Flathead: not enough piston to head clearance: WHAT NOW?

[I have a big dome fly cutter for this type of work - but it is NOT easy to use! You better have a big-ass mill that is very tight in the ways and you better REALLY mount the head well. It is hard to get them to cut nicely and not chatter unless you are all jigged up and have everything just right (including a sharp cutter).

Bear40 on here emailed me picturs of his flycutter setup like you explained. I had initally thought of the same thing but rejected the idea because I was skeptical to take such a big cut at one time and my Deckel mill is really too small for doing that kind of work.

Will look into the solid copper shim gaskets.

http://www.headgasket.com/gaskets.html

 

Call me I can redome the heads 818-248-2371.

 

I like this method but here is another i have used in the past.You stated earlier you were a machinist so this should be no problem for you.Use a piece of .375 to .500 aluminum plate and mimick a connecting rod .050-.060 longer.use it as a mock-up rod,in conjuntion with a degree wheel and simple math you can figure out material removal needed.Either at the piston or head you can find the clearence you need.
I would be happy with .040-.045 on a flathead,the piston is fairly light and rod streach as well as wrist pin deflection is not that great with the rpm that they spin at.The tighter the better for squish needed to help a real poor chamber design.
Some say remove from the head some the piston,two ways to skin the cat but if piston change is done keep in mind ballancing comes into play.When or if piston replacement ever does reoccur doing the head means doing it once.
Gary

 

I believe Cometic will make thicker gaskets on order.

 

Spoke with them today. They are $148 /pc! - Ouch

 

Sounds pretty dear, but considering the options, probably the cheapest way to go and by far the quickest way to go.

 

Ditch those shitty egge pistons and order Ross pistons.



-Jason

 

Earlier Egge pistons had a curve that was totally different from stock...almost conical. These hit in the middle and would seem to have necessarily had too much clearance elswhere, since the shape did not match the chamber. Can you post pics?? Major differences in shape can be very bad, as the areas with too much clearance kill the quench and make the engine tend toward knocking from the bad chamber effects. Ford meant for the things to run with very closely corresponding shapes of piston and head, and as with any engine poor quench degrades performance in several ways.

 

My SCAT Kit came with Ross Forged Pistons.No issues.

 

I don't have any pictures yet. The clay blob ended up being a good inch in diameter and had thickness was only margianlly bigger on the perimeter than the center.
I want to get a better measurement tonight.

Speedway advertises their big bore copper gaskets as being .070 compressed, which might give me enough additional clearance. Seems most composite gaskets are around .050 installed from what I can tell.

 

Try solder or balled up aluminum foil...they are stable once smashed and can be accurately measured. Ideally you want curves to match exactly...probably too much to hope for with aftermarket parts made decades after production, but you at least do not want great variation or large clearances. Engine will ping and knock at lower compression, be less responsive. If thicker gaskets would give good clearance at center but too much at edges, it is time to pull pistons and cut the high area. Do multiple measurements and try to get an idea of differences in clearance in different places to choose a route to best possible compromise.

 

Ok everybody. Did some homework and took some crappy cell phone pictures...

First off, I measured the depth of the combustion chamber and the pop up of the pistons above deck height. I just used the dial indicator and the stand. You'll get the idea...



Piston 8 had the most pop up @ .235" above deck (about .010" more than some of the the others). Edge of piston is still below deck, except in the relief.

The combustion chamber depth (Edelbrock Block Letter heads, new) measured .188"

The compostie gaskets measure .063" uncompressed and should compress to about .050"

Calcualted piston to head clearance with installed gasket:

.188"-.235 + .050=.003"

Okay, lets' take an actual measurement: I used a 2" long piece of 1/16" rosin core solder (softer than the solid kind), put a dab of greas on the piston to hold it in place. Installed trial gasket and head and gave it a few cranks, then removed the head.


Here is what it looks like:





The second picture shows the measurement from the center to get an idea of the two diverging radii. Don't really know if this is acceptable or not - opinions?

The center thicknesses for cylinders 5,6,7,8 were as follows:
.025"; .028"; .030"; .016"

The discrepancy between the calcualted and actual values is that when you apply an actual compression force you take out the slack and start to strech materials. Actual vlaue is probably somwhere in between the two.

Here is my plan. I am not going to go with other pistons for the following reaons. Block has been machined for these pistons. ROSS or ARIAS pistons for the 4" stroke start at .060 or .0125 over standard. I only have a .030 overbore. Don't even know if I could return the pistons and rings at this time.

I found several companies that can make custom copper shims or even full thckness copper gaskets. I know it's not recommended to stack gaskets but a solid (lets say .032") shim on the block side using copper spray (between block and shim only) should give me a pretty stable surface to use a modern composite gasket on top.

