13 replies to this topic

[Java_Green]

Is there a default deck height value to trust for a previously non-opened A+ 1275? I need a start value to combine components for a suitable CR and I don't have any pistons.

[Rick Anderson]

Did you ever get an answer to this question? I am wondering the same thing. More specifically, and in order to calculate compression ratio, what should the top of the piston to deck top be with a stock block and a piston pin height of 1.498? 

 

 

Is there a default deck height value to trust for a previously non-opened A+ 1275? I need a start value to combine components for a suitable CR and I don't have any pistons.

[nicklouse]

You measure the height during your dry build. That gives you the volumes you need for your calculation.

 

yes there is most likely a measurement given from crank axis to block surface somewhere.

 

but an un opened engine does not mean it is “correct” many were reworked.

[Java_Green]

Did you ever get an answer to this question? I am wondering the same thing. More specifically, and in order to calculate compression ratio, what should the top of the piston to deck top be with a stock block and a piston pin height of 1.498?

Is there a default deck height value to trust for a previously non-opened A+ 1275? I need a start value to combine components for a suitable CR and I don't have any pistons.

I did not, and I am still wondering... There are people saying that the way to go for making out a suitable CR is to measure deck height during dry built. Honestly, I cannot see how that should be performed without pistons and in my case choice of pistons is derived by present deck height. I rather not "dig out" the combustion chambers in a new Al head and would like to skim the block as little as possible. So, this is my "moment 22".... (I have a lot of other things to sort out as well, so right now this is pit aside)

[alex-95]

 

Did you ever get an answer to this question? I am wondering the same thing. More specifically, and in order to calculate compression ratio, what should the top of the piston to deck top be with a stock block and a piston pin height of 1.498?
 

Is there a default deck height value to trust for a previously non-opened A+ 1275? I need a start value to combine components for a suitable CR and I don't have any pistons.

I did not, and I am still wondering... There are people saying that the way to go for making out a suitable CR is to measure deck height during dry built. Honestly, I cannot see how that should be performed without pistons and in my case choice of pistons is derived by present deck height. I rather not "dig out" the combustion chambers in a new Al head and would like to skim the block as little as possible. So, this is my "moment 22".... (I have a lot of other things to sort out as well, so right now this is pit aside)

 

I can't help with figures as such but to give you an idea of how different deck heights change the compression ratio I stuck some different deck heights into my compression ratio calculator for a 1293 bore size and other random figures. 0mm deck height would give 10.62:1 , 0.25mm deck height gives 10.55:1, 0.5mm gives 10.48:1, 1mm gives 10.35:1 ratio. I think the deck height was around 0.1mm on my 1275 before I rebuilt it.

[Cooperman]

As a rough guide, every 10 thou down the bore is 1 cc.
When you do the 'trial build' you measure what you actually have. Then calculate what CR that gives with 3.8 cc gasket volume, the piston dish volume and whatever your head has.
From that and having decided what CR you want, you can then calculate how much to skim the block deck.
For a performance engine, the pistons should be at deck level at TDC and the head chambers ground out for best gas flow and for chamber volume.

[Spider]

The stock 1275 Block is a nominal 8.886" from Sump Rail to Deck.

[Java_Green]

The stock 1275 Block is a nominal 8.886" from Sump Rail to Deck.

Thank you spider.

[Java_Green]

With a nominal deck height of 8.886", 5.75" rods, 81.28mm stroke and compression height of 1.498" (omega, swiftune etc.) leaves me with 38 thou in deck clearance. This with a non-skimmed block. Not much left for skimming if you ask me. Does this make sense to you more experienced people within the subject?

[imack]

I would have guesstimated that standard components would typically give a piston to deck clearance of 30-40 thou.
How much do you want to skim? Ideally you want pistons flush with the deck, or even some protrusion.

[ACDodd]

30 to 40 ish thou is normal and plenty of material to play with, you can also get a variation of 0.020” on stroke length, a few thou variaton on rod centres, and the top of the block may also not be square with the bearing tunnels. Therefore a trial build, is essential to work out where the actual heights are. If it’s a 73.5mm bore you don’t want the piston flush with the block. It’s also completely acceptable to skim the piston crowns a little if more deck height is required.

Ac

[Java_Green]

30 to 40 ish thou is normal and plenty of material to play with, you can also get a variation of 0.020” on stroke length, a few thou variaton on rod centres, and the top of the block may also not be square with the bearing tunnels. Therefore a trial build, is essential to work out where the actual heights are. If it’s a 73.5mm bore you don’t want the piston flush with the block. It’s also completely acceptable to skim the piston crowns a little if more deck height is required.

Ac

Thank you for all provided information. I am fully aware of the different tolerances and how it will effect. My aim is to work out which pistons I should buy (mainly decided by available dish). It is a 1275 block which I will "re-sleeved" and there are some pitting in the head mating surface. So yes, the block will be skimmed to some extent. Preferably would be to go for standard bore (70.60mm) so I have margin for another bore in these liners. Second option is to go for +20 thou right away (might be tricky to get standard dimension pistons suitable for the purpose).

The combustion chamber volume is specified to 23cc with 33/29mm valves. I will go for 35/31mm valves so most likely on the short side of 23cc. All in all, the aim is a CR on the short side of 9.5 (boosted....., X-flow head...., port fuel injected...., electronic ignition....). 

 

Based on what I have found as available piston wise. Std dim pistons with 6.5cc dish (I have not found 10cc dish available) seems to end up a bit proud of CR=9.5. Of course, a dry built will be performed. I just do not want buy a set of pistons that leads me into a dead end during dry built.

 

Again, thank you for provided information. I will see what the combustion chamber volume will end up at after making room for the larger valves, and go from there.  

[ACDodd]

https://www.med-engi...ast-pistons-020

These are available in 10cc dish

They also list an 18cc dish option for forced induction.

Ac

[Java_Green]

https://www.med-engi...ast-pistons-020

These are available in 10cc dish

They also list an 18cc dish option for forced induction.

Ac

Those are plus 020 and for that size there are more options. I would still prefer STD size.