Piston to Head clearance

4

47Merc

Senior Member
Hi Guys
I am rebuilding a 59A engine (my 1st time) and have had the block bored to .125 over and decked.
I have read on FB many times that the piston to head clearance should be 40 thou, but I'm not sure if this is measured with or without the gasket in place. Looking at the engine I think the machine shop has taken quite a lot off when it was decked but they assure me they haven't. I have Offy 400 heads and a standard 3 3/4 inch crank

As it stands, with the heads on and no gasket the pistons hit the heads and I am unable to turn the engine over. I therefore assume that the measurement should be with gaskets on but I have been told to never assume anything.
Any advice would be appreciated.
 
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yes, the desired .040" clearance is with head gaskets fully torqued down as in final assembly and retorqued after repeated heat cycles. The way I do it is to grind the interference spots in the cyl head combustion chambers , checked with NO gaskets in place, until I get no interference anywhere. Then, I can be assured that after head gaskets are installed and fully compressed after the numerous heat cycles/retorques, that I've got the .040" clearance. This assumes compressed gasket thickness is .040"
 
Yes, .040 is a "standard" but, you can get away with as little as .035.

Since you bolted the head down, you can't tell what the clearance is so, I'll suggest tw ways.

1. Put the head gasket on and some wadded tinfoil, about 1/8 thick, on top of the piston, turn the engine over, then measure the foil (Ron's method)

2. Measure the head, for the center of the piston and mark that location on the top of the head. Put the head on, with the bolts loose and put a dial indicator at the mark for the piston center and then turn it over and measure the amount the head rises. Subtract that from the torqued thickness of the head gasket, for your clearance.

The head gasket thickness can vary, between manufactures. My Victors measure .060 after torquing. Some others measure .050 and less.

It won't hurt to torque the gasket, pull it off and measure it. It can be re-used at the same location, when putting the engine together.
 
Dab those tin foil wads in grease to keep them in place. I put about 5 on each piston. Put head in place without gasket and slowly hand crank engine until all pistons have gone through a cycle. Remove head and measure the crushed wads to see where you may need to grind a bit. Chap
 
As others noted, you want about .040 with the head gasket on and torqued down. On most of the head gaskets I've measured, they are usually around .050 thickness (after installation). Hopefully your machine shop made sure that the deck heights are the same on both sides and didn't take a lot more out of one side than the other - due to the valves being about .040 higher on the Driver's side (which I've seen shops do who are not versed in flathead valve angles). Make sure you measure BOTH sides for piston to head clearance - as well as valve clearance up by the top of the "eyebrows" in the heads.
 
The Offy 400 heads willl not clear an L-100 cam which has a net lift of .355" +/- .010". Now I haven't checked one in 10 years or so, but bent valve could ruin you day.
Gramps
 
I have done this on three 8BA's and believe it is the single most important thing you can do to improve the efficiency of a Ford flathead. One thing mentioned in passing should be emphasized. Each bank must be done separately, as the heads and even blocks vary side to side. On one set of heads, each had to be milled different amounts to get to a consistent starting point before final finish grinding. On another, one head to be milled more in the front than the back, a result of a poorly done previous decking. (It was a lot of fun finding someone who understood what I needed and was able to do it.)

I use the "foil ball and grease" method with a used gasket. You measure each piston head and the valves. I use 6-8 balls per piston and write down the results on a crude chart. Then sit down and analyze the chart and decide how much must be taken off each head to get to a good starting point and then have the heads milled. I will admit I sat and stared at the chart and scratched my head a couple of times when I saw the chart that showed a nominal .060" on the rearmost cylinder, increasing to .110" on the front.
 
Kids' Playdoh or some sort of clay is what I've always used. You can knead a strip any way you want, lay it across the dome, then after cycling you can slice the playdoh anywhere you want to check thickness with a caliper. Seems like that would be an easier method to measure than using tin foil.



And Tubman is right, keeping the quench as tight as possible greatly improves combustion efficiency of any engine, but especially true on flatheads. You just need to keep overall compression in mind when figuring volume.
 
Ol' Ron said:
The Offy 400 heads willl not clear an L-100 cam which has a net lift of .355" +/- .010". Now I haven't checked one in 10 years or so, but bent valve could ruin you day.
Gramps
Click to expand...


Hmmm, I plan on doing the Playdough thing thing when fitting the heads but, I have an Isky 1007b that Pete ground for me and it has a listed 365 lift, so that will not clear my new Offy 400 heads?
 
On the 77B cam I think the lifter clearance is .016 but I haven;t done one of those cams in 20 years, You have to subtract that from the cam lift. However, Your looking at this from the wrong information. I suggested the fact that the L-100 wouldn't clear thes heads many years ago. They may be different now. but YOU HAVE TO MEASURE EVERYTHING. Don't just take my word for it, or anyone els. They are just telling you what they found when they did it. This information is important, because it gives you a starting point on what to checki. Especially when it comes to pistons and valves.
Take care
Gramps/
 
Ol' Ron said:
On the 77B cam I think the lifter clearance is .016 but I haven;t done one of those cams in 20 years, You have to subtract that from the cam lift. However, Your looking at this from the wrong information. I suggested the fact that the L-100 wouldn't clear thes heads many years ago. They may be different now. but YOU HAVE TO MEASURE EVERYTHING. Don't just take my word for it, or anyone els. They are just telling you what they found when they did it. This information is important, because it gives you a starting point on what to checki. Especially when it comes to pistons and valves.
Take care
Gramps/
Click to expand...


You are so right Ron, I appreciate your wise words. The cam I have is an Isky 1007B. Thanks.
 
That is a really good street cam, better than the Max #1. It was very popular back in the day
G
 
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