pecanflyboy
Well Known Member
I'm hoping to put a bunch of information into one place, as I've slowly pieced together this puzzle after a lot of reading. I'm not here to debate whether the oil bypass plunger is better than the vernatherm. Instead, I want to share the information I've gathered into one place. Much of this is opinion mixed with fact (isn't it always! Ha!).
What is the oil bypass plunger? Without the vernatherm, Lycoming uses a viscous PRESSURE plunger to control the oil to the oil cooler. The philosophy is that when the oil pressure is high (cold oil), the oil stays in the engine until the pressure reduces (hot oil), and then it is routed through the oil cooler. This is a simple device that uses a spring to exert force on the plunger. When the cracking PRESSURE is high enough to push the plunger off the seat (cold oil), most of the oil stays in the engine and does not go through the cooler. When the pressure is below the cracking pressure (hot oil), the plunger seals on the seat and most of the oil is sent through the oil cooler. The plunger seems to seal better than the venatherm and does not wear out and become less efficient over time. Apparently, the plunger can send as much as 30% more oil to the cooler resulting in more oil cooling.
The vernatherm uses the same kind of oil porting, but is activated by temperature and not pressure (unless there is a blockage in the oil cooler). Lycoming Service Instruction No. 1008C explains how to remove the plunger system and install the vernatherm. It also shows all the part numbers.
https://www.lycoming.com/sites/default/files/SI1008C Bypass Valve Installation.pdf
Temperature moves the sealing surface into and out of the seat by extending and contracting the venatherm. While this is a proven and automatic system, users have reported that a new vernatherm can let as much as 30% of the oil bypass the oil cooler when fully extended (hot). The sealing efficiency is also affected by the aging of the mechanism. So, you will read about a user whose oil temperature has become higher than normal, installs the plunger system after removing the vernatherm, and immediately has cooler oil temperature. The oil that was bypassing the cooler because of a poor vernatherm seal is now being routed into the cooler system.
There is an issue with the plunger system that must be addressed in cooler weather. When the engine is warmed up, all of the oil will be sent to the cooler regardless of temperature. This will result in cooler than desired oil temperature during cold OAT. The solution is to install a ball valve in the oil cooler line that is controlled from the cockpit using a push-pull cable. A good way to do this is to install the control near the throttle/prop/mixture controller. Full forward opens the ball valve allowing full flow to the oil cooler. On takeoff, all of the knobs are pushed fully forward. Oil temperature is now manually controlled and must be monitored by the pilot. I?ve not yet found installation pictures of this ball valve, so any advice is greatly appreciated. It has also been noted that you don?t want to completely block flow to the oil cooler. The ?closed? stop should be adjusted to allow a small amount of oil to still flow.
The decision comes down to replacing the vernatherm or removing it and installing the plunger system for a fraction of the price. When working properly, the verantherm provides automatic oil temperature control. The plunger system is cheaper, maybe more efficient, requires the pilot to manually manage oil temperature, and adds a ball valve and fittings to the system that are potential failure points.
If you decide on the plunger system, here are the parts you will need. Most you can find at Aircraft Spruce:
Plug (may already be installed): 62417
Plunger: 62415
Spring: 69436
Gasket: MS35769-11
Push-Pull: A-700-BL-0960
Ball Valve: McMaster 4256K52 (3/8? NPT, PTFE seat and packing, brass body, stainless ball)
Any help with ball valve installation greatly appreciated.
What is the oil bypass plunger? Without the vernatherm, Lycoming uses a viscous PRESSURE plunger to control the oil to the oil cooler. The philosophy is that when the oil pressure is high (cold oil), the oil stays in the engine until the pressure reduces (hot oil), and then it is routed through the oil cooler. This is a simple device that uses a spring to exert force on the plunger. When the cracking PRESSURE is high enough to push the plunger off the seat (cold oil), most of the oil stays in the engine and does not go through the cooler. When the pressure is below the cracking pressure (hot oil), the plunger seals on the seat and most of the oil is sent through the oil cooler. The plunger seems to seal better than the venatherm and does not wear out and become less efficient over time. Apparently, the plunger can send as much as 30% more oil to the cooler resulting in more oil cooling.
The vernatherm uses the same kind of oil porting, but is activated by temperature and not pressure (unless there is a blockage in the oil cooler). Lycoming Service Instruction No. 1008C explains how to remove the plunger system and install the vernatherm. It also shows all the part numbers.
https://www.lycoming.com/sites/default/files/SI1008C Bypass Valve Installation.pdf
Temperature moves the sealing surface into and out of the seat by extending and contracting the venatherm. While this is a proven and automatic system, users have reported that a new vernatherm can let as much as 30% of the oil bypass the oil cooler when fully extended (hot). The sealing efficiency is also affected by the aging of the mechanism. So, you will read about a user whose oil temperature has become higher than normal, installs the plunger system after removing the vernatherm, and immediately has cooler oil temperature. The oil that was bypassing the cooler because of a poor vernatherm seal is now being routed into the cooler system.
There is an issue with the plunger system that must be addressed in cooler weather. When the engine is warmed up, all of the oil will be sent to the cooler regardless of temperature. This will result in cooler than desired oil temperature during cold OAT. The solution is to install a ball valve in the oil cooler line that is controlled from the cockpit using a push-pull cable. A good way to do this is to install the control near the throttle/prop/mixture controller. Full forward opens the ball valve allowing full flow to the oil cooler. On takeoff, all of the knobs are pushed fully forward. Oil temperature is now manually controlled and must be monitored by the pilot. I?ve not yet found installation pictures of this ball valve, so any advice is greatly appreciated. It has also been noted that you don?t want to completely block flow to the oil cooler. The ?closed? stop should be adjusted to allow a small amount of oil to still flow.
The decision comes down to replacing the vernatherm or removing it and installing the plunger system for a fraction of the price. When working properly, the verantherm provides automatic oil temperature control. The plunger system is cheaper, maybe more efficient, requires the pilot to manually manage oil temperature, and adds a ball valve and fittings to the system that are potential failure points.
If you decide on the plunger system, here are the parts you will need. Most you can find at Aircraft Spruce:
Plug (may already be installed): 62417
Plunger: 62415
Spring: 69436
Gasket: MS35769-11
Push-Pull: A-700-BL-0960
Ball Valve: McMaster 4256K52 (3/8? NPT, PTFE seat and packing, brass body, stainless ball)
Any help with ball valve installation greatly appreciated.