10 replies to this topic

[Stu1961]

Hi All,

 

Have just come across a bit of info on the Mini Spares web site regarding combustion chamber volumes. It states all 1275 heads left the factory with a volume of 21.4cc

does anyone know how accurate this figure is, and were heads produced consistently to this spec.

 

Only reason I ask is because I recently measured the volume of my head and it came out at 22.5cc, so I have just measured out 1cc of fluid and it isn't a lot so I would definitely question the accuracy of the measurement  I have taken.

 

This 1cc difference makes quite a big difference to the compression ratio something like 0.3 in my case, and on top of this the head has had a minimal skim to clean it up so the compression ratio figure for my engine worked out using the figure 22.5cc is almost exactly 10:1 but using the figure of 21.4cc it works out to 10.3:1 and with the minimal skim the ratio would be even greater.

 

Anyone have any thoughts on the figure of 21.4cc and its accuracy?

 

Hope that lot makes sense.

 

Cheers

Stuart  

Edited by Stu1961, 04 January 2014 - 10:26 PM.

[100ev8]

how are you measuring the volume ?

[minimadles]

That's why it's always prudent to measure the head and engine you are using and calculate the cr from that there are to many tolerances involved so best practice is always measure your engine and calculate accordingly

[Stu1961]

I used a clear sheet of thin styrene (1.5mm thick) with half dozen  air holes in it with a thin layer of grease to hold it in place (minimal amount only) filled chamber with WD40 through a 30mml syringe. In my opinion it would be very easy to overfill even with the holes in the sheet.   

Edited by Stu1961, 04 January 2014 - 10:43 PM.

[Stu1961]

There is a better and probably more accurate way of taking the measurement and if I had thought about it at the time I would have done it in the following way.

 

Fill the combustion chamber with something pliable such as plastiscene level it of with a straight edge, remove it from the combustion chamber then drop it into a container with a set volume of fluid in it and measure the amount of fluid displaced.

 

Obviously the smaller the divisions on the measuring container used the more accurate the reading would be and you would also need to use something that sinks to fill the chamber with in the first place not sure if plastiscene is buoyant or not.

Edited by Stu1961, 04 January 2014 - 11:03 PM.

[ACDodd]

The later unleaded factory 1275 heads were 22.5 to 23cc.

Ac

[Stu1961]

My engine is A+ and came from a 1990 Metro and has a unleaded head so I will assume my figure of 22.5cc is reasonably accurate?

 

Stuart 

[ACDodd]

Yes.

Ac

[Stu1961]

Thanks Ac any chance I can PM you about re bushing my primary gear? all dimensions are in my original topic started yesterday.

 

Cheers

Stuart

[ACDodd]

PM away.

 

AC

[Vipernoir]

I always measure with a spark plug of the type I use installed - there probably isn't much difference, but it makes me feel better.