colonel Posted December 30, 2013 Share Posted December 30, 2013 Greetings All: Can't seem to get anywhere using search feature so will ask all you engine builders out there what you feel comfortable with when it comes to piston to cylinder head clearance AKA:Quench. Was trying to build a BBC with stock stroke, 6.535 rod and stroker piston with 1.27 compression hieght. You add all that up and you get 9.805 with a stock deck height on a BBC of 9.8 and using an MLS head gasket that ends up at .037 it only leaves .032. Is that enough with these engines?? Reading up on this it sounds like .040 is a common standard to safely plan on using steel rods and <6000 rpm. Outside that question, I know I am not likely to find an engine that will still have a 9.8 deck as it will likely need to be trued up so that number could easily change. My old block was only rebuilt once and was down to 9.79 which would put the piston out of the hole by .015 which is I believe a tad to far out to be comfortable with but even if it was only .010 it could be made up with a .050 head gasket and all should be OK if you buy into the acceptable clearance number of .040. All this is in an effort to gain .150 in con rod length. A back up plan does allow me to keep the stock stroke, use a 6.385 rod and a piston made specifically for this combo that would net out to only a total height of 9.78 though. So is the potential gain of the slightly longer rod worth the hassle and effort to find a block that is tall and true enough to enable keeping piston only .010 out of the hole and using a thicker gasket or not???? What you figure???? Quote Link to comment Share on other sites More sharing options...
Sam (Bones) Posted December 30, 2013 Share Posted December 30, 2013 not in my book I built a 468 using a 6.385 rod and a JE piston, the block had been decked and I had a zero deck height, and with a cometic .040 headgasket I was where I wanted to be Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted December 30, 2013 Share Posted December 30, 2013 Cometic makes a variety of gaskets from .027 to .120 for a BBC if you are .005 out the hole use a .045 gasket. I never get the head gaskets until I do a trial assembly and make final clearance checks. The piston that you are talking about is for a 4.25 stroke and a .250 long rod. The common compression distance for a 4 inch stroke is between 1.240 (Ross) and 1.245 (CP/Carillo) Quote Link to comment Share on other sites More sharing options...
colonel Posted December 30, 2013 Author Share Posted December 30, 2013 OK, so you both agree with the .040 piston to cylinder head clearance then as being the number to shoot for when building the engine. Basically I am converting the .250 stroke from the 489 kit to a .150 longer rod but the difference is still .025 over the standard 9.78 kit combination which is where the .005 out of the hole comes from. Being out of the hole before decking is where it develops some concern as the block I use will likely need to be decked but until it is done I don't know how much it will need to true it up. The pistons I am looking at using are Probe with a compression height of 1.27. You say there are other pistons out there that have a clearance height of 1.24. That would give me the room I need to make the deck height what I want rather than hope it isn't too low and leave me with more out of the hole than I am comfortable with. I will look into these other pistons. Thanks. Quote Link to comment Share on other sites More sharing options...
colonel Posted December 31, 2013 Author Share Posted December 31, 2013 Had a look at both of those piston manufacturers and they both make pistons that are designed to be used with 6.535 rods but they will create 12 - 13:1 compression ratios with 47 and 51 cc domes which is not exactly streetable in my opinion so that option is out. Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted December 31, 2013 Share Posted December 31, 2013 The manufacturers may figure that if you are going with that long of rod, you have performance plans in the works. What are your goals? Quote Link to comment Share on other sites More sharing options...
colonel Posted January 10, 2014 Author Share Posted January 10, 2014 Just trying to do something a bit different and see how it all works. It has become evident that I could measure blocks all I want but even the one I was able to buy that measured good enough to try was unfortunately twisted and will end up with a deck of 9.79 anyway. Being .0015 out of the hole is beyond my comfort zone and I am not willing to buy a new block to give this a try so have had to go back to a 6.385 rod and either keep it a 460 or splurge and get a 489 stroker kit. Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted January 10, 2014 Share Posted January 10, 2014 .0015 out of the hole would not bother me at all. Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted January 10, 2014 Share Posted January 10, 2014 just to throw my bit in when we built my 509 we used a 6.535 rod with the stock (502) 4" stroke. The theory being this would allow the use of a shorter (lighter) Mahle piston as well as reduce the side loading on the cylinder walls. We did put the pistons 10 thou out of the hole and used a Cometic .051 gasket to get a .041 quench....seems to work okay... Quote Link to comment Share on other sites More sharing options...
colonel Posted January 11, 2014 Author Share Posted January 11, 2014 Picking up my block today after decking and aligning mains and deck ended up at 9.787 so I would be .0018 out of the hole with the Probe pistons. Will look at some other manufacturers and see if I can find a piston to make thing fit closer to 0 out of the hole with out too much compression. Is .0015 the limit for out of the hole numbers generally?? Quote Link to comment Share on other sites More sharing options...
CrazyDavey Posted January 12, 2014 Share Posted January 12, 2014 I would say the out of the hole is generally dictated by the position of the top ring grove...that would be a good question for the piston manufacturer... Quote Link to comment Share on other sites More sharing options...
Mike 57 Posted January 12, 2014 Share Posted January 12, 2014 I am glad that you are using the math, but I would mock it up. Do you have a piston and rod available? Like Dave Said, check with Probe see what they have to say. If you are not going to spray it I don't see an issue. You may want to ask them about ring gap with the top ring closer to the deck. What compression are you shooting for? Quote Link to comment Share on other sites More sharing options...
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