A few weeks ago there was a discussion here regarding how to repair a block that had been broken in the area of the starter mounting. This is the sort of calamity that presents great challenges, not the least of which is the engine has to be removed from the car and disassembled to perform a repair.
Obviously, the need to perform such a repair is to be avoided. A very important item to be aware of towards that end is to use ONLY correctly configured starter bolts for starter attachment. Starter bolts are not just 'plain, old bolts'. They are very special bolts and have several special features which are very important to their function. First, they are of specific and special lengths. Second, they have a very non-standard unthreaded shank-to-thread length. Third, they have a KNURLED section just above the threaded section. Fourth, for certain applications, they have a flanged head.
The special lengths of the bolts used is critical. The bolts must be long enough to accommodate the length of the starter nose hole which they are designed to work with, but not so long as to bottom out in the block tappings. Two of the 3 starter bolt attaching holes on most blocks are blind-tapped, so exact bolt length is critical for these positions.
Starters must be precisely located on the block if the starter drive-to-ring gear relationship is to function properly. However, starters have no obvious indexing feature (e.g. dowel pin(s)) to ensure precise location. However, there IS an indexing feature which is not so obvious to many. I'll get to this in a minute after explaining some other starter mounting features.
The block's starter mounting pad is fitted with 3 tapped holes. The 2 holes closest the rear of the block are used for mounting the ALUMINUM starter nose used for all 12-3/4" flywheel/flexplate applications. This starter nose uses 2 attching bolts of different lengths. One bolt is 1.84" long and the other is 4.66" long. Each of these bolts has a flanged head since the head bears on aluminum material.
An additional tapped hole is located about 1-3/4" to the rear of the other holes and not perpendicular to either of them. This tapping is used in conjunction with the outermost of the other two tappings to mount the CAST IRON and ALUMINUM noses used for starters used with 14" flywheels or flexplates. The cast iron starter nose uses 2 bolts of 3.63" length; the aluminum nose uses 2 bolts of 4.66" length.
With a single exception, over the course of 1963-96 Corvettes, only 3 basic design starter bolts have been used on Corvettes-----the 1.84" bolt, the 4.66" bolt, and the 3.63" bolt. There have been 7 different part numbers for the 1.84" bolt, 5 different part numbers for the 4.66" bolt, and only 1 part number for the 3.63" bolt. Within a length, all are interchangeable and virtually identical except earlier versions of the 1.84" and 4.66" bolts are SAE grade 8 while later versions are SAE grade 5. The 3.63" bolt is SAE grade 5. All bolts use 3/8-16 thread size.
So, how does the starter indexing work? Well, first of all, there are the tappings in the block. If you examine these carefully, you will find that these are more than just a tapped bolt hole. There is a 1/4" deep, 0.381-0.382" counterbore for each of the holes.
Second, the lowermost portion of the bolt holes in the starter nose are the same OD as the block bolt hole counterbores.
Third, there is the knurled area on the bolts. The knurling creates an effective OD of 0.380". So, the combination of the knurled bolts, starter nose lower hole OD and the counterbores in the block effectively indexes the starter to the exact mounting location necessary. Of course, for all this to work as designed, the proper bolts have to be used.
In addition to the proper bolts, the bolts have to be installed correctly. This means NO lock or flat washers under the bolt head. If one installs washers, it will cause the short length of knurled engagement in the block counterbores to be reduced to "0". This means the indexing feature is lost. (Note: in case you're wondering, the thin eye terminal on the grounding wire attached to one of the bolts causes no problem).
So, in case you're wondering what the ONE exception is to the 3 bolts described above for all 63-96 Corvette applications, the exception is 1982. 1982 Corvettes, which, incidentally, used the same starter nose as all 1970-81 Corvettes with auto trans, used a different starter bolt. It was originally GM #14037730 and later GM #12337924. This bolt is 4.66" in length, very similar to the 4.66" bolts used for many 1963-81 applications and uses 3/8-16 thread size like all other 63-96 starter bolts. However, there are 2 important differences. First, the shank OD is slightly larger at 0.390". Second, the knurled area is a TWO-STAGE OD. The upper portion which contacts the bore of the starter nose is 0.399" OD and the lower portion, which contacts the block counterbore is 0.380" (like the knurling on all of the other bolts). Although the only year this bolt was used on a 63-96 Corvette was 1982, it was used on many other GM applications of the 1982+ period. Why was this done? I have no idea.
