VAG-Slag]

engines do wear out

What, even VAG engines? . .

 

[antwan64og]

Or you could get it bored out to 81.5mm (for 1803cc) or 82mm (for 1825cc) and then get Wiseco pistons for under £450 posted from Germany in 81.5mm or 82mm.

Allegedly VW do 81.5mm 1.8T OEM mahle pistons for cars that need boring after cylinder problems but I can't find a part number for them.

 

[VAG-Slag]

No Antwan, just no... I do not need an excuse to spend another 500 quid on this motor!

And I did actually phone TPS and asked if they could supply over sized pistons but they weren't able to... Or least they said they couldn't anyway!

Hopefully this other block will be good, apparently it's got super low miles on it. . . Third time lucky ey!?

 

[<tuffty/>]

If boring the block and fitting new pistons it would be rude not to get a stroker crank as well ;P

<tuffty/>

 

[badger5]

What, even VAG engines? . .

i know.. shocking is'nt it

hehe

 

[VAG-Slag]

If boring the block and fitting new pistons it would be rude not to get a stroker crank as well ;P

<tuffty/>

Don't you start as well!

 

[<tuffty/>]

Don't you start as well!

<tuffty/>

 

[VAG-Slag]

 

[<tuffty/>]

Don't just point to it in awe... get it done!! ;P

<tuffty/>