Topic: Decking to control “squish/quench”

[Sano]

  I have been lurking for months and trying to soak up all the impressive knowledge of the SOHC brain trust.  Thanks to all of you who share your valuable experience.  I have been making progress on my basket case resto-mod 550 but am a bit apprehensive about getting the machine work started.
  I want to increase my compression ratio and optimize the “squish” dimension while installing a set of flat top .5mm overbore (David Silver) pistons.  After doing a solder dimension check of the combustion chamber I have come up with some seemingly large dimensions.  I am hoping some of you more experienced engine builders can offer me a little guidance. 
  There is slight variation in measurements (.003”) but I have listed all minimum clearances below:

Top of piston below cylinder deck with base gasket and .040” MLS head gasket installed, no rings on piston
(torqued to 15 foot pounds                                                                                .029”

Piston crown to outside perimeter of combustion chamber at wrist pin axis.           .080”

 I am understanding from others comments that .040” squish would be a good dimension to aim for but that seems like a lot of material I would need to remove to achieve this.  Could I get a reality check here?
  Do these dimensions sound correct for the 550 motor and if so should I split the .40” material removal between the cylinder block and head.  Thanks to all who can offer some assistance.

PS:I am using the cb650 camshaft and “zeroing” the cam with an adjustable cam sprocket.

 

[calj737]

You might get better attention in the HiPo section than Engine.

If it’s any consolation, my 500 motor had pistons 0.032 below the deck. Mulled to eleven, used a 0.040 MLS and torqued to achieve 0.032 final squish.

I doubt a 650 cam needs an adjustable sprocket to time it. As long as your intake valves are stock diameter, you should have no clearance issues.

[Sano]

Thanks Cal,  you have been one of my most reliable sources of quality information so thanks for that and your timely response to my question.  I do have a question though about your comments.  If you milled the head to get the piston tops to .011” below top of cylinder barrels and used a .040” head gasket how do you get a squish dimension of .032”?  Would it not be .043” (sum of dimension of crown below deck and the thickness of the head gasket).  Maybe I am not understanding this process correctly.  I measured the clearance directly with solder and the head torqued down with all gaskets in place.  I will try asking this question in th hi- PO section (once I find it!). Thank you for the suggestion.

[calj737]

I decked the block to bring the surface level to the top of the pistons. That equals 0.000. Then a 0.040 gasket torqued yields 0.032 space between head and piston tops (squishy-squish).

[Sano]

Well, finally got .024” taken off my cylinders to essentially get the new piston tops to the top of the cylinders so the squish will be about .032” after the MLS head gasket is torqued down.  One unanticipated result of taking off this much material from the top of the cylinder block is that the o-ring recess needs to be counterbored to return the needed depth. You need at least .030” o-ring squish, .040” is better.