Pictured below are the three 63-96 bolts (except 1982). Also pictured is the 1982-only bolt.
Obviously, the need to perform such a repair is to be avoided. A very important item to be aware of towards that end is to use ONLY correctly configured starter bolts for starter attachment. Starter bolts are not just 'plain, old bolts'. They are very special bolts and have several special features which are very important to their function. First, they are of specific and special lengths. Second, they have a very non-standard unthreaded shank-to-thread length. Third, they have a KNURLED section just above the threaded section. Fourth, for certain applications, they have a flanged head.
The special lengths of the bolts used is critical. The bolts must be long enough to accommodate the length of the starter nose hole which they are designed to work with, but not so long as to bottom out in the block tappings. Two of the 3 starter bolt attaching holes on most blocks are blind-tapped, so exact bolt length is critical for these positions.
Starters must be precisely located on the block if the starter drive-to-ring gear relationship is to function properly. However, starters have no obvious indexing feature (e.g. dowel pin(s)) to ensure precise location. However, there IS an indexing feature which is not so obvious to many. I'll get to this in a minute after explaining some other starter mounting features.
The block's starter mounting pad is fitted with 3 tapped holes. The 2 holes closest the rear of the block are used for mounting the ALUMINUM starter nose used for all 12-3/4" flywheel/flexplate applications. This starter nose uses 2 attching bolts of different lengths. One bolt is 1.84" long and the other is 4.66" long. Each of these bolts has a flanged head since the head bears on aluminum material.
An additional tapped hole is located about 1-3/4" to the rear of the other holes and not perpendicular to either of them. This tapping is used in conjunction with the outermost of the other two tappings to mount the CAST IRON and ALUMINUM noses used for starters used with 14" flywheels or flexplates. The cast iron starter nose uses 2 bolts of 3.63" length; the aluminum nose uses 2 bolts of 4.66" length.
With a single exception, over the course of 1963-96 Corvettes, only 3 basic design starter bolts have been used on Corvettes-----the 1.84" bolt, the 4.66" bolt, and the 3.63" bolt. There have been 7 different part numbers for the 1.84" bolt, 5 different part numbers for the 4.66" bolt, and only 1 part number for the 3.63" bolt. Within a length, all are interchangeable and virtually identical except earlier versions of the 1.84" and 4.66" bolts are SAE grade 8 while later versions are SAE grade 5. The 3.63" bolt is SAE grade 5. All bolts use 3/8-16 thread size.
So, how does the starter indexing work? Well, first of all, there are the tappings in the block. If you examine these carefully, you will find that these are more than just a tapped bolt hole. There is a 1/4" deep, 0.381-0.382" counterbore for each of the holes.
Second, the lowermost portion of the bolt holes in the starter nose are the same OD as the block bolt hole counterbores.
Third, there is the knurled area on the bolts. The knurling creates an effective OD of 0.380". So, the combination of the knurled bolts, starter nose lower hole OD and the counterbores in the block effectively indexes the starter to the exact mounting location necessary. Of course, for all this to work as designed, the proper bolts have to be used.
In addition to the proper bolts, the bolts have to be installed correctly. This means NO lock or flat washers under the bolt head. If one installs washers, it will cause the short length of knurled engagement in the block counterbores to be reduced to "0". This means the indexing feature is lost. (Note: in case you're wondering, the thin eye terminal on the grounding wire attached to one of the bolts causes no problem).
So, in case you're wondering what the ONE exception is to the 3 bolts described above for all 63-96 Corvette applications, the exception is 1982. 1982 Corvettes, which, incidentally, used the same starter nose as all 1970-81 Corvettes with auto trans, used a different starter bolt. It was originally GM #14037730 and later GM #12337924. This bolt is 4.66" in length, very similar to the 4.66" bolts used for many 1963-81 applications and uses 3/8-16 thread size like all other 63-96 starter bolts. However, there are 2 important differences. First, the shank OD is slightly larger at 0.390". Second, the knurled area is a TWO-STAGE OD. The upper portion which contacts the bore of the starter nose is 0.399" OD and the lower portion, which contacts the block counterbore is 0.380" (like the knurling on all of the other bolts). Although the only year this bolt was used on a 63-96 Corvette was 1982, it was used on many other GM applications of the 1982+ period. Why was this done? I have no idea.
Pictured below are the three 63-96 bolts (except 1982). Also pictured is the 1982-only bolt.
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