Amount to take off block?

[AxeSeth6]

I am in the process of rebuilding a 250 and have read a lot of post on quench and see that to have optimum quench the piston would be about .018 above the deck using a felpro head gasket of .038 crush leaving the desired .020 distance between the flat part of the head and piston. So the questions are.
1. Useing a cam and rocker set-up to achieve .450" valve opening (csc-274-hsp-10) is there an equation that I could use to determine if the valves are going to hit the top of the piston.
2. I have read that if I do not use the .020" gap the the quench effect is is minimalized or non existant is this true?
I want to put this engine together with as many benifits as I can but the though of taking too much off is killing me. Currently the pistons should be apporx .027 down the hole using 255 pistons and both deck and head are going to be machined. The head is only going to be cleaned to flat leaving the valves as high as possible.

 

[MustangSix]

I've read that generally you lose the quench effect at .o50" and larger. If your pistons are down in the bore .027" adding a .050" gasket pushes the distance to .077" - too far. If you zero deck a 250 you have to machine dishes in the pistons or relieve the chambers to get the CR to a manageable level. On pump gas, that's about 9.5:1 with an iron head. With a tighter quench distance, you might get away with a bit more compression.

 

[Lazy JW]

.020" is pretty tight, generally accepted practice is to run .040" and anything over .050" loses effectiveness.

You had best have done your homework if you plan on running .020", I believe it's too close.
Joe

 

[AxeSeth6]

That is the problem with no math I will have to play it safe. Many post in this section have that .020" number but noone seems to run it or have any math avail. to determine if you can do it at all. At this point I am going zero deck with as thin a head gasket as I can get, but would still like some math to determine if its feasible.

 

[MustangSix]

Zero deck with a .045 - .055 head gasket will work and is safe. Just keep an eye on the CR.

With the currently available composite gaskets it's hard to hit .020" without having the piston above the deck.

As far a piston to valve clearance, it's tough to accurately calculate because it's a dynamic relationship and its dependent on cam timing as well as lift. It could be done, but you would need to measure some things anyway, so the best, easiest way is to use the time-tested clay method. Drop in two solid lifters in #1, put a lump of clay on the piston, bolt down the head and gasket (a used gasket is fine), and rotate the engine at least two revolutions. Remove the head, slice the clay and measure how thick it is where the valves come closest.

 

[Lazy JW]

My 300 has the pistons coming OUT of the hole .007" and has run nicely for about 60,000 miles. Do your homework first, this means measuring things carefully and doing a full mock-up.
Joe

 

[rbohm]

8) my advice is to zero deck the block and run a modern composite gasket. that will give you a decent quench area, and plenty of clearance between the head and the piston.

 

[CZLN6]

Howdy Seth and All:

Check the compressed thickness of the FelPro head gasket you are using. The FelPro 7916 PT-1 typically measures .050" for a compressed thickness. The thinnest commonly available gasket is the Victor 3910VC at .044" compressed thickness. So, with currently available head gaskets, you may find it necessary to follow Joe's solution of having the pistons come above the deck surface to attain an optimum quench height.

Have you measured your deck height on the 250 engine? Using your figures indicates that your engine has a deck height of .115". That is very tight for a typical 250 engine. Typical stock deck height on most OEM 250s is closer to .150".

My reccommend is to not mill the head at all until you have all the parts selected and the block accurately measured and machined- Do all measurements with new bearings and pistons in place. New replacement pistons, in overbore sizes, are typically built with slightly less pin height.

What is your compression ratio goal? Given optimize quench and more cam duration more than 9:1 is reasonable. What is your elevation?

.035" to .040" is a reasonable deck clearance for good quench effect. Milling a "D" shaped dish into the top of the 255 pistons, mirroring the shape of the combustion chamber will maximize the bore to quench area ratio as well as helping to achieve a more reasonable CR.

I'm going to run some numbers on the CR calculator and will get back with them later today.

Adios, David

 

[CZLN6]

Howdy Back Seth and all:

Here's the calculations-
*assuming an overbore of .030", a zero deck height, a Victor head gasket (3910VC) with a compressed thickness of .045", 62 cc chambers and 255 V8, flat-topped pistons the CR comes out to 10.8:1. (NOTE- there are already eyebrow valve reliefs in the 255 pistons that have some volume (?) so the actual CR is something less than listed)

*Same as above but with a "D" shaped dish milled into the top of the 255 pistons with 7 cc of volume- 9.9:1 CR.

