Hi Fellas,

Some may remember my post from a couple of months ago "261 teardown and reassembly" detailing my low mile 261 and conversion to full flow oiling. Well after the teardown and trip to the machine shop, it wasn't in as good shape as I had hoped. The bores were tapered and the rod bearings and crank were out of spec.

So after a .030" overbore, .010" crank regrind, new pistons, rings, main, rod and cam bearings, and reconditioned rods, I'm ready to put this thing back together!

I picked up everything last Friday in a rush and finally got to look over my parts collection this weekend. Everything looks great but I noticed the machinist did not deck the block.

I put a straight edge (not a machinist's straight edge) across the block every which way today, and noticed two places I could slide a .002" feeler gauge under the straight edge. This bothered me a little so I looked in my shop manual to try and find specs on this, but came up empty.

So my question is should I take the block back to the machinist and have it decked, or is .002" perfectly acceptable? The block is bare however the new cam bearings have been installed. The machinist who did the work is very experienced with vintage engines and has built several 261s that I know of.

Any opinions are, as always, greatly appreciated!

Did he surface the head? I bet he checked the block and felt it was OK. I'd run a good Felpro gasket and not worry.

Ron

Well since you didn't use a precision ground straight edge, I would recheck the deck with one and see if your readings change for the better or worse. Since you have gone to the expense of boring the block, and regrinding the crank and rod work to get them back into spec, I would probably go ahead and get the block resurfaced as well. Just make sure they pressure wash the block really good afterwards to make sure no issues come from the cam bearings. I've done it many times without problems for customers that weren't wanting to remove the cam bearings. As long as it doesn't exceed .002 when checking it with a precision straight edge, you should be ok if you choose not to resurface the block.

Thanks for the input so far Ron and CNC-Dude. Like CNC said, my thinking is that since I've gone this far I should probably get the deck milled. Out of curiosity, what kind of gap would absolutely require the deck to be milled?

Oh and he didn't mess with the head. I'm planning to take the 848 head from the 235 in my car now and drop it onto the 261. I'll check out how flat it is when I do the swap.

I agree, .002" is probably within the gasket's compression range, but it's not a happy mistake to cure afterward.
Unless you can either accurately measure all bits (block height, piston CD, and gasket thickness), or assemble the block, you don't know how much to take off.
Personally, I would try to get the quench down to .040", which may leave almost no dome to deck distance.

Thanks Panic. So I should really put the crank, pistons and rods into the block to determine how much room I have to play with? I'm going to talk to the machinist today to see if it was an oversight or if like you guys have mentioned, he felt it was OK to run as is.

He should have resurfaced the block on a rebuild, but since he didn't, check it like Panic said then determine what you need to do.

Be sure to check piston-valve clearance since the valves open straight down toward the piston. This is especially true if the cam has more than stock lift or you have rockers with a larger ratio.

Check EVERYTHING at least twice and ask questions if you have any doubts.

Ron

Fellas to follow up on this one, I bought a dial indicator and assembled the crank and pistons in the block. I measured the block deck to piston top and came with .045" at one end and .050" at the other. Since I'm going to be decking the block I'd like to gain a little extra compression too. If I have the block decked .020" will I have enough valve clearance?
Thanks again for everyone's input so far.

will I have enough valve clearance

For what cam?

The cam is one of Tom Langdon's Bulldog cams. I don't have the specs on hand at the moment but I can check tonight.

You have to check it first to see where you are with clearance to begin with. It also depends on how the cam is installed compared to the cam card. If its "advanced" from the cam card, you will have less intake valve clearance than if its installed per the specs, so unless you check it you cant really guess if it will be good or not.

So do I need to assemble the whole engine; crank, pistons, cam, lifters, pushrods and head, in order to check the valve to piston clearances?

Usually, you can get by just with doing the #1 cylinder. The valves in the head for that cylinder are generally the shallowest when prepped for racing, so you know if those have adequate clearance, then the rest should be ok. So, the next question is are the valve seat depths in the cylinder head consistant, or do they vary a whole lot. If they vary a lot, you might need to loosely assemble the engine, and clay the tops of all the pistons to see if any are closer than others to eliminate that one that seems to always come back and bite you because you assumed or overlooked it.

Wow, this keeps getting more and more involved! But better to check now and get everything correct than have to pull it apart later and replace a bunch of bent valves right?

Yeah, plus its good practice for when someone you might run into needs to know how to do it, you can tell them or show'em. You might likely have no trouble whatsoever, but if you don't check it, you don't know how close to the edge you are in case you want a bigger cam at a later time, or cut the head some more for more compression.

Yes that all makes a lot of sense. So one final question: how do I check the space between the piston and valves? You mentioned clay? Is it time to steal some of my daughters Play-Doh and have at it?

