Post by tasilloracing76 on Jan 10, 2006 17:20:34 GMT -5
Tech Tip #2 documented the procedure to check valve lash. For any Slingshot engine that is run at or close to 6000 RPM, the valve train takes a tremendous pounding and the lash can be expected to open up with time. Additionally, the valve springs can be expected to loose compression force over time. Our experience has been in the 6 to 8 race timeframe. As inexpensive as the valve springs are, we generally replace them whenever we check the lash.
As the valve springs loose compression strength, the tendency is for the valve to lift off of the rocker at the maximum height of lift. This results in the valve opening a little higher than spec and remaining open a little longer. In our stock microd days, we used to run what was known as a slapper cam. In order to get more flow through the engine, a slapper cam will intentionally launch the valve higher than it's static lift and remain open longer in order to improve flow through the engine and thus increase horsepower. It was very tricky to get it right and involved matching the cam, the valve spring strengths, and header to work togethor at a certain RPM. This is ""NOT"" the objective with the Slingshot engine. The better the valve stays in contact with the rocker, the better the performance. In fact Tobias even sells washers that sit below the spring and 'increase' the spring compression force to insure that the valve does not launch at excessive RPMS. I've never used the washers and cannot comment on their effectiveness. Instead, we keep fresh valve springs in the engine.
The springs are tricky buggers to change since it involves compressing the spring in order to free the automotive style valve keepers which are a pair of locking devices that sit down into the round valve spring cap. If you're not an octopus, this is a 2 man operation.
CAUTION - VERY IMPORTANT - NOTE WELL: If the piston is not at top dead center on the cylinder that you are removing the valve spring keeper, you will loose the valve down into the cylinder. I can't imagine getting the valve back into position if that happens. PLEASE also note that top dead center on one cylinder is not the same as TDC on the other. You must insure that the piston is up as close to the top of the head as it will go FOR THE CYLINDER YOU ARE RELEASING THE KEEPER ON. And ALWAYS ONLY WORK ON 1 VALVE SPRING AT A TIME AND COMPLETE THE OPERATION BEFORE MOVING ON TO THE NEXT VALVE !
Step 1: With the sparkplug removed, manually rotate the flywheel such that the piston is up as far as it can go ! ! ! This will prevent the valve from falling into the cylinder. Work on only one valve spring on one cylinder at a time and do not allow the piston on the cylinder you are working on to be anywhere other than top dead center before you attempt to remove the valve keepers. My mechanic buddy recommends feeding a length of nylon rope into the sparkplug hole after the piston is a top dead center in order to take up any gap between the valve and the piston as well as provide a cushion to protect the valve and piston from making hard contact with each other.
Step 2: Remove the rocker and push rod to entirely expose the valve spring assembly.
Step 3: Use an overhead valve spring compressor to close up the spring which will allow enough clearance for the valve spring cap to be pressed down to expose the 2 keepers. Many times the spring cap and keepers are wedged together pretty tight and require the spring cap to be tapped on with a light plastic hammer to free it up. Once again you can see the value of feeding the nylon rope into the cylinder since the valve may want to make contact with the piston top. Never hammer on the valve spring cap with excessive force. I usually use a wrench socket placed squarely on the valve spring cap and tap the end of the socket.
Step 4: Remove the 2 keepers, valve spring cap and compressed valve spring.
Step 5: Compress the new valve spring and place over the valve stem. Reinstall the valve spring cap and with another pair of hands, position the 2 keepers into
the slot around the valve stem. Once in place, slide the cap up to capture the keepers and gently release the valve spring compressor to allow the spring to expand up against the cap.
Step 6: Reassemble the pushrod and rocker.
Step 7: Adjust the lash ( as described in Tech Tip #2 )
And that's pretty much it. You need an overhead valve spring compressor tool but that's pretty much the only special tool required.
PLEASE PLEASE PLEASE make sure that the piston is at top dead center and you take up the space between the piston and head with nylon rope before you release the valve spring cap and keepers. And don't ever turn the flywheel while the valve spring keepers are removed.
Best Regards, Ed T.
As the valve springs loose compression strength, the tendency is for the valve to lift off of the rocker at the maximum height of lift. This results in the valve opening a little higher than spec and remaining open a little longer. In our stock microd days, we used to run what was known as a slapper cam. In order to get more flow through the engine, a slapper cam will intentionally launch the valve higher than it's static lift and remain open longer in order to improve flow through the engine and thus increase horsepower. It was very tricky to get it right and involved matching the cam, the valve spring strengths, and header to work togethor at a certain RPM. This is ""NOT"" the objective with the Slingshot engine. The better the valve stays in contact with the rocker, the better the performance. In fact Tobias even sells washers that sit below the spring and 'increase' the spring compression force to insure that the valve does not launch at excessive RPMS. I've never used the washers and cannot comment on their effectiveness. Instead, we keep fresh valve springs in the engine.
The springs are tricky buggers to change since it involves compressing the spring in order to free the automotive style valve keepers which are a pair of locking devices that sit down into the round valve spring cap. If you're not an octopus, this is a 2 man operation.
CAUTION - VERY IMPORTANT - NOTE WELL: If the piston is not at top dead center on the cylinder that you are removing the valve spring keeper, you will loose the valve down into the cylinder. I can't imagine getting the valve back into position if that happens. PLEASE also note that top dead center on one cylinder is not the same as TDC on the other. You must insure that the piston is up as close to the top of the head as it will go FOR THE CYLINDER YOU ARE RELEASING THE KEEPER ON. And ALWAYS ONLY WORK ON 1 VALVE SPRING AT A TIME AND COMPLETE THE OPERATION BEFORE MOVING ON TO THE NEXT VALVE !
Step 1: With the sparkplug removed, manually rotate the flywheel such that the piston is up as far as it can go ! ! ! This will prevent the valve from falling into the cylinder. Work on only one valve spring on one cylinder at a time and do not allow the piston on the cylinder you are working on to be anywhere other than top dead center before you attempt to remove the valve keepers. My mechanic buddy recommends feeding a length of nylon rope into the sparkplug hole after the piston is a top dead center in order to take up any gap between the valve and the piston as well as provide a cushion to protect the valve and piston from making hard contact with each other.
Step 2: Remove the rocker and push rod to entirely expose the valve spring assembly.
Step 3: Use an overhead valve spring compressor to close up the spring which will allow enough clearance for the valve spring cap to be pressed down to expose the 2 keepers. Many times the spring cap and keepers are wedged together pretty tight and require the spring cap to be tapped on with a light plastic hammer to free it up. Once again you can see the value of feeding the nylon rope into the cylinder since the valve may want to make contact with the piston top. Never hammer on the valve spring cap with excessive force. I usually use a wrench socket placed squarely on the valve spring cap and tap the end of the socket.
Step 4: Remove the 2 keepers, valve spring cap and compressed valve spring.
Step 5: Compress the new valve spring and place over the valve stem. Reinstall the valve spring cap and with another pair of hands, position the 2 keepers into
the slot around the valve stem. Once in place, slide the cap up to capture the keepers and gently release the valve spring compressor to allow the spring to expand up against the cap.
Step 6: Reassemble the pushrod and rocker.
Step 7: Adjust the lash ( as described in Tech Tip #2 )
And that's pretty much it. You need an overhead valve spring compressor tool but that's pretty much the only special tool required.
PLEASE PLEASE PLEASE make sure that the piston is at top dead center and you take up the space between the piston and head with nylon rope before you release the valve spring cap and keepers. And don't ever turn the flywheel while the valve spring keepers are removed.
Best Regards, Ed T.