Post by justyfied on Jan 18, 2014 13:33:41 GMT -4
Okay, This thread is about listing critical internal engine dimensions and how to measure them. Some of these measurements are extremely hard to find on the internet and should be verified for yourself. That is because in some cases you cannot get new parts or you may be able to substitute better (high compression) Pistons or stronger and longer connecting rods from a totally different engine. Some of these can be had for a very reasonable price, gently used from Ebay for example.
Just because an engine has the same bore of a Sherpa it doesn't mean that it is automatically a candidate for a second source of a replacement piston.
For example there is a Diesel engine that has the same bore of 78 mm as a Sherpa but these pistons are twice as long as a gasoline burning Sherpa piston which is 58 mm long on the thrust side and 90 degrees to that on the pin axis it measures 45 mm from the top of the piston to the bottom. There are numerous 2 stroke engine pistons that are not suitable also.
Anyway even if you do find a car gasoline burning engine that has the same bore as your car. The distance from the top of the piston to the center of the wrist pin (Gudgeon pin) will be different in most cases. This means that if by fluke it had the same size wrist pin as a Sherpa which uses a very rare 18mm pin, It might stick way up the bore and hit the cylinder head before the end of the full revolution.
This is called compression height and here is a picture showing my lousy artistic skills. It is the picture I made for the EF12 piston which has the same compression height.
To start with the EK42 comes with either pressed pin or full floating piston pins (Gudgeon). It is much easier to remove the piston from the rod if you have the ones held in with little clips in the piston next to the pins. You need to do this to get an accurate measurement of the con rod length.
With a big end bearing in place and with no pin on the small end use your vernier caliper to measure the length from the bottom edge of the piston pin hole to the top edge of the big end including the bearing shell.
For the Sherpa EK42 that is 90 mm. Then add 1/2 the pin size which is 9 mm plus 1/2 of the crank-- rod journal size, 21 mm for a total of 120 mm.
We all ready now know the piston compression height which is 28.2 mm. If we add that to the known 1/2 of the crankshaft stroke of 69.6mm we come up with the unit measurement of : 34.8mm + 120mm + 28.2mm = 183mm.
So after we subtract that from the measurements taken from the block crankcase and bore we have a remainder of .3mm which is how far the piston top is down from being flush with the deck. If you get the block surface ground down it could result in the piston sticking above the deck. Thankfully the head gasket is fairly thick.
I obtained the bore measurement by measuring from the deck down to the bottom of the bore where the crankcase starts. Remember that this is a bare block without a crank or pistons in place but the bearing shells are in and the main caps bolted in.
I had a short metal ruler across the bottom of the bore and with another larger steel ruler down the bore I could clearly see the measurement because the block I have has a large window in it the size of a fist. Thanks to a con rod blowing the balance shaft through the side of the block.
If you have a large vernier caliper it would be best as long as it shallow enough to fit in the 78 mm -3.07 inch bore!
Next I put the larger steel ruler through the crank journals in the block where the crank would sit. I made a bridge between pairs of journals and I used my small ruler across the bottom of the bore and placed a 3rd ruler from there to the ruler where the crank sits. This measurement is 43mm. Then you need to add 1/2 the diameter of the crankshaft main journal which happens to be the same size as the throw: 42mm. This gave a measurement of 64mm from the center line of the crank to the bottom of the bore in the top of the crankcase.
Now if we add the length of the cylinder the piston rides in which is 118mm to the 65 mm we get 183mm subtract from the 183mm of the crank -rod - piston we have the same distance as the piston is not below the deck.
Now that all of these measurements are present we can start plugging different numbers in for longer con rods and shorter piston compression height's to have a better combination for more power
Just because an engine has the same bore of a Sherpa it doesn't mean that it is automatically a candidate for a second source of a replacement piston.
For example there is a Diesel engine that has the same bore of 78 mm as a Sherpa but these pistons are twice as long as a gasoline burning Sherpa piston which is 58 mm long on the thrust side and 90 degrees to that on the pin axis it measures 45 mm from the top of the piston to the bottom. There are numerous 2 stroke engine pistons that are not suitable also.
Anyway even if you do find a car gasoline burning engine that has the same bore as your car. The distance from the top of the piston to the center of the wrist pin (Gudgeon pin) will be different in most cases. This means that if by fluke it had the same size wrist pin as a Sherpa which uses a very rare 18mm pin, It might stick way up the bore and hit the cylinder head before the end of the full revolution.
This is called compression height and here is a picture showing my lousy artistic skills. It is the picture I made for the EF12 piston which has the same compression height.
To start with the EK42 comes with either pressed pin or full floating piston pins (Gudgeon). It is much easier to remove the piston from the rod if you have the ones held in with little clips in the piston next to the pins. You need to do this to get an accurate measurement of the con rod length.
With a big end bearing in place and with no pin on the small end use your vernier caliper to measure the length from the bottom edge of the piston pin hole to the top edge of the big end including the bearing shell.
For the Sherpa EK42 that is 90 mm. Then add 1/2 the pin size which is 9 mm plus 1/2 of the crank-- rod journal size, 21 mm for a total of 120 mm.
We all ready now know the piston compression height which is 28.2 mm. If we add that to the known 1/2 of the crankshaft stroke of 69.6mm we come up with the unit measurement of : 34.8mm + 120mm + 28.2mm = 183mm.
So after we subtract that from the measurements taken from the block crankcase and bore we have a remainder of .3mm which is how far the piston top is down from being flush with the deck. If you get the block surface ground down it could result in the piston sticking above the deck. Thankfully the head gasket is fairly thick.
I obtained the bore measurement by measuring from the deck down to the bottom of the bore where the crankcase starts. Remember that this is a bare block without a crank or pistons in place but the bearing shells are in and the main caps bolted in.
I had a short metal ruler across the bottom of the bore and with another larger steel ruler down the bore I could clearly see the measurement because the block I have has a large window in it the size of a fist. Thanks to a con rod blowing the balance shaft through the side of the block.
If you have a large vernier caliper it would be best as long as it shallow enough to fit in the 78 mm -3.07 inch bore!Next I put the larger steel ruler through the crank journals in the block where the crank would sit. I made a bridge between pairs of journals and I used my small ruler across the bottom of the bore and placed a 3rd ruler from there to the ruler where the crank sits. This measurement is 43mm. Then you need to add 1/2 the diameter of the crankshaft main journal which happens to be the same size as the throw: 42mm. This gave a measurement of 64mm from the center line of the crank to the bottom of the bore in the top of the crankcase.
Now if we add the length of the cylinder the piston rides in which is 118mm to the 65 mm we get 183mm subtract from the 183mm of the crank -rod - piston we have the same distance as the piston is not below the deck.
Now that all of these measurements are present we can start plugging different numbers in for longer con rods and shorter piston compression height's to have a better combination for more power
