Hope some of you more experienced guys can answer a few questions for me.

I am putting together my motor out in the garage. Today I started working on the heads. I tried to check P to V clearance and it brought up a few questions.

First off its a 355 with Eagle forged crank and 6" rods. JE domed nitrous pistons. I am running a felpro .039 gasket and I have TFS heads with 63 cc chambers. I am running a set of Comp endurex solid roller lifters on a custom cam motion cam. 263/270 duration at .050, .674/.686 lift.

So I dropped some clay on the piston and bolted on a head. Suddenly I realized my measurements might not be right due to the fact that I dont have the correct push rods yet. I do however have 1 adjustable pushrod so I dropped it in there and put on a set of lash caps, and dropped the rocker on. I adjusted it to 0 lash and rolled the engine over. I then swapped the adjustable push rod over to the exhaust and did that one.

I pulled the head and heres what I found. intake I have .110" clearance and exhaust I have .205".

Now I know the exhaust is good and I think the intake will be ok. Its a race engine and not going to see the street at all. is the .110 ok? I searched the internet and came up with a couple different "correct" measurements.

My next question is this. How critical is push rod length when checking this? I dont see how it would make much difference since lift is lift and the pushrod length should have no affect on valve lift but I just wasnt sure. I dont have anything long enough with me to get an accurate measurement on the adjustable pushrod so i am not sure it if was the same from intake to exhaust. I just adjusted it so that the rocker was up off the spring and riding some what close to the center of the lash cap.

does this sound right?


1 more question. the cam was ground on 112lsa, its already got +4* in it. The cam card says it should be installed at 108ICL. I degreed it and it comes up at between106-107. Is that close enough?

 

You'll be fine.
I build 305 econo-sprint engines that must use a flat top piston and a spec head. In an effort to increase compression, we cut our own valve pockets, and normally target .045" int. & .075" ex. P to V clearance. Granted, this is on the very edge, but we have also tested extensively in order to keep the valvetrain under control.

The pushrod length and ICL will both effect P to V, but with the clearance you have, I wouldn't be too concerned. I would however degree the cam correctly, and recheck just for your own peace of mind

 

That make me feel better. I was afraid of having to pull the pistons out of this thing to have them cut. I really didnt want to do that.

I can see how ICL would affect p to v clearance. I checked it 2 times and cam up with 106+ the first time and just about 107 the second time.I figure I was pretty good since I came up with almost the same numbers both times.

I have read alot about people changing cam position on an LT1 and from what I have read it seems that unless its way out its easier to leave it because changing the cam position will mess with timing. I am going to be running the stock ecm and opti being driven from the cam. I was leaning away from changing it, but now I dont know. If I advance the cam to 108 wont that also advance my ignition timing by 1-2 degrees? That seems to be the debate I have read about in many searches here.

Can you explain how the pushrod length will affect p to v clearance? I must be missing something because I just cant picture in my head how it has an effect.

thanks for your response... im learning more about this every day. This is my first SBC build up.

 

I shoot for a minimum of .080", .100" preferred. I've done less, but not in a high performance engine that's going to see some RPMs.

Aftermarket performance pistons typically have much larger valve reliefs than a stock replacement or cheap-o piston, which can be a godsend with big cams.

Did you check your radial clearance around the edge of the valve as well?

 

Well not all the way around the valve relief but on the top side I did and I have plenty of room. The valve relief that is below the flat part of the piston is realy big. I was supprised as to how much room there is since i am running 2.08/1.625 valves. I was under the impression that was a pretty big valve for a 4.030 bore.

I'm going to check it again tomorrow on the other side just to be safe but it looks like I am safe with ptvc...

I was really worried about that because I know this is a rather large cam, especially for a 355 but what the heck, its a track only car. The JE pistons have very large reliefs. I'm sticking with the stock ecm for now so this motor should only see 7000 rpm's. With this cam it will want to go much more I am sure.

 

oh, and for degreeing it, I have read so many mixed reviews i am just totaly lost. I'll redegree it again tomorrow. I can change it a bit. I am using a Cloyes 3145 chain and it had 3 keyways to change crank by 4*'s that would be *2 cam timing if I am not mistaken. I can change it but I already put the front cover on an pan on... got a little excited today and just kept on going

 

Since you can only move the cam in 2 degree increments, if it's at 106, I'd retard it. If it's at 107, I'd leave it alone.

