26 replies to this topic

[jessieparsons1]

ok here it is, just gto my self a 73.5mm rebore and when i was looking over how to buildd it back up i found somthign refering to deck height, there was not much information and i jst wounderd what this was on about and what values i need to obtain before i go ahead with skimming anything off the block..!

and if any one knows anything please could they help me.... thank u x

[Rosslin Racing]

put the crank in and rods/pistons without rings. you will see how far the piston is from the top of the block (deck) depending how much compression you want you can remove from the deck, "have the block skimmed". but beware it depends on what sort of pistons you are using as to how much you can remove. and dont spin the crank with out oil.

[mini_turbo_pete]

put the crank in and rods/pistons without rings. you will see how far the piston is from the top of the block (deck) depending how much compression you want you can remove from the deck, "have the block skimmed". but beware it depends on what sort of pistons you are using as to how much you can remove. and dont spin the crank with out oil.

And make sure you what you measure it in is made clear to your machine shop...

One of my mates years ago measured his block and wanted 1mm taken off the block, they took 100 thou off (thats about 3mm) He got it all back started to build it then realised what had happened... he then took a big file to his pistons and filed them down to deck height.. lol.. the engine rean really well considering..

[Ethel]

Deck height is just the distance between the top of the pistons and the head gasket mating face of the block at TDC. You want the pistons, particularly at their edges, to always stay inside the bore, bearing in mind that conrods and cranks stretch a little when the revs go up. The deck height will also make up part of the combustion chamber volume so you need to know what it is to calculate your compression ratio....

or how much to file off your pistons!!!

Edited by Ethel, 19 March 2008 - 11:56 AM.

[pogie]

The deck height will also make up part of the combustion chamber volume so you need to know what it is to calculate your compression ratio....

No, you're getting deck height mixed up with unswept volume. As said, the deck height is the distance the top of the piston is from the top surface of the block when the piston is at TDC.

Unswept volume is

Volume of cylinder created by deck height +
Piston dish volume +
Compressed head gasket volume +
Combustion chamber volume.


To get compression ratio, you add these together and divide total into the volume of 1 cylinder of your engine.

Edited by pogie, 19 March 2008 - 12:15 PM.

[Ethel]

Pogie,

You've just disagreed with me then gone on to explain how what I said is correct.

"Unswept volume is

Volume of cylinder created by deck height +"

[pogie]

Pogie,

You've just disagreed with me then gone on to explain how what I said is correct.

"Unswept volume is

Volume of cylinder created by deck height +"

No you said

"The deck height will also make up part of the combustion chamber volume so you need to know what it is to calculate your compression ratio...."

The volume of the combustion chamber is the volume of the head and completely separate from the volume created in the cylinder by the deck height.

Edited by pogie, 19 March 2008 - 12:32 PM.

[Ethel]

Well I'd consider everything that that makes up the space where combustion is initiated to be the combustion chamber. It's possible to have a cylinder head with no volume and pistons that protrude in the combustion chamber volume of the head.

Yes, cylinder heads have combustion chambers but that doesn't mean all combustion chambers are in cylinder heads.

[pogie]

Well I'd consider everything that that makes up the space where combustion is initiated to be the combustion chamber. It's possible to have a cylinder head with no volume and pistons that protrude in the combustion chamber volume of the head.

Yes, cylinder heads have combustion chambers but that doesn't mean all combustion chambers are in cylinder heads.

I see what you're saying, but when you are calculating CRs and what deck height you need you will find that the term combustion chamber only refers to the volume of the chamber in the head. If someone was to use your definition of combustion chamber and then look at a tuning manual (Vizards is one) they may well end up getting their calculation wrong. Basically what I said in my first reply stands, you are getting combustion chamber volume mixed up with total unswept volume.

Written English isn't my strongest point and I hope I am getting my message across, but a picture paints a thousand words so here's an CR calculator that shows what I am saying - http://www.rbracing-...staticcalc.html

Here's another calculator that will let you use metric for bore size - http://www.csgnetwor...m/compcalc.html

[Ethel]

It's only semantics, if someone follows your advice they'd end up with a correctly built engine so I've no real argument with it.

I can't quite see how they'd have a problem if they they took my definition (might even save them some confusion - if they were building an O series, for example.

[pogie]

It's only semantics, if someone follows your advice they'd end up with a correctly built engine so I've no real argument with it.

I can't quite see how they'd have a problem if they they took my definition (might even save them some confusion - if they were building an O series, for example.

OK, imagine you have done a dry build of your engine so you can find out what deck height you have. You measure the height and then strip the engine down and take the block to a machine shop to get the final deck skim done. If you were not sure how much to have removed but knew what CR you wanted and gave the shop the volumes of the piston dish and gasket and then gave the volume of the combustion chamber using your definition it would lead to too much material being removed.

I have been working out the build for my engine and the if I gave the total unswept volume (37cc) as the combustion chamber size instead of the actual volume (20cc) the machinist would have real problems getting the 10.5:1 CR that I am planning on having. In fact the pistons would end up sticking out of the block by a couple of mm, which as we know doesn't work with Mini engines.

Edited by pogie, 19 March 2008 - 03:25 PM.

[haz]

is it worth putting all pistons in and measuring the TDC deck height of each or just say pistons 1 & 3... or just any?

[Ethel]

is it worth putting all pistons in and measuring the TDC deck height of each or just say pistons 1 & 3... or just any?

Lol, back on topic

Yeah check 'em all - if you're building an engine you can't overcheck anything

Edited by Ethel, 19 March 2008 - 10:59 PM.

[pogie]

is it worth putting all pistons in and measuring the TDC deck height of each or just say pistons 1 & 3... or just any?

It depends how far you want to go. I intend to measure the deck height of each piston and unless they are miles out I will probably go with the average and get the block machined down to suit. Hopefully modern production techniques will mean that the pistons should be pretty well matched.

The ultimate would be to find out which piston is lowest at TDC, then you would machine the tops of the other 3 pistons to take them down to the same height as the lowest and then you would have the block skimmed to suit.

Edited by pogie, 19 March 2008 - 08:40 PM.

[pogie]

Lol, back on topic

Yeah check 'em - all if your building an engine you can't overcheck anything

Ah you're back. I'm not stalking you, honest. Your reply popped up as I was typing mine out.