measuring compression height on sohc dome piston

[hbstang]

so i am use to measuring flat tops or dish,but sohc dome pistons are not the same.do you measure to the break where it transitions to dome?just need to know for sure,see pic.

[jayb]

That is correct - Jay

[hbstang]

thanks jay.with the stock ford aluminum heads and 4.350 bore and 4.375 stroke what would compression be with those pistons,as they are flat top dome of about .215 .i dont know chamber size on heads but thought you would if anybody knows.
i am going to email you a bunch of questions.

[blykins]

Do you have a piston volume? 

[hbstang]

no,i have no paperwork on pistons.

[jayb]

Those are fairly small dome by SOHC standards, a typical dome for an SOHC will be around 38cc.  Those look much smaller, but I really have no idea what they are...

[hbstang]

so i did some more checking and i think the dome volume on the pistons is 28 cc,or so.but with a chamber volume of 119 or so and bore 4.35,stroke 4.375 i get 10.87 compression .all of that maybe off some!
also bringing this up about combustion chambers,pond originally had a heart shape chamber on his but went back to the sohc open chamber. piston dome maybe only 20cc or so.low 10-1.want 11-1 or so.
also bore is 4.350 and piston skirts are 4.340,were for a blower combo,but how would they do for n/a 520 sohc?


https://fepower.net/simplemachinesforum/index.php?topic=514.0

[jayb]

Gasket bore on your calculator should probably be 4.400", not 4.500".  Also if the piston is .015" in the hole you could go to a .041" head gasket, instead of the .051" that you are showing on the calculator sheet.  That would probably get you to 11:1 - Jay

[hbstang]

thanks jay,just trying to decide if i can use the pistons on a n/a build,but .010 piston to wall is not ideal.have you bought custom pistons from race tech for your sohc builds?

[jayb]

Never used Racetec, I always get them from CP or Diamond.

[WConley]

Also, you should double check that C/R calculator before ordering pistons.  I very carefully measured out my original iron head street engine with TRW domed pistons (much bigger).  My chambers are 120 cc with slightly tuliped Donovan valves, and with an 0.040" head gasket I got 10.7:1 compression.

No way that little dome gives so much compression.  Those must be blower pistons.

[blykins]

I've been buying my cammer pistons from Mahle.  They have a nice lightening program, come coated, have a nice ring package.

[hbstang]

thanks every one for your replys.