4 1/8 stroke standards piston head clearance

Hey everyone I recently ran into an issue ***embling my Flathead 8. Fresh rebuild 1cm crank that I had thought was 4” stroke but may be 1/8 bigger as my pistons stick up out of the block about 1/8” they are domed pretty good but while installing my Fenton heads that have been milled the head literally rocks on the dome of the piston without a gasket on so I know once I tork the heads down I’m running a very high risk of making contact. I have a set of the stock Mercury heads that I’ve been told have the biggest chamber of all the flatheads so I’m wondering if I should go that route or do the clay trick and double gasket or possibly get the .125 over gaskets…

Any and all help/advice is appreciated. My first Flathead build. I’ll take a little better picture of the heads next chance I get

 

Show us a picture of the piston sticking out of the block at top dead center.

 

Why not roll it over and measure the stroke? If you were sent pistons for a 3-3/4 stoke crank, they would sit 1/8th inch higher with a 4 inch stroke.

 

It is much more likely that you have the wrong pistons than a stroked crankshaft. Mercury pistons have the pin located 1/8" higher in the body of the piston than Ford pistons. This was done so they could both use the same connecting rods. Lemme see, a Mercury has a 1/4" longer stroke and you say your pistons stick out of the block 1/8". Does that suggest anything to you?

 

Do you have 21A or 91A rods? Which use floating bearings? If you do then there is the chance it's 4.125". If you have regular 8BA rods then you don't have a stroked crank.

That pic doesn't really look like it sticks out a full eighth. Just looks like the deck has been cut a lot.

 

The clay 'trick', as you call it, has been found to work easier with little balls of tin foil.

 

You know the dome area of the head sometimes has to be cut with a bore increase.

 

Play dough works best and you can play with it when done.

 

Next time I’m at the shop I’ll get some better pictures and get a tdc profile for you. This is all I got right now

 

Yes I’ll need to do that. Either way I’m trying not to tear the engine apart to resolve the issue but rather, if I should / could run the stock 8cm heads to “fix” it

 

So a back story the connecting rods I used came out of an old race motor that had a cracked block the pistons were Jahns 3 3/8, 1/8LC if memory serves me right they were 8ba connecting rods, I used the connecting rods from that engine and another 1cm crank I had. Current pistons are Egge .040 pistons but the machine shop had ordered then I am unaware if they were for a Merc or ford. Just going off of what I see and my current issues. Either way I’m wondering if running stock 8cm heads would “fix” the issue or should I go about getting thicker gaskets or double gaskets. Trying to refrain from tearing the motor back apart

 

I believe they were 8ba rods but could have been 91a as that does sound very familiar. they came out of an old race motor with a cracked block. Pistons were Jahns 3 3/8, 1/8LC with a 1cm crank, now running Egge .040 pistons unsure wether they are for Merc or ford as the machine shop had ordered them.

 

here are the heads in question…they’ve been milled quite a lot but it feels as there is still heavy casting left I could probably have them machined slightly more to clear the dome of the pistons

 

I'm no expert (though the checks I wrote building my flathead were more than my first year of college so maybe I am more an expert than I give myself college credit for) but on an unrelieved block those pistons of yours look an awful long way out of the hole! If it was mine I 'd be sliding a piston out and double checking what is in there (rods, bearings, pistons).

I'm taking you didn't do the ***embly? If not, and the machine shop that did used ford pistons atop 8BA rods on a merc crank, well that no bueno! I don't know if you can machine the heads enough to clearance but these are 70+ year old engines, why re-write the book?

Just trying to help, that and I'm not sleepy yet.

 

Like I said before any and all help is appreciated and this is my first Flathead build so my knowledge is sp****. I ***embled the rotating ***embly in the engine I had to bring the block back to the shop for a separate reason and asked them to check it over (just incase) didn’t mention the pistons sticking up too far. From my knowledge they didn’t mill any off the block but rather just surfaced it (from what they said it was a really nice block) I suppose the pistons are on me since I installed them although they did provide the parts/kit. Looks like I’ll be going the stock Merc 8CM head route for now and give the plumbers puddy a try.

 

Think ahead a bit…I have heard of folks using two head gaskets before. But consider you need an 1/8th inch for clearance, and that 1/8 inch gives a good quench, no valve to piston interference, ect.
***ume your compressed gasket is .040. That means you might get by with stacking 3 gaskets up…(.120), which I can’t see being a good idea. So then you’d be on the hunt, or pay dearly(imo) for a set of head gaskets that would be .125 compressed height.
My opinion is to figure out which parts are not compatible and correct it with the right parts.
Yes, more time and maybe more money than a bandaid, but it will the operate worry free, without the bandaid coming apart.