In reality, you'd need to do controlled testing to find out what the engine actually wants. At 107, you have a reasonable starting point.

 

I guess I need to retest it untill I can repeat it with the same reading. I'll try it again this morning.

 

It would have much less effect than ICL. If the pushrod was too long or too short it could effect your valve movement at certain points of the rocker sweep due to the position of the pushrod cup relative to the roller tip.

 

Thanks for your help, I really appreciate it.

I think I got it now. The cam is in at 107 so I am going to leave it alone. My problem yesterday and all this morning was this. I wasn't getting a good accurate set on the degree wheel for TDC. I made my own piston stop from an old head bolt, a spacer and a large washer to hit the domed portion of the piston. Every time I tried to set TDC on the wheel I would check the cam timing and it was different from the previous time. I think the whole problem was that I must have been getting some deflection in my piston stop.

So to correct the problem I set up my dial indicator on the piston and turned till it stopped moving. set it to 0 and then backed off. Then I checked for TDC just as you would check for the cam center line. Moved to .050 before and after and set my degree wheel off those readings. Now I am getting repeatable cam timing readings. Its in at 107. I have done it 3 times with the same result so now I belive the numbers.

I agree with you after doing some more reasearch and I think it will be safe to just leave it there rather then try to adjust it by 1 degree.

As for PTVC I am going to recheck that again just to be certain I didnt mess that up too. This time I am going to try and set my adjustable pushrod as close to the actual length the push rods will be and see how it looks. I was not expecting to see the exhaust side with .200"+ clearance. I am more concerned with the intake side now, but I think I am going to be safe. Its pretty funny to drop in a stock pushrod... they are about 1 inch too short

Thanks again, you guys have really made me more comfortable about this assembly. I didnt build the short block. I had my machinist put that together but I opted to assemble evrything else.

 

Chain will stretch that 1* once you've got it running. Trust me, they all stretch just a smidge- even the good ones.

Feel like driving yourself bonkers for no good reason...... go try to repeat your readings on #6 cylinder. It should reach TDC at the exact same time as #1, it's just that the cam is always 2 strokes behind #1 (360* of crankshaft rotation). It SHOULD drop out exactly the same results as #1 without touching anything in the setup. But WILL it? Go try- start with checking to see if TDC on #6 is happening at exactly the same instant as #1. That should already make you a little nervous when you find it's off by a degree or two. Then check your cam events again. Then imagine the spread of results you might get if you checked all 8 holes the same way.

By the time you're done you'll understand why agonizing over a single degree of cam timing is probably not the most productive activity for an weekend wrench like you or me with our home-built motors using off-the-shelf mass-produced parts.

 

oh god no.... lol. I'm happy now that I know its close. I actually did this with a new Cloyes chain. I had an LT4 ED chain on the old motor and I tried to reuse it. There was a ton of slack in it and I know I would have had a hell of a time repeating my numbers. There was way too much slack in the chain for my comfort so I bought a new chain.

I figure I have repeated it several times now so its probably right. I'm not going to sweat 1*. Like already stated if it were off by 2 then I could adjust for it relatively easy. I would be there for a week if I tried to check all 8 hole let alone worry about weather or not they are all correct.

I think degreeing a cam is more for checking to make sure someting isnt way out. If it were off by 6-7* I would know there was a problem.

I have other problems to deal with now. Got the heads on and found the front oil return hole in the head is blocked by the block. I have to pull the head back off and do a little clearancing to the block. Never knew this was an issue till I got the heads on and bolted down.

God I hate to have to grind on a brand new motor...

 

After trying to use the clay, I found it was MUCH easier to find P to V clearance by putting a cheap lightweight spring (made by Comp Cams for this purpose) on the valve and then when the piston is up and the valve is open, use a feeler gauge to measure how far you can press the valve down, until it contacts the piston. Hope that helps.

 

Wouldn't you want to check P to V at the points in the piston's travel where the valves are closest to it as determined by the cam? That wouldn't necessarily be at TDC.

 

The clay works very well actually. You just have to get the right kind of clay. Moddeling clay is good for this.

I used clay for the exact reason OldSStroker just mentioned. Now if I could figure out how to acurately measure the adjustable pushrod I would be set.