Melted Piston

[Dirrt]

Afternoon Gentleman

OK so I've melted a piston on the G and the head has completely eroded away and needs welding and mending… 2 plugs are absolutely fubared like they’ve been raped 

Just to re cap, I changed the sparks back to one pins and provided it with a new lambda and blue sensor, this was then running sweeter than ever before, 3 hours later… Oil spat out the dipstick!

It has definitely been under fuelling and running lean. I need to double check the history when I get home to find out what exactly was rebuilt at 113k

What causes am I looking at here?
Weak fuel pumps and dodgy injectors?

Could you tell me please what the standard size of a piston is? Sorry, I did try the search button with fail - 75.485mm?

Is there anything else you know off, what we can try, look for and investigate?
All help, thoughts, personal experiences and opinions muchly appreciated as always.

Thank you
Laura

[PeteG40]

Wiring loom, fuel pumps and injectors need checking - PPP can check/clean the injectors, maybe one or two had a sh1t spray pattern.

Also what chip were you running for the modifications?  maybe it was too lean?

thought stock pistons were just 75mm?

[Dirrt]

Cheers Pete
Notes taken and passed on!
Must be 75mm then I'm not the one under the hood, he measured it up and told me to find out as I wasn't sure what standard size was... What other size pistons can you get mate?
Yeah he deffo reckons it was running too lean!

[vwmk3jon]

Don't sound Good L.

Im not technically minded (as many people know!) but i hope yoiu get it fixed up soon.

Jon.

[PeteG40]

Piston sizes, you can get 75 to 77mm (77mm effectively giving 1341cc) i think - check accralite or wossner sites.


Ensure your pumps and injectors are tip top and the condition of yr wiring loom.  Also I'd be tempted to get someone to code read your chip, to ensure its sufficient for the spec of the car (or buy a new decent chip!)  Its been known that people have been running stock injectors on chips for 250cc injectors, therefore running very lean!

Pete

[Dirrt]

Yeah I'm sure I've read that somewhere before Pete, I think you're right!

Will do. Thank you 

[swiftg40]

not sure about the piston meltage but oil spitting out the dipstick could be the crankcase pressurising...check/clean out the breather on the back of the crankcase

[lance]

thats a bit shit dirt! yea might as well rebore it to 1341 now. there is a few choices of pistons depending on what comp ratio you want to run

[Nick_S]

That is bad news after having already thrown ££££'s at it

Wossner do four sizes of forged pistons for the G40, all with a comp ratio of 8:1

Standard 75mm 1272cc
75.5mm 1289cc
76mm 1307cc
77m 1341cc

Chap who built my engine has an account with Wossner if you get stuck sourcing some.

[Yoof]

Worthwhile checking your coolant system for blockages in the cooling jackets, this can also cause a piston to melt.

I can clean your injectors and read the code of the chip- PM me if you want to get this done.

As a minimum I'd flow test the injectors, fit new fuel pumps, and replace the fuel filter and lift pump gauze.

Your lambda is now probably f**ked as you'll have burnt plenty of oil.

For me there's too much coincidence between you changing the plugs and this happening, what did you change them to?

Need to find out what went first, piston or plug. Have you got pictures of all 4 plugs?

I've got the following engines:

1) 80k strong engine, headgasket and cambelt done recently along with water pump

2) Fully rebuilt G40 motor, new gaskets etc done 10k made 161bhp

Both have standard heads and no ancilaries- I'd like £400 for the first one, and £500 for the second.

Cheers

Pete

[Dirrt]

Good news
I got some good luck for once but I'm not holding my breathe, I always expect the worse with me and this car There are no marking on the bores so replacing the piston for now is gonna do the job
I don't have the money to do anything else yet, so boring it out will have to wait until after Christmas where I shall be starting a new G fund and putting money by for lots of things. It's a long process but if the G's running whilst I'm saving, I'm happy  

My old G guy (that I won't be using again) didn't put the Lambda sensor in properly, Matt was able to unscrew it with his fingers, should it be like that? Could that have caused the melt from wrong readings? The sensor itself is fine though and not damaged!

Everything else is being checked...

Right, where's the best place for me to source a 75.5mm standard piston (.5 oversize and not forged as it wont match the rest!) and how much should I be paying for one?

[Dirrt]

I think you read something wrong? I'm female and even I'm not that stupid, they are all this size 

[grayg40]

if you are only replacing 1 piston you will have to weigh all pistons and make them all the same weight for balance reasons. you will have to find a non structual part of the inside of the piston to buff away, plus hone the bore to get rid of the shine.

[Dirrt]

Erm... If the pistons are all 75.5mm and the same make why would they be out of balance 

[PeteG40]

if they all are 75.5mm then it would suggest an overbore at somepoint (perhaps an issue like this in the past!)

and therefore if a non-stock piston size (if they are vag oversize pistons) then back to vw -vagcat says about 165 euro - similar £165 i'd expect per piston.