Maximum Lift - Clearance

Matthew68

Well-known member
Hi All,

For those who chimed in on my DCR thread - thanks! I settled on the 270/270 110 cam from Schneider and talking to Jerry about my build. In the cam specs (custom grind), he stayed the valve lift would be .485” with my YT 1.65 roller rockers. I recall a post by X (I think) that anything over .490” would have piston to valve clearance issues. Do I need to re-evaluate my cam choice? I haven’t purchased it yet. My motor specs below, based on these am I running a high risk of piston/valve interference? Note that this is all planned machine work - haven’t had it done yet so I can course correct if recommended.

1968 200ci block - bore TBD (20 or 30 over)
Planned zero deck
Flat top pistons (I haven’t found any with reliefs)
Planned 10:1 static compression ratio
Aluminum head/intake with 1.84” intake valves
53cc combustion chamber size
Victor head gasket (.044” thickness compressed)

I realize my questions are all theoretical and must be measured which I plan to do upon assembly. However, I don’t want to be in a situation where I’m dangerously close and it’s too late to leave a little meat on the deck, change to a felpro gasket, etc to give me more room.

Thoughts? Appreciate any advice.

Cheers,
Matthew
 
mathew, as you have already noted, any combination you come up with will have to be checked before final assembly. this is why mock up is so important.

so start by finding out what your current v/p clearance is. you will also need the current compressed gasket thickness, and the depth the piston is down in the cylinder.

now install the cam you are going to use and get the v/p clearance for that combination. now you can play with the numbers and figure out how much you need to clearance the pistons before you rebuild the short block. it owuld be a good idea to build the head like you want it before checking the v/p clearance so you can use it to figure what you need to do to the new pistons.
 
must B some way to strategise here...
no paper method? Charts, tables, formula?
Didn't I C a guy on here shave a piston top on
(I think it wuz) a Bridgeport a yr ago/or wuz it
2? Least the table looked like one. Now let's see

1/2 the small end + the C-rod + to the piston pin C/L +
the squish...
:unsure:
:p
:shock:
:oops:
 
rbohm":1vu36bh2 said:
mathew, as you have already noted, any combination you come up with will have to be checked before final assembly. this is why mock up is so important.

so start by finding out what your current v/p clearance is. you will also need the current compressed gasket thickness, and the depth the piston is down in the cylinder.

now install the cam you are going to use and get the v/p clearance for that combination. now you can play with the numbers and figure out how much you need to clearance the pistons before you rebuild the short block. it owuld be a good idea to build the head like you want it before checking the v/p clearance so you can use it to figure what you need to do to the new pistons.

Thanks, but my setup I took apart was a stock 68 head and block. That head has long been since removed and had the stock cam and stock 6.5cc dished pistons. I’ll be using the aluminum cylinder head for the first time and I’ll be dropping the block off for all machine work. They’ll also disassemble and assemble. I suppose I could take the head and have them measure, or I could wait until. Have the head here and do it before drop off. The only thing is I’m planning to not use the dished pistons to gain some compression so I’m not sure what my best course of action is here.
 
Howdy Matt and All:

My first thought is why not hold off on the 1.65 rocker arms and start with 1.5. You can alway add higher lift rocker arms later- if you think you still need more lift. You didn't say what the lift at the cam is? The extra lift from the 1.65 compared to 1.5 isn't much, but it may minimize clearance issues. Just a thought.

Adios, David
 
CZLN6":icujscy8 said:
Howdy Matt and All:

My first thought is why not hold off on the 1.65 rocker arms and start with 1.5. You can alway add higher lift rocker arms later- if you think you still need more lift. You didn't say what the lift at the cam is? The extra lift from the 1.65 compared to 1.5 isn't much, but it may minimize clearance issues. Just a thought.

Adios, David

Hi David,

Here are the cam specs:

270-H
270 DURATION
214 @ .050"
.485" LIFT WITH 1.65 ROCKER
110 LOBE CENTER
 
There will not be a valve to piston clearance issue.
The intake valve sits at least .250" in the chamber.

If you do the math, if you advance the 270H cam 3 degrees the intake lobe center will be at 107* ATDC which puts the .050" lobe lift right at TDC.
.050" lobe lift times the 1.65 ratio rocker opens the intake valve .082" at TDC.
The tightest point between intake valve to piston occurs between 10 and 15 degrees ATDC.
That particular cam lobe has a lobe lift of .100" at that point which puts the valve lift at .165" but the piston is now .038" down in the hole for a difference of .127"
The piston is now .045" closer to the valve but there is still plenty of clearance.

Now add the .044" thick head gasket for additional clearance.

Do the check once you get the cam and head to verify.
 
I also agree with Mr. Muller, you will not have a piston to valve issue...But always verify.
The biggest issue on these engines is the short valves, that is where high lift can start to be a problem at about .500 ...The bottom of the retainer can contact the seal and guide...Easy to measure with the spring off...I do not think that you will have a problem, but verify.
Another thing to watch for is spring coil bind....I am sure that your cam guy gave you springs that will work, but verify on your head .
 
CZLN6":3rxb3c76 said:
Howdy Matt and All:

My first thought is why not hold off on the 1.65 rocker arms and start with 1.5. You can alway add higher lift rocker arms later- if you think you still need more lift. You didn't say what the lift at the cam is? The extra lift from the 1.65 compared to 1.5 isn't much, but it may minimize clearance issues. Just a thought.

Adios, David
Cannot do this the 1.65 Yella-terra rockers require V-8 oil through lifters & pushrods The oil galley on the LR of the of the block has to be blocked to prevent an oil leak.
You will have no clearance problems.
If you really want to be sure put a ball of clay on the piston top & turn the engine over.
Don't worry you are good to go.
 
"...require V-8 oil through lifters & pushrods The oil galley on the LR of the of the block has to be blocked to prevent an oil leak...."
even on the alu head Bill? I think that's in use here~
 
chad":2avyao42 said:
"...require V-8 oil through lifters & pushrods The oil galley on the LR of the of the block has to be blocked to prevent an oil leak...."
even on the alu head Bill? I think that's in use here~

It’s for the Yella Terra rockers
 
Back
Top