 

Just read my post again got to say that cutting the pistons would probably be the cleanest solution. I am just tired of messing with this thing. What hsould have been a paintjob and replacing the heads turned into a $1500 rebuild....maybe even more reason to do it right...I dunno anymore.

I would probably cut the pistons to put a larger radius that intersects with the current edge and is .030" lower in the center. This would also make the piston follow the chamber more closely.

 

sounds to me like you're best bet would be to determine the correct radius to match the heads then cut it from the top of the piston. since the dome currently doesn't match the heads. your buddies with a cnc should be able to help you with this pretty easy once you determine a closer radius. while you are at it you can custom cut each piston so they are equal.

BTW I've read this problem coming up before and someone did re radius the head or the piston, you may be able to search it out to find a starting point for your radius size.

 

Up for the evening crowd.

Contacted Egge about the problem and asked them to chime in and if they were willing to recrown the pistons for me. Let's see if they respond. Gave them the link to this thread.

 

You have a great degree of variation ranging from .016 to .030,the problem may lie in the connecting rod.mesure center to center and see if someone took too much off the rod end when they reconed them.This will give you variations in the rod length.A tobin arp pin boring machine corrects this at the bushing end to enshure that the rods are all the same length.I have been at this a while now and i never seen a piston be off THAT far from each other.If the rods check ok then check the stroke on the crank if it was cut.ALWAYS check there work,i cannot tell you how many cranks i got back from reputable grinders that measure off one way or another.

 

It won't be rod length out that much as the rod bolts are not removable from the rods on a 8ba rod so most of the correction will come off the cap which will not shorten the rod that much. His measurements suggest to little clearance which would be a long rod.
To confirm your measurements switch the rod and piston assy. to another hole to confirm dimension change

 

When you recondition a rod weather or not you are able to remove the rod bolts you have to grind at least one cap,agree? So a sunnen rod hone works off the center of a hole,shorten the cap .010 and the hole center just moved,open it up to the corrected size and now your rod length is shortened.
Do this one or two times in the rods life and things go arye.The way to correct it is bore the bushing to make up the loss in length then final hone to size.Engine rebuilding 101

 

Reconditioning a rod will shorten it. That would place the piston farther down the hole at top dead center. Time to check all dimensions. If pistons are wrong, then manufacturer should make good.

 

No room to relocate the pin hole in the rod no material to spare the pins are only .750" rods are bushed from factory. Rods are not his problem. Your response is correct i don't have a problem with that.Moving pin hole to shorten rod,what do you use for new pin bushings?Your correction works with big rods in a diesel and others not with these small rods.He has to look elsewhere for a correction fly cut pistons is an option.

 

My 32 ran 2 head gaskets on one side because one Edelbrock head was shaved to far down, it ran for 20 years with no problems. It works in a pinch however, I feel that is not the right awnser. One of my Grancor heads is shaved down a lot also and I think the best way for me is to do it right and have the head domes redone. I did the test where I set the heads on and turned the engine over by hand and everything cleared, so I maybe ok, we'll see. Good luck and keep us posted I would really like to see how this pans out.

 

FWIW stock French pistons have as stock a "conical" dome. You can see the shape mirrored in the stock French heads. I had the same piston / head clearance problem when I put Offenhauser heads on my engine. As I was using older used parts I just filed the lumps off the tops of the pistons and licked a little out of the heads with the angle grinder. Wouldn't recommend it as the right way to do it, but it sure worked well for me.

Mart.

 

Pin bushins range from .035 thick on a flat head replace them recon the big end and use a tobin arp pin boring machine to correct the length,when there all the same length then check the crank persons work.These are good engine building practices and are applied to little flatheads as well as big diesels.When this is established time to check the decks.Again i havent seen pistons varry as much as he has so with a min of 58 years of service the possibility of these parts living there entire life together is slim.When you kicked a rod out you went to the pile and got a new one.How many times was that seviced 2 or 3 times that will give you different heights.

 

yup

 

I am learning new stuff here. Anyway, I had the pistons cnc machined to take off .030 in the center and blend in at the edge. Reassembled everything and measured all clearances with the solder wire again and came up with measurements between .042 and .055". Actual "under air" clearance will probably a little less due to the compression force to crush the solder wire. No need to split hairs, just want to avoid a costly self destruction.
To recap: this is a french block with 4" crank and Ford rods and Egge pistons.
Final judgement will be reserved until this thing is back in the roadster which is not going to happen for a couple months.

 

You did well

 

Ok mike, how did it pan out?

 

I had similar problem today with Egge "domed" pistons hitting Offenhauser heads. Not wanting to pull new engine apart. After considering options. I removed plastic base from my wood router so it would fit over piston dome. Taped everything up to keep clean. Made a small test cut, then set wood router to depth to remove .035 off top of piston. Removed an area about the size of a quarter. I have a milling machine, but this worked great and was supper simple. I know, I know, but it worked.

 

4 year old thread, hope he got it going a few years ago.