You can use your dial indicator, and place it on the side of the retainer, but you will need to have the ability to put light tension checking springs on the cylinders you are checking. If you dont have a degree wheel, you will have to check the distance by rotating the crank in slight increments at and around TDC in overlap. The valve basically becomes an extension of your dial indicator, and you would zero the dial before starting, and push the valve further open each time you rotated the crank, noting if the valve appears to get closer to the piston or farther away from it, trying to locate the spot that it is the closest to the piston. An older method, is to place modeling clay on the piston tops, and then rotate the crank over several complete revolutions, and then remove the head. Of course, you will need to install all the pistons and adjust the valves for zero lash when doing this, and you can omit the head gasket, and just lightly snug the head down with a couple of head bolts. Once you remove the head after rotating the crank over, you can take an exacto knife and cut thru the clay to see how thick the impression is from the valves touching it. The problem with this method is that if you are real close with your P/V clearance, you could inadvertantly cause them to hit the piston if you rotate it over too fast before you realize it. You would also add to the thickness of the clay, the gasket thickness and valve lash if its a mechanical lift cam. The degree wheel method lets you sneak up on it by letting you see if you are getting too close before you have contact, plus you can see in thousandths, how much clearance you have.

It's a PITA to notch the pistons while the block is assembled, but it's been done.
The usual warnings about greasing the edge of the dome, using Mor-Tite etc. to prevent chips from getting loose etc. apply.
You use an old lathe bit re-ground to a 1/16" radius nose and brazed onto an old valve (stem must be same or slightly smaller than the actual intake valve). The cutter nose should be at the actual valve radius + .030" minimum, I prefer .050" to make the actual relief the head diameter +.100". 4 bits make a smoother cut (less chatter), but harder to get the radius to match.
Because of the close distance, the piston has to be away from TDC to be cut (a lathe bit is too thick to fit between the seat and dome near TDC).
When assembled, measure the stem height above the head - use this as a reference for depth of cut.
Lot of grease on the stem, turn the stem very slowly with a drill while applying very light pressure.
The stock dome is thick enough that cutting down .060" won't hurt. Mark the stem at .060" above the guide etc. or you can clamp something on as a stop. If I have to do a few of the same size, I make a brass shaft instead of an actual stem (less wear in the guide) only .001" smaller than the guide ID, and thread the upper end 5/16-24 NF for a fine-thread large diameter nut as a stop, each hex of the nut is .007" of movement.

I think I'm going down the modeling clay road. All of the math involved with the degree wheel etc. makes my head hurt no matter how many times I read the posts!

Doc Frohmader uses the degree wheel technique in his Webrodder series, but again I'm having a hard time trying to make sense of it all.

What is the minimum piston to valve clearance to aim for? I read that around .090"-.100" is a safe bet? Any smaller and problems could arise— does that sound correct?

That is good for the intake, I would make the exhaust a little more like .125 though.

The exhaust isn't going to need any work - the exhaust pocket is very deep, while the intake seat is nearly flush with the deck.

Somewhere in all of the great posts on this valve clearance topic it became easy to forget the OP was talkin' 261! Good info boys! Seen lots of engine that the 'first time running her hard' method was used (well, never thought about that!). Easy to measure valve depth into piston, no clay needed!

OK to follow up on my valve to piston squish test, and to get final reassurance before I get my block decked, here are my findings.

I finally assembled the block last night using no. 1 cylinder for the squish test. The Bulldog cam was in place but I decided not to use the head gasket. After a couple of crank revolutions and cutting the modeling clay through the thinnest point, I have .065" to .070". My head gasket is a FelPro from the Best Gasket set but I don't know what the compressed height is? I thought I read another post saying .045"?

If so that gives me .115" total clearance. So could I have the block decked .020"-.025", given that the current piston to deck distance is .045". That would give me a final piston to valve clearance of .090"-.095".

Does this sound right to you guys? Especially the part about the compressed head gasket thickness?

Thanks again in advance!

Is the camshaft a hydraulic or solid lift, and did you adjust the lash to zero when doing this checking. If its a solid lift and you used zero lash, you can also add the lash to the clearance you already have.

The cam is solid lift and suggested lash is .012". I had zero lash at TDC on #1 cylinder. Hopefully I did this correctly! If so I can then add the .012" to my total and be safe with decking the block at .030"?

Quench over .060" is pretty much gone.

Hi Panic, I'm not sure what you mean. Can you explain in a little more detail for this rookie!

Edit: I read a little online about quench and it's effects. So the quench measurement is from the top of the piston to the top of the combustion chamber? And this measurement differs from the valve to piston clearance. So ideally you still want .090" valve to piston clearance with a quench of .040"-.050", factoring in a compressed head gasket? Have I finally got this straight?

Quench is from the dome to the closest flat area on the head's deck surface. It's only related to P/V by accident. .040"-060" is pretty much the entire working range for engines of this size, with .040" preferred.