1800 8v piston advice again please

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Dayknight-VW
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1800 8v piston advice again please

Post by Dayknight-VW »

Howsit guys…

Ok so Im almost done with my build from 1.4 to 1.8. Bottom end is assembled and everything looking good.

I’m just a bit confused regarding the piston height.

I got the 1800 piston with a slight hump on one side of the piston. I wanted to pick up compression ratio a little so I decked the block 0.2mm. The flat part of the piston sits slightly below the surface of the block, basically inline with the bevel on the top of the cyclinder, I assumed this flat part will protrude 0.2mm out of the block after decking it but it doesn’t. The squish area protrudes a lil out of the block.

Is this normal or is there something not right here? I’ve got standard rods, a 1800 crank and a new set of those pistons.

Can I possible spilt the metal head gasket and use the thin layer only to bump the compression up a bit. Trying to get about 10.8 or 10.9 compression ratio.
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missioner
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Car Model: '87 MK1 Golf Citi

Re: 1800 8v piston advice again please

Post by missioner »

DO NOT INTERFERE WITH THE GASKET.

Take it from me high compression isn't as fun as it sounds.

You will need to do something to get your ignition system to work, aka ditch the dizzy.
Second imagine spending R250 to R300 extra every tank for OB.

Your plan with the gasket will end badly.

Remember that the gasket will compress to somewhere between 1.1 and 1.2 mm when the head is torqued down. If the squish pad protrudes more than 0.4mm the pistons WILL touch the head at high RPM. This is because the rod stretches due to piston inertia.

I made this mistake and had to redo my clearances. Rebuild the engine a month after rebuilding it. It sounded like it was permanently pinging.

Let's do some simple maths,

Gasket compressed thickness = 1.1mm
Piston protrusion height = 0.4mm (at the top of the squish pad)
Conrod stretch = up to 0.5mm
(Don't believe me look it up)
Equals 0.2mm clearance. Just enough for heat expansion of the piston to remove.

Ideal clearance is around 0.8mm, or more, from cylinder head to piston at its maximum height, to be safe.
You could get away with 0.7, but less than that will result in issues.

Just to repeat the gasket is very much NOT the place to look at for "solving" a compression issue.

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'87 Golf 1 Olde Skewl OEM+
Dayknight-VW
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Re: 1800 8v piston advice again please

Post by Dayknight-VW »

Thanks a lot. U really saved me from messing up as I’m on a tight budget. Can’t really afford to rebuild the motor again.

But I’m still puzzled as to why is the flat part of the piston sitting slightly below deck height? Is this normal?
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Re: 1800 8v piston advice again please

Post by missioner »

Dayknight-VW wrote:Thanks a lot. U really saved me from messing up as I’m on a tight budget. Can’t really afford to rebuild the motor again.

But I’m still puzzled as to why is the flat part of the piston sitting slightly below deck height? Is this normal?
Perfectly.

A 1.8 still runs at 10.5ish to 1 compression ratio. It's plenty for 95RON fuel.

My 1.9l was slightly over 11:1 and I couldn't drive the car at the coast without retarding the timing to 1°BTDC. Absolute PITA. At home it was better but anything more than 6° was ping city. EFI and ethanol would have been the solution, but funds did not allow.

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'87 Golf 1 Olde Skewl OEM+
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