Also, ***uming you can open up the heads adequately for clearance, you then have to source spark plugs that will also not stick through too far.

 

Rolled up balls of aluminum foil is more accurate and much easier to measure!

 

Check 4 things.
Measure how far the piston moves from TDC to BDC
Measure the piston pin height
Measure the rod length between centers
Measure the deck height of the block
That should tell you if you have the correct parts.
I have a feeling if you try to use what you have that those flat heads won't be flat anymore
Too much time and money to take a chance.............

 

Well I have to say you make a great point and as much as I don’t want to tear that damn engine apart again I did have it built so I wouldn’t have to worry about it…I appreciate all the info and tips. Ill be contacting the machine shop in the morning

 

I can tell just by looking that your heads have been surfaced. Every time they are cut the dome gets smaller diameter. When they were new the dome in the head was probably right around 3-3/8" diameter.

You said your block wasn't cut, but just surfaced. What ITH does that mean? They just sanded it a little with some sandpaper? No, they cut it with a stone. Maybe not much, but every bit counts. And if it wasn't the first time being cut, then those add up. As I said up above, it sure doesn't look like the piston is sticking up 1/8" above the deck (on the edge is where it's measured from, not the middle of the piston). I'd lay my money on you having the right parts, just having bad luck of the deck being cut too much. The answer is probably getting the dome cut back into your heads, and maybe finding some thick head gaskets.

You also might want to check the clearance over the valves while you are at it.

 

The heads were definitely taken down quite a bit. And by “surfaced” yes it was cut with a stone but not heavily at all from my understanding, basically a light surface grind to ensure it’s true. I know for a fact that this was the first time this block was rebuilt or even messed with as the ‘50 Merc it came out of had been sitting in the original owners garage parked since the early 70s. I like your idea of milling the heads back to stock bore I just wonder if there’s enough meat left that it won’t cause overheating issues and have eventual casting failure.

 

My 8ba block block is .125 over and it has a Merc crank offset ground so it has 4 1/8" stroke using 21A rods and full floating rod bearings. Pistons are 4" stroke and pop out of the bore 1/16" We fly cut the combustion chamber to clear the pistons. I would not cut an 1/8" out of the combustion chambers if it was me. I think that's getting too thin. Mine have been running for many years but it's half the cut at the most as you are talking.

Dave

 

Called the machine shop they had no idea about different pin locations on ford vs Merc pistons. He’s going to call the supplier to see which kit he bought. Hopefully they cover some of the fix. Labor alone would be nice if I buy the parts. I can only ***ume it’s running ford pistons. I did ask about how much they milled off the decks he said around 6 thousandths they don’t do anything more then 8 typically. So with the correct Merc pistons and maybe a slight chamber size increase to get the chamber to the correct .075 clearance that should resolve the issue I’ll touch base when I get a sold answer on the pistons that were bought

 

I am looking forward to hearing what the machine shop finds out. When I built my flathead I found out that most shops had no clue. I even bought Joe Abbins book on supercharging a flathead and gave it to the guy I chose and told him to follow the recipe verbatim. I hope they cover the expense of the labor at least.

 

You think you have problems now? They are nothing compared to what you will have if you try to "band-aid" this together (multiple head gaskets?) and it grenades on you, possibly ruining a good block. Hopefully, the machine shop will stand up and make it right for you, which should ease the pain. If the pistons are new, there should a ready market for them.

Do it once and do it RIGHT.

 

I'm with Tubman. Do it right the first time.

 

"Which kit HE bought"? Sounds to me like this is entirely their fault and they should stand behind it and pay the entire cost. Then, don't go back there.

 

So I got word back and it turns out they did buy ford pistons, I was pleasantly surprised when he said to bring the block back and they will resolve the issue at no cost to me. I really thought I’d have to cough up some money…big relief. New Mercury pistons are on the way and I’m dropping the Fenton heads off with the motor so they can get the correct clearances without the “band-aid” highly agree with you fellas about do it right the first time. Thanks everyone for all the info and help. I’ll post back up when I get the final results of head gaskets and clearances to show the difference the correct pistons make.

 

Cool glad to know about the pistons

 

Although these guys are doing the right thing and taking responsibility for their mistake, they have also shown a profound ignorance of flathead Fords. I highly doubt that they will know the proper way to get the proper clearance with the heads. Ask them out of the blue what quench they would shoot for over the piston dome; their answer will tell a lot. Actually it's quite easy to do your self, possibly requiring one milling to get them close, and then some work with a die grinder to get the clearances just right. The grinder work is easy, fun, and good for the soul.