Head swap facts revisted

[Anonymous]

I would like to add a few points here. Milling the heads and block requires the pushrod length or rocker shaft to be shimed to maintain proper rocker geometry. The lifters may pump up and compensate and the "slipper" style rocker design is quite forgiving to extreme tollerance varations. Why because with out it you will change the rocker geometry instead of 1.5 ratio you might see 1.45 (dont remember the the small 6 rato off hand) which means less than advertized lift. Two because your valve tip to rocker contact patch is moved off center greater side loads are placed on the valve causing quicker wear of the valve guides. I prefer shimming the rocker stands over custom push rods. Just be shure to make the hole in the spacer for the stand that feeds the oil to the shaft is big enough to allow oil to pass. I also recomend the adjustable rocker shaft assembly.

 

[addo]

So, would you shim with ground steel plate, or grade 8 washers, or milled spacers of some other sort?

Your comments point to my favourite issue - roller tipped rockers diminish side load significantly. This means you can shorten the protrusion of the guide boss into the ports, and gain a litle extra flow. The prepped Argentine heads have guide bosses 100 thou shorter, so they are obviously taking this benefit and "running with it".

Adam.

 

[CZLN6]

Howdy Turbo2256:

Here's the facts; I decked my 250 block .070", milled the modified '78 head .030" and used a Felpro head gasket at about .050" compressed. That's .100 less through machining, but .025" back for the difference in head gaskets. That should have meant I had .075" less and would need to shim the rocker stanchons to get everything lined up again.

I assembled with new hydraulic lifters and a recon 144 adjustible rocker arm assembly with custom pushrods from Clifford. Everything lined up perfectly, NO Shims. A variable you missed in your facts is the adjustibile rocker arm adjustment. That tends to add or subtract length to the valve train. Apparently, it can compensate for .075".

The stock 1.52:1 ratio is a constant. It is determined by the ratio of fulcrum to pushrod contact to fulcrum to valve tip contact. Nothing, neither milling, wear nor adding material, will change that. What may change is where the rocker tip contacts the valve tip. That contact must be correct for the full arch of the rockers motion. That detail must be checked and corrected if necessary.

Shimming may not hurt anything as long as the oil delivery is not impeded and a base is solid, but I have yet to see a case where shimming was necessary. Yours may be the one.

That's my 2 cents.

Adios, David

 

[61Futura]

I don't think that milling or decking would alter the geometry either. Shiming the rocker stand would move the point of contact of the rocker to the valve stem. That can't be good. Shorter push rods shouldn't be a problem, but is it really necessary.

I'm starting a rebuild on a '68 200 that I was planning on putting a 250 M head from a '69 250. After looking at the combustion chambers, I'm not so sure. I'll have to cc them to be sure, but I think there will be quite a bit of difference. I guess I'll have to buy a handbook from Dennis instead of just trying to figure this all out with logic. The further I tear the engine down, the more questions I have. Not to mention the more tools I have to borrow from Dennis

Steve

 

[Anonymous]

The ratio of the rocker arm is deturmined by the distance between the center of the pushrod cup and the center of the rocker shaft divided into the distance between the center of the rocker shaft and the valve contact patch. Thusly if your push rod is to short the distance between the valve contact point and the center of the rocker shaft is less which gives you less lift. Too long of a push rod has the opposite effect. Except for perty much bone stock engines pushrods are the last part I order and I dont care if its a 16 cyl engine I will set up a dial indicator and measure the actual lift intake and exhaust for each cyl. Adjustable rockers can take up some of the slack so do the slipper style valve contact points on the rockers. If your talking roller tip rockers one has to be very careful as to were the roller sits at half valve lift. Some of these issues dont even matter much over the short run but premature valve train wear and some valve train instabilites might occur. I built a Boss 302 for a 84 Mustang I had it was a nightmare with it. Push rod lenghts were 1/2 inch longer than stock on the intake. 5/8 inch longer on the exhaust to get the roller centerd on the valve tip. I ended up dissassembling half the engine looking for the cause and couldnt find a part out of speck and I have the Ford prints at my disposal.

 

[61Futura]

On an I6, the adjustment is by moving the the pushrod cup and not the fulcrum so milling the head will no affect the rocker to valve contact point. If the adjustment was moving the fulrum up or down, it would affect the point that the valve stem meets the rocker. If adjustable rockers are used on the I6, moving the cup during readjustment would make no difference if the pushrod to rocker angle was at 90 degrees. IIRC the angle is not too far from 90 degrees. If non adjustable rockers are not used and you rely on the lifters to collapse a little more, there would be no difference on that side either.