*Same as above, but with the chambers opened to achieve 64 cc of volume- 9.7:1 CR. With your planned cam, optimized quench, chamber smoothing and optimizing F/A ratio, this is a liveable combination for a street engine. You may need to reduce initial advance and/or add a tank of higher octane gas during the heat of summer, but it is doable. Polishing the chamber and the piston top will not reduce CR, but it will eliminate hot spot opportunities, will reflect heat and the dish will improve the quench effect, all will help to safely run a higher CR.

It is most likely that you will need to mill the head a small amount to achieve a true mating surface, but only do the absolute minimum. Then look to open the chambers by unshrouding the intake and exhaust valves, which will increase flow and reduce CR.

When planning to machine a "D" shaped dish into the top of the pistons, plan to stay at least an eighth of an inch (.125) in from the piston edge. The shape and depth will determine volume. Too much and you will weaken the piston. I tend to favor error on the safer side of risk. I have toyed with various combos of shape and depth of the dish but have no exact reccommendation. Know that the depth of a stock OEM piston's dish is .010", The diameter of the dish is 2" at the bottom and 2.125" at the top, with a bevel of aproximately 45 degrees. I use these generalities to estimate the volume of a "D" shaped dish. I would encourage you to keep notes and to take pictures. Someone will want to know how you did it. I have only that the opportunity to play with a couple of used 255 pistons so know that I have no real experience with this approach. I believe it is a reasonable, workable solution and will try it someday.

Jack's advice on assessing valve-to-piston clearances is right on. No amount of math can make up for putting it together and rotating it with a bit of clay to tell the tale.

That's my two cents, for what it's worth.

Adios, David

 

[drag-200stang]

Axe,

To ease your mind about the piston to valve clearance, as long as the cam is timed to the crank I don't think its possible to build one of these engines and have the pistons hit the valves. But always check with with clay as suggested. Please let us know how much piston to valve clearance you ended up with.

P.S. whenever building an engine, don't take anything for granted, check it.

 

[AxeSeth6]

Syracuse elevation is 500ft above sea level and normally a humid climate. I have planned to run a high static compression ratio 10.7:1 or so with the cam I selected with intake valve closed 67degrees bdc and zero decked would give me 8.5:1 dynamic comp ratio putting me at the bottom of high octane giving me room in either direction. I am going to do a mock up with clay but I was hopping to find some math to make sure I did not take off too much before I mocked it up. Once the material is gone I cant put it back on. At this point the block cleaned up at .02 over (leaving meat for the next one ) I plan to take .025 off the block leaving .003 margine at top of piston. The head will just be cleaned to flat after unshrouding the new valves 1.75int 1.5 exhust. I can still D shape the piston if i have to but with that cam selection I am hoping not to have to.

 

[Lazy JW]

I did the mockup BEFORE having the block decked. Yes, it takes longer, yes, it means two trips to the machine shop but if you are planning to run things tight it pays off in the long run.
Joe

 

[CZLN6]

Howdy:

What is the new volume of the chambers after unshrouding?

Adios, David

 

[AxeSeth6]

Sorry I'm not that far with the head yet but I am aiming for stock 62cc or slightly larger.

 

[jahearne]

Any progress?
I'm about to start on a similar project myself only running on pump gas staying at or under 9.5 CR; no matter what I'm going to have to dish the 255 pistons. Mike informed me that on a late model head the most we can open up the chambers to is 66 cc keeping as much of the orginal D-shape as possible to maintain optimal quench providing we get the pistons that close. Clearance measured .1155 ~ 0.12, so that will leave me .020 to .025 deck hieght after cleaning it up a bit. Going to leave the deck hieght because this might get rebuilt again someday and the CR really jumps up there if you go zero deck. Who knows maybe some day I'll try Dave's trick offsetting the rod journals or we can hold out for MSL gaskets @ .027. With 66 cc chambers and Cortec head gasket, the pistons will require 3-4 cc dish. We're going to leave it up to the machinist wether or not to increase the dish size and go with smaller chambers.

 

[AxeSeth6]

Progress is slow right now just got a new job and it is 2nd shift. I hope to get the block back this week and I had him take .020 off the block as a start. If I have to I can mess with the pistons on the mill I work on. I was stopping in at the machine shop every week to move him along but havn't for the last two so I dont know if hes done yet.