CORVETTE RESTORATION UPDATES

JUNE 25, 2006 - CYLINDER HEAD INSTALLATION AND DETERMINING PUSHROD LENGTH

 

 

 

 

   

Well, as usual progress has been a little slow over the past few weeks but nonetheless things are still moving in the right direction...

The simplest thing I've done since my last update was installing the harmonic balancer. Getting the balancer in place on the crankshaft snout can be a bit of a challenge since it's press-fit, but with the proper installation tool mine went on pretty smoothly. I also heated the balancer in an oven at low temperature (250 degrees or so) which made the balancer expand and helped it go into place more easily.

Copyright - Tim Cote 2007

Next, in preparation for installing the cylinder head studs, I used a tap to chase the threads of all the stud holes. This simple task is a must to ensure that the head studs are accurately torqued during installation and to help prevent any coolant leaks down the road (the holes penetrate the water coolant passages). It's really surprising how much crud can collect in the threads.

Copyright - Tim Cote 2007

With the holes all chased I coated the threads of each stud with some Permatex No. 2 non-hardening sealer. This is absolutely essential since all of the cylinder head studs penetrate into the coolant passages. Failing to add some sealant here could lead to some major problems down the road.

Copyright - Tim Cote 2007

Here are the installed studs ready for a head gasket and the cylinder heads. I should take a moment to note that the stock motor used bolts rather than studs. I opted for the studs since they provide more uniform clamping pressure and can help prevent head gasket failures. With my aluminum heads and increased compression ratio I though the extra cost was a worthwhile investment.

Copyright - Tim Cote 2007

So here are the new aluminum cylinder heads I ordered. They've actually been sitting in my garage for the better part of 6 weeks now, I just haven't gotten to them yet. I decided on Airflow Research 305's for my motor which feature rectangle ports and an open combustion chamber design. These heads flow many times better than stock and will really wake up the motor. The large rectangle ports make great top end but they do sacrifice a bit of torque at low RPM's when compared to smaller oval port heads. I will say that for my current setup the smaller oval port heads would actually have been ideal. However, I eventually plan to swap out my rear gears and transmission to get some better gearing. Once I've made those changes I'll really be able to take full advantage of the great flow numbers these heads provide.

Copyright - Tim Cote 2007

Copyright - Tim Cote 2007

Since I'm changing cylinder heads, as well as lifter type, it was necessary to determine the required pushrod length in order to maintain proper valve train geometry. The goal of this process is to come up with a pushrod length that centers the movement of the rocker arm tip over the center of the valve stem. In order to do this properly I had to remove the valve springs from one intake and one exhaust valve and put a lightweight checking spring in their place. I'll explain why this is needed a little further down the page...

Copyright - Tim Cote 2007

Here's one of the checking springs I used. It's simply a very light weight spring about 3 inches long and about 5/8" in diameter that I bought at the local hardware store. The spring needs to be able to produce just enough pressure to hold the valve closed.

Copyright - Tim Cote 2007

Below you can see that a checking spring has been installed on one intake valve and one exhaust valve and that the head has been mounted to the block. While I was determining pushrod length I installed one of the old cylinder head gaskets to avoid damaging one of my new ones - a bit of quick research told me that both gaskets had similar compressed heights so substituting an old gasket for a new one wouldn't create errors in my calculated pushrod length. With the head on the block I'll install the new lifters and then move onto actually determining the pushrod length.

Copyright - Tim Cote 2007

Here are the new retro roller hydraulic lifters I'm using. You can see the difference between the new lifters (on the left) and the stock type hydraulic flat tappets (on the right). These lifters are the reason that the lightweight checking springs are needed. Both types of lifters have springs inside that compress to "preload" the lifter. When the motor is running, the lifters fill with oil and the hydraulic pressure from the oil prevents the lifter from collapsing further. However, when the motor is not running the valve would collapse completely under the load of a normal valve spring. The light weight checking spring ensures that the hydraulic lifter does not collapse and ensures that the correct pushrod length is determined.

Copyright - Tim Cote 2007

Comp cams recommended letting the new lifters soak in oil for 24 hours prior to installation to ensure proper lubrication. So, after a day in a tub of oil it was time to get them installed in the block.

Copyright - Tim Cote 2007

Here's one set of installed lifters. In the photo you can see how the rollers ride on the camshaft lobes. The link bar is necessary to ensure the proper orientation of the rollers is maintained.

Copyright - Tim Cote 2007

Copyright - Tim Cote 2007

Ok, so now I've got the heads installed and torqued down and the new lifters have been installed. Now it's time to figure the proper pushrod length. To accomplish this a pushrod length checker is used. A pushrod length checker is basically a pushrod that can be adjusted to a variety of lengths.

Copyright - Tim Cote 2007

One last thing needed to determine pushrod length are the rocker arms. I decided on a set of full roller Crane Cams rocker arms with a 1.7:1 length ratio. This means that the valve lift will be 1.7 times greater than the cam lobe lift.

Copyright - Tim Cote 2007

So, with the checking pushrod placed in the lifter for the intake valve - the one with the checking spring installed, I put a rocker arm in place and tightened the nut until the valve was set to zero lash. Zero lash is found by rotating the pushrod while tightening the rocker arm nut until a slight resistance can be felt on the pushrod. This is the point where there is no play in the lifter/pushrod/rocker arm assembly.

Copyright - Tim Cote 2007

Here's a view of the checking pushrod, rocker arm, and checking spring installed. Now it's time to rotate the motor through several combustion cycles in order to mark the movement range of the rocker arms roller tip on the valve tip. The goal is to have the movement centered on the valve tip. To be able to tell where the roller tip travels I colored the top of each valve tip with black marker prior to installing the rocker.

Copyright - Tim Cote 2007

Despite the fact that I was using a checking spring I was still concerned that the hydraulic lifter was collapsing and giving me false readings. To verify this wasn't happening I set up my dial gage on the valve spring retainer and verified the max valve opening. It turns out the checking spring was serving its purpose and that I was getting the proper valve lift (for my intake valves this was 0.350" * 1.7 = 0.595").

Copyright - Tim Cote 2007

After finishing the intake valve, I repeated the same process for the exhaust valve. Below is a photo of the valve tips after dialing in the proper pushrod length. You can see where the movement range of the roller tips removed the marker from the valve tips.

Copyright - Tim Cote 2007

Here's a close-up of one of the valve tips. You can see that the movement is nearly centered on the valve tip which is very desirable. If the movement range is centered to far forward or back on the valve tip (pushrod to long or to short) it results in high loads on the valve guides and leads to premature wear of the guides.

Copyright - Tim Cote 2007

After getting the movement range centered on each valve tip I then measured the adjustable pushrod. To this length I then added 0.06" to come up with the final pushrod length. The extra 0.06" is necessary to compensate for lifter preload in the final installation. While determining pushrod length the valve lash was set to zero. However, in the final installation the rocker nut will be tightened an additional 1/2 turn past zero lash which compresses the lifter. The 0.06" compensates for this lifter preload. In the end my final pushrod lengths were 7.900" for intake valves and 8.750" for exhaust valves.

That's all for this update! The new pushrods have been ordered, now it's just a matter of waiting for them to arrive so I can continue with my project.

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Total time spent on restoration to date:

374 HOURS

JUMP TO THE NEXT UPDATE -  CHECKING PISTON TO VALVE CLEARANCE

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