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Low Comp Head Machining

Started by fenz , Aug 24 2016 09:52 AM

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#26 _oldjohnno_

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Posted 05 September 2016 - 08:37 PM

Sorry, I misunderstood. To know how much to remove you first have to know the area of the opening or bowl in the head. And as it turns out there is an old-school method for measuring the area of an irregular shape that is easy and accurate and once again involves kitchen scales.

First you need some material that is easy to cut but also has some weight to it - lead flashing would be good, or maybe some rubber sheet, heavy gasket jointing or something like that. First cut out a square 10cm x 10cm and weigh it, then write the number down.

Next cut a piece that fits neatly into the chamber opening and weigh it. We know that the square piece had an area of 100cm² so if the shaped piece has a weight of say 60% of the square then the area will also be 60% of the square, or 60cm². With me so far? In this case if we machine off 1mm (a tenth of a cm) then we'll reduce the volume by 6cc, or a tenth of the area in cm². In other words for each millimetre you remove you'll reduce the volume by a figure equal to one tenth of the chamber area.

It actually doesn't take long to do and will tell you exactly how much to machine off for your particular head. I think the method was originally used to measure the area of irregularly shaped areas of maps.

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#27 fenz

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Name:Aaron
Location:South Australia
Car:LJ 2 door and UC V8 4door
Joined: 26-July 09

Posted 05 September 2016 - 09:56 PM

Yep I get where your coming from.

Have some 3mm rubber gasket that might do the job.

Think I will still try to get a measurement using water and a depth micrometer (if anything it will be a comparison)

The old measure twice and cut once.

I understand that casting irregularities from the factory will make each head slightly unique in small amounts and I will go through and cc all chambers to see what the difference is and correct if needed.

Plan on getting as much as I can sorted with this head while it is off the motor.

Not sure on the history of yella terra heads but would have thought that they saw more low comp heads than high???

This motor was a reasonable street motor before so with a few more tweaks hopefully it will liven it up some more.

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#28 fenz

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Name:Aaron
Location:South Australia
Car:LJ 2 door and UC V8 4door
Joined: 26-July 09

Posted 06 September 2016 - 08:43 PM

OK did some measuring using the above methods and came up .040" off standard 55cc to bring it back to 50cc.

The measure of the insert in the chamber came up at 50% of the known weight so 5cc equates to 1mm (0.040")

Using depth micrometer measuring down to fluid level came out at 0.033" so pretty happy to settle at 0.040"

They may not be perfect methods but in this way I get a good idea of where I am at and how much I can safely remove with out stuffing the head for future builds.

Final compression ratio is still lowish 9.2:1 but if I remove engine at a later date and zero deck I wll be around 10.2:1.

Will still check volume of chamber after machining to confirm that all of the above is correct.

Got to check cam lift and duration now as spec is unknown.......might be time for a new cam hmmmmmm the plot thickens.