Post by Garrett on Apr 25, 2005 14:54:20 GMT -5
After getting the right camshaft for your application and getting the best carburetor you can possibly obtain, the third most vital piece in setting up a Briggs stock class motor is making it loose. Loose is fast but you must also have good ring seal so as to not lose compression. Fun huh.
THE FOLLOWING RECOMMENDATIONS ARE FOR THE OLD STYLE PISTON AND NOT THE NEWER RAPTER III**********(See out new book for the new pistons recommendations)
As Bob mentioned in an earlier article get the block line bored by a good Kart shop. This is very important in obtaining a round cylinder. After this operation hone the block for piston clearance. On cool bore motors you can go to .007/.008 and on the I/Cs keep it to .006/.007. This will give you enough clearance for heat expansion. You can buy very expensive bore gauges to measure this with OR simply use an old engine builder trick by using long feeler gauges. These gauges are available from MSC supply mentioned in an earlier article and are very quick and accurate.
Now you are ready to setup the piston rings. The larger single contributor to piston drag is the oil ring. Now that WKA has dropped the check for minimum inside diameter it is no longer best to use cut rings. The best method for reducing drag is to heat shrink the ring. You can do this by placing the ring into an old cylinder making sure it is square in the bore. I use the piston with the second ring installed to push the oil ring down just far enough to square it up. Now take a propane torch and heat the ring to a dull read. You can start in one place and as the ring heats up slowly move around the ring, obtaining the dull red color as you go. Using the Block to do this is very slow, as it takes quite a while to heat the block.
The best method if you are going to do many of these, is to obtain a replacement sleeve for the Briggs from a supply house. You will find that this will heat up much quicker and yet retain the roundness you need. It is very important to use a round fixture for this no matter whether you use the block or a sleeve. I always use a standard size Briggs oil ring in all my motors no matter the bore size (up to .030). Always use a new oil ring for this operation. If you compare a new ring against an older used ring you will note the difference in the size of the actual surface contacting the bore. The new ring will have very small edges and thus contribute to less drag after we shrink it.
You can also shrink the second ring on the Briggs in the same fixture with the same method. The top ring gets a little tricky. Most builders will use an over size ring for the bore size and simply cut the end gap down to .005 or so. The problem here is that we have a lot of drag caused by the oversize rings wall tension. You can shrink an oversize ring down to fit but you must be careful not to go too far. You will need a larger fixture to do this in as a standard sleeve will be too small to accept even a .010 over ring. I simply have taken the sleeve and had it cut by a machine shop into several pieces and then honed the pieced to the exact size needed for each bore size marking the piece as to .010, .020 etc. You will want to leave the ring just a little tight so you can cut the end gap to the exact size you want (.005 for a stock class) (.010 for restricted).
When cutting the end gap on the top ring don't use a file. It is a rare person that can actually file one size and keep it square with the other. I actually use my Dremel tool with a sanding disc installed. I simply squeeze the ring together on the rotating disc. This will cut both sides at the same time and keep them concentric.
Another area to work on is the rod and crank clearance. Typically a new crankshaft from Briggs comes in at .999 or .998 in diameter. A standard rod out of the box will usually be at 1.000 to 1.001. So you can see that you can have from .001 to .003 clearance if you do not check the components. Using a combination of a quality dial caliper and micrometer check the clearance. I work to have .003 on a new motor. If you have above .006 you are going to start braking parts(rod!!). If the crank is down to .995 use it in the fun motor on your neighbors Kart!!! It seems expensive but really a replacement crank is cheaper than a whole new motor!. Also use a hardened crank. This is just a durability issue no speed here. If you can find some of the older Briggs heat treated cranks from their industrial motors of days gone buy get them. You can recognize them by the dark edges around the crank journal. These are hard as a rock and just never seem to wear!. I have nothing positive to say about the new cranks from Briggs that are cyrogenically treated. They seem to wear just as quickly as a standard crank.. I also like to use the old style crank in all of my motors. They are lighter and cheaper.
As to how to obtain the proper clearance, I use the flexi-hone mentioned in my carb article. These can be obtained in 1" as well as 1/2" for use on the large and small ends of the rod.. Take the rod and torque the bolts down to exactly 100 inch lbs. before honing the journal end. This will insure roundness after you are finished. Simply use plain motor oil mixed with kerosene for honing oil. Hone the journal end for the .003 clearance we talked about and then hone the piston pin send of the rod. This end should be honed just enough to allow the pin easy travel back and forth. Do the same operation to the piston. Hone the pin holes just enough with the 1/2" hone to obtain free movement of the pin. This will give you a free floating pin.
As to maintenance of a Briggs there are as many opinions as engine builders. I take my personal motors and go through them after three races. A properly prepared motor will begin to fall off after that. What is happening is the top rings end gap will be wearing to the point of losing compression and these are torque motors no matter what. It doesn't hurt to check the other components in the motor at the same time, such as valves and your spring retainers. Use good oil (I like cool power) and change it often. It's cheap insurance. Don't worry about all of these fancy fast? oils as there is very little to be obtained here. The only really fast oils have illegal additives and are probably dangerous to your health any way.
One last area concerns the flywheel and type of cranker. It has always amused me that people will spend lots of time and money looking for the lightest possible flywheel from older motors and still put on the old style cranker. Just look at the rotating weight difference between the two styles of crankers!!! I have always used the new style with no problems. I also don't have to carry a cranker in my hip pocket all day.
