13 replies to this topic

[purple_fly06]

Hi All,

 

Just looking for a bit of advice as to what my deck clearance will be? I'm trying to calculate it but am getting brain fog. I'm mainly following the Ultimate mini 1380 engine guide so will be using the following:

 

Offset rebore to 73.5mm
Skim top of block 0.025" (25 thou)
I will be using the standard 1275 A+ crank & Conrods reground, balanced etc.

Omega forged 73.5mm 7cc dished pistons

 

If anyone is able to help that would be greatly appreciated, I'm going to get my head worked and the deck clearance is required to obtain the desired compression ratio 10.5:1.

A lot of the calculators online don't seem to make a great deal of sense to me.

 

Cheers,

Josh

 

 

[nicklouse]

Errm measure it?

 

it is what ever you need to get the CR you need with the head you have. Or you measure it and then have the head made/modified to suit.

[GraemeC]

As Nick says, best to measure.

The piston pin to crown height will dictate it, but then there are a lot of tolerances that will stack up to alter it by a few thou which will make a difference.

 

Or......

 

Specify what you want it to be and then give that figure to your head man and your block machine shop.  Best performance would be with pistons flush to deck (ie 0 clearance) but I'd recommend at least 0.005" to give some margin of safety.

[purple_fly06]

I'll struggle to measure it as the block hasn't been machined yet, but i was hoping there was some math to figure out what is best. I've quickly done the below following examples and online info and have got a compression ration of 10.34:1, I'm looking for 10.5:1 so once everything is tightened down, variance in chamber volume's and hot it should be somewhere near so 0.025" deck clearance would be correct? Thoughts?

 

Edited by purple_fly06, 22 April 2020 - 10:21 AM.

[nicklouse]

Is does not matter if it has be machined or not. It will change when machining happens. 
 

you have some strange measurements there. And some missing. Ring land volume?  Have you measured what the distance between the top of the piston and the deck now?

[whistler]

You need to have the block finished and a dry build with pistons in. Then you can measure the deck. I used a dti for tdc and then a feeler gauge between the piston crown and a steel straight edge placed across the bore. I then told the head modifier (Keith Calver) the clearance and he worked out the rest, the pistons were from KC as well.

Others may have a different method.

[nicklouse]

https://youtu.be/lZCIW2TuOQY

https://youtu.be/qHoOWEu7IlA

 Might help.

[purple_fly06]

Great info, I'll change my plan slightly and get the block machined and skimmed, dry build and go from there. It makes sense to do so - I was hoping that someone else has done a similar build and had already had the figures (was probably being too optimistic).

[whistler]

Great info, I'll change my plan slightly and get the block machined and skimmed, dry build and go from there. It makes sense to do so - I was hoping that someone else has done a similar build and had already had the figures (was probably being too optimistic).

After the dry build the block was machined 10 thou to get the deck clearance down to 5 thou.

[nicklouse]

Great info, I'll change my plan slightly and get the block machined and skimmed, dry build and go from there. It makes sense to do so - I was hoping that someone else has done a similar build and had already had the figures (was probably being too optimistic).

Way to optimistic as you have no idea if your block has been machined at any time.

[purple_fly06]

 

Great info, I'll change my plan slightly and get the block machined and skimmed, dry build and go from there. It makes sense to do so - I was hoping that someone else has done a similar build and had already had the figures (was probably being too optimistic).

Way to optimistic as you have no idea if your block has been machined at any time.

 

 

Only thing I have to go off is that there is still an 'A' stamped on the top of the block which would mean it hasn't been machined before? but will still do a dry build first and measure etc.

 

Also how can places sell off the shelf modified heads if all of this information is needed upfront? Is it literally chalk and cheese when it comes to the end result?

[nicklouse]

Most people don’t know and don’t care. Or it will be close enough.

one of the good things about doing this pre internet. You had to get the books and read up and go and talk to the engine builder.

[purple_fly06]

Thanks for the help, this is my first A series engine build so want to make sure I do the best I can don't want to cut corners. Glad i asked... Now just to find a machine shop that is open.

[Turbo Phil]

If the block is unskimmed just specify to the machine shop what you want the clearance to be. As suggested above, 0.005” is fine.

Phil.