I am betting that your experience with the 302 was that the adjustment was on the fulcrum. I haven't been under the valve covers of a 302, but I seem to recall reading that they are adjusted with the engine running and therefore would have to be fulcrum adjusted.

 

[rbohm]

8) 61futura is right. since the I6 rockers are shaft mounted, you have to either change the length of the pushrod or move the contact point up or down on the rocker. rocker arm ratio is the same all the time, it does not change. however rocker arm geometry does affect valve lift, but only in a minor amount.

 

[Anonymous]

Ok guys. I have been through this and measured it in the physical world and in the world of CAD CAM its what I do for a living. If your push rod is to short you will loose lift because the contact point of the valve tip moves towards the fulcrum of shaft as you will. If the pushrod is too long it moves away from the fulcrum or rocker shaft increasing lift. As for the Boss 302 or any of the Cleveland style heads the rocker geometry for the intake and the exhaust is different and actually require two different rocker arm configurations to have correct geometry.

Look at it this way. lets say from the center of the push rod cup to the center of the shaft/fulcrum is 1 inch. The distance from the shaft fulcrum to the center of the designed contact pach and valve is 1.5 inches. You mill the head the push rods become longer rotating the rocker arm towards the valve. this moves the designed contact point off the center of the valve thusly increasing the 1.5 inches to 1.55. Next instead of milling the head we will put a thicker head gasket on it. Now the contact point is moved closer to the fulcrum because the pusrod is now to short the 1.50 distance becomes 1.45. Now we have 3 different ratios here 1.45 , 1.5 and 1.55. In a production engine all these situations could appear in the same motor due to production tollerances. A lot of other things move the contact path too valve installed height, cam base circle etc. usually these issues are corrected by changing push rod length when blue printing an engine. If you dont believe me get a dial indicator a solid lifter an adjustable pushrod and check it out. When that Boss 302 was finally assembled it had a differance of plus or minus .00025 lift between cylinders. Another point the rocker shaft although the most stable of the fulcrum designs is also the least forgiving to tollerance varyations.

 

[61Futura]

Well, I don't want to irritate you, but,,,,,,,,,, the rocker arm will not move. If it did, it would open the valve. The lifter must collapse, or the height of the cup on the rocker must be raised. The rocker arm stays exactly where it was, just resting on the valve stem.

I tried not to respond again, but I just had to. I have thought thru what you have said, and it makes sense for the adjustable fulcrum style rocker. And I will remember that if I work with a V8. But the position of the rocker will not move on the I6. I will concede that the shape of the rocker is altered on the adjustable rocker assembly on the push rod side, but since the angle of the rocker to pushrod is about 90 degrees, it's no big deal.

I hope you know that if you prove me wrong, I might kill myself

 

[addo]

I'm sure it changes. Maybe not by much, but than again, cams are ground in thou, not hundredths or tenths. Visualising a cross-section is how I came to this conclusion.

61, don't do it.

 

[61Futura]

The weak point in my logic would be the angle of the pushrod to rocker arm. The angle does change with movement, and the more lift your cam has, the more movement you get. I can see that raising the contact cup will lower the 1.5 ratio by a very very very small amount. Meaning a very small change in lift. A computer program would tell me exactly how much if the exact dimensions and angles were entered.

I was more concerned with the contact point on the valve stem being altered. I don't see that changing. And saying that the less lift means the contact point when the valve is fully open will be different doesn't count. The starting point will be the same.

 

[MustangSix]

Actually you are both correct .

On a little six, because the rocker fulcrum is on a shaft, the relationship between the rocker and the valve stem does not change when you mill the head or adjust the valve as long as the rocker stand does not move in relationship to the valve stem.

If you start shimming or altering the rocker stands or altering the lenght of the vavle stem, then the geometry changes because the fulcrum has now moved in relation to the valve. On a stud mounted rocker where the adjustment is at the fulcrum such as on a SBC or SBF, it can make a considerable difference in rocker to valve contact area and even the rocker geometry.

On a non-adjustable shaft mount rocker assembly, milling the head or decking the block only alters lifter preload because all the points are still in the same relative location with each other and cannot move.

But on an adjustable rocker assembly, the contact point between the rocker and pushrod is being moved up or down, altering its relationship with the arc it travels thru. That potentially changes the geometry and the rocker ratio.

It's generally such a small amount that on a stock engine it makes little or no difference. You'd be hard pressed to dyno the result. But on a high perf engine with bigger lift and longer durations it might be enough to add up to some real power.

 

[rbohm]

8) the only way to change the rocker arm ratio is to change the rocker arm itself. a 1.6 rocker is always a 1.6 rocker. however a change in the rocker arm geometry will change the valve lift as you are changing the angle at which the rocker sits, but you are not changing the rocker arm ratio.