Well there you go . My ideas on setting up the rings, rod and piston. Keep in mind to use good parts(new) when building or rebuilding a Briggs and you will have little problems on race day. That will give you time to work on the most important part of Kart racing - handling!
THE FOLLOWING RECOMMENDATIONS ARE FOR THE OLD STYLE PISTON AND NOT THE NEWER RAPTER III**********(See out new book for the new pistons recommendations)
As Bob mentioned in an earlier article get the block line bored by a good Kart shop. This is very important in obtaining a round cylinder. After this operation hone the block for piston clearance. On cool bore motors you can go to .007/.008 and on the I/Cs keep it to .006/.007. This will give you enough clearance for heat expansion. You can buy very expensive bore gauges to measure this with OR simply use an old engine builder trick by using long feeler gauges. These gauges are available from MSC supply mentioned in an earlier article and are very quick and accurate.
Now you are ready to setup the piston rings. The larger single contributor to piston drag is the oil ring. Now that WKA has dropped the check for minimum inside diameter it is no longer best to use cut rings. The best method for reducing drag is to heat shrink the ring. You can do this by placing the ring into an old cylinder making sure it is square in the bore. I use the piston with the second ring installed to push the oil ring down just far enough to square it up. Now take a propane torch and heat the ring to a dull read. You can start in one place and as the ring heats up slowly move around the ring, obtaining the dull red color as you go. Using the Block to do this is very slow, as it takes quite a while to heat the block.
The best method if you are going to do many of these, is to obtain a replacement sleeve for the Briggs from a supply house. You will find that this will heat up much quicker and yet retain the roundness you need. It is very important to use a round fixture for this no matter whether you use the block or a sleeve. I always use a standard size Briggs oil ring in all my motors no matter the bore size (up to .030). Always use a new oil ring for this operation. If you compare a new ring against an older used ring you will note the difference in the size of the actual surface contacting the bore. The new ring will have very small edges and thus contribute to less drag after we shrink it.
You can also shrink the second ring on the Briggs in the same fixture with the same method. The top ring gets a little tricky. Most builders will use an over size ring for the bore size and simply cut the end gap down to .005 or so. The problem here is that we have a lot of drag caused by the oversize rings wall tension. You can shrink an oversize ring down to fit but you must be careful not to go too far. You will need a larger fixture to do this in as a standard sleeve will be too small to accept even a .010 over ring. I simply have taken the sleeve and had it cut by a machine shop into several pieces and then honed the pieced to the exact size needed for each bore size marking the piece as to .010, .020 etc. You will want to leave the ring just a little tight so you can cut the end gap to the exact size you want (.005 for a stock class) (.010 for restricted).
When cutting the end gap on the top ring don't use a file. It is a rare person that can actually file one size and keep it square with the other. I actually use my Dremel tool with a sanding disc installed. I simply squeeze the ring together on the rotating disc. This will cut both sides at the same time and keep them concentric.
Another area to work on is the rod and crank clearance. Typically a new crankshaft from Briggs comes in at .999 or .998 in diameter. A standard rod out of the box will usually be at 1.000 to 1.001. So you can see that you can have from .001 to .003 clearance if you do not check the components. Using a combination of a quality dial caliper and micrometer check the clearance. I work to have .003 on a new motor. If you have above .006 you are going to start braking parts(rod!!). If the crank is down to .995 use it in the fun motor on your neighbors Kart!!! It seems expensive but really a replacement crank is cheaper than a whole new motor!. Also use a hardened crank. This is just a durability issue no speed here. If you can find some of the older Briggs heat treated cranks from their industrial motors of days gone buy get them. You can recognize them by the dark edges around the crank journal. These are hard as a rock and just never seem to wear!. I have nothing positive to say about the new cranks from Briggs that are cyrogenically treated. They seem to wear just as quickly as a standard crank.. I also like to use the old style crank in all of my motors. They are lighter and cheaper.
As to how to obtain the proper clearance, I use the flexi-hone mentioned in my carb article. These can be obtained in 1" as well as 1/2" for use on the large and small ends of the rod.. Take the rod and torque the bolts down to exactly 100 inch lbs. before honing the journal end. This will insure roundness after you are finished. Simply use plain motor oil mixed with kerosene for honing oil. Hone the journal end for the .003 clearance we talked about and then hone the piston pin send of the rod. This end should be honed just enough to allow the pin easy travel back and forth. Do the same operation to the piston. Hone the pin holes just enough with the 1/2" hone to obtain free movement of the pin. This will give you a free floating pin.
As to maintenance of a Briggs there are as many opinions as engine builders. I take my personal motors and go through them after three races. A properly prepared motor will begin to fall off after that. What is happening is the top rings end gap will be wearing to the point of losing compression and these are torque motors no matter what. It doesn't hurt to check the other components in the motor at the same time, such as valves and your spring retainers. Use good oil (I like cool power) and change it often. It's cheap insurance. Don't worry about all of these fancy fast? oils as there is very little to be obtained here. The only really fast oils have illegal additives and are probably dangerous to your health any way.
One last area concerns the flywheel and type of cranker. It has always amused me that people will spend lots of time and money looking for the lightest possible flywheel from older motors and still put on the old style cranker. Just look at the rotating weight difference between the two styles of crankers!!! I have always used the new style with no problems. I also don't have to carry a cranker in my hip pocket all day.
Well there you go . My ideas on setting up the rings, rod and piston. Keep in mind to use good parts(new) when building or rebuilding a Briggs and you will have little problems on race day. That will give you time to work on the most important part of Kart racing - handling!