Saturday, June 29, 2013

Understanding The 235 Hydraulic Lifter

Understanding The 235 Hydraulic Lifter

There is a lot of misunderstanding when it comes to 235 hydraulic lifters. 235 lifters are not like most lifters that have a hole in the pushrod seat and a hollow pushrod.
Lifters with a hole in the pushrod seat and hollow pushrods do two things. First they send oil to the rocker to keep the top end lubricated. Second and just as important they bleed air out of the lifter.
When Chevrolet started using hydraulic lifters in the 235 in 1950 they did have a hole in the pushrod seat and instead of a hollow pushrod they had a groove across the bottom face of the pushrod to allow air to escape. This lifter and pushrod style was used from 1950-52.

Starting in 1953 Chevrolet dropped the hole in the seat and grooved pushrod.
The 1953-62 Chevrolet 235 lifter, the style without a hole in the pushrod seat,  has no place for the air in the lifter to escape as the oil enters on initial filling( if installed without prefilling). Most Chevrolet shop manuals give you instructions on disassembling each lifter and prefilling them to assure that there is no trapped air, but most people mistakenly skip this step. 

 If you study a 235 hydraulic lifter you will see an oil supply hole in the side of the lifter body. Oil coming from the engine oil galley enters this hole. This hole lines up with a groove around the lifter plunger. The groove in the lifter plunger has a hole in it so the oil can flow into the center reservoir. From there it is drawn down through the check valve into the lower working area of the lifter each time the plunger makes an upward movement inside the body. This happens every time the valve it has opened closes. The spring inside of the lifter pushes the plunger back up as far as it will go.

Every time the engine is shut off all the lifters that are still holding a valve open bleed the oil from the lower working area up between the plunger and the body (this due to valve spring pressure). As the oil reaches the groove around the plunger it is directed back through the supply hole into the reservoir.  This next part is very important! This also happens a little bit every time a valve is opened while running.

The problem comes in when all the air is not removed on assembly.
What appears to happen at higher RPMs is that as the lifter reaches the maximum height of the cam lobe and starts back down inertia keeps the oil moving towards the top of the reservoir. If there is air trapped in the reservoir it will move to the bottom of the reservoir as it is lighter than oil. From there it will enter through the check valve into the lower working area as the lower working area of the lifter recharges on the downward travel. This gets the lifter ready for the next valve opening. This air that has now entered the lower working area of the lifter makes the lifter spongy and creates lifter clack.
Once the RPM's are reduced and the oil again stays in the bottom of the reservoir, the air bleeds back up from the lower working area between the plunger and body, back into the reservoir, and the lifter will quiet back down.

As I stated above 1953-62 Chevrolet lifters do not have a hole in the pushrod seat, but most aftermarket replacement lifters do.

There is an easy fix for engines that have lifters with a hole in the pushrod seat.
You can do it with the lifters still in the engine.
By making a simple groove from the center of the bottom of each pushrod out to the edge any trapped air can escape.  The grooves do not need to be very deep, just enough to allow air to escape.

I have started venting lifters that do not have a hole in the pushrod seat. 
This can be done with a cutoff wheel on a Dremel. Here are some pictures.


A second problem lifters can have

Oil never bleeds out of a lifter when the engine is off.
They do bleed down but the oil just goes back into the top reservoir area of the lifter.
The first time the valve closes that a lifter is holding open, the spring in the bottom of that lifter pushes the plunger back up and oil is drawn through the check valve back into the working area and the lifter is ready to go.
If they click on start up it is because the plunger gets stuck in the bottom of the body after they bleed down while the engine is off and cools. As the lifter warms the body expands more than the plunger and releases the plunger and the lifter starts working correctly. This can happen with new lifters where the plunger is too tight in the body, and it can happen on old lifters that have gummed up internally.
If this is happening with new lifters they will wear in after some break in time and work correctly.
Used lifters would need to be disassembled and cleaned.

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