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Old 08-17-2019, 08:32 PM
ed heiser ed heiser is offline
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Join Date: Aug 2019
Boat: mastercraft 1993 prostar 205
Location: CA
Posts: 8
Why I needed a micrometer to change my alternator belt

The 1993 Prostar 205 I'm fixing needed a new alternator and belt, easy peasy right!
I can see now why the belt had never been changed. In order to put the new belt over the crankshaft pulley, you have to remove the raw water pump bracket! The 2, 5/8th inch bolt heads on the block are straightforward, but the allen head bolt on the bottom is very tight, and took a shot with every allen wrench I own to figure out the correct size. Turns out to be 1/4 inch. A 6 mm will strip it, a 7 won't fit. All my socket allen heads are metric of course.
I have a dial micrometer that measures in thousandths. I made a chart long ago that converts this to inches and mm. 1/4 inch is 250 thousandths (duh), 6 mm is 240, 7 mm is 280.
This shows why a 6 mm is just a little (10 thousandths) too small, and can strip a really tight bolt that's been sitting 26 years.
Strange design from a maintenance perspective, but not the first time I've seen changing a belt requiring disassembly.
This might help someone in the future reading this after I'm dead, so in a small way this makes me immortal.
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Old 08-17-2019, 10:00 PM
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sp00ky sp00ky is offline
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Join Date: Jul 2011
Boat: 2010 x-15, for sale:1994 prostar 205
Location: southeast
Posts: 849
Quote:
Originally Posted by ed heiser View Post
The 1993 Prostar 205 I'm fixing needed a new alternator and belt, easy peasy right!
I can see now why the belt had never been changed. In order to put the new belt over the crankshaft pulley, you have to remove the raw water pump bracket! The 2, 5/8th inch bolt heads on the block are straightforward, but the allen head bolt on the bottom is very tight, and took a shot with every allen wrench I own to figure out the correct size. Turns out to be 1/4 inch. A 6 mm will strip it, a 7 won't fit. All my socket allen heads are metric of course.
I have a dial micrometer that measures in thousandths. I made a chart long ago that converts this to inches and mm. 1/4 inch is 250 thousandths (duh), 6 mm is 240, 7 mm is 280.
This shows why a 6 mm is just a little (10 thousandths) too small, and can strip a really tight bolt that's been sitting 26 years.
Strange design from a maintenance perspective, but not the first time I've seen changing a belt requiring disassembly.
This might help someone in the future reading this after I'm dead, so in a small way this makes me immortal.
strange my 94 just had the 5/8 bolt for both ends. My buddy's 96 LT1 has 2 different size allen bolts. ***? they just throw whatever bolts they had lying around?
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Old 08-18-2019, 09:17 AM
ed heiser ed heiser is offline
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Join Date: Aug 2019
Boat: mastercraft 1993 prostar 205
Location: CA
Posts: 8
different sizes

Apparently so. Not surprising I guess, these are not a mass produced product. The monster tower I just installed had 4 set screw allen heads, two 5mm, two 4.5mm.
I've climbed in and out of this boat so many times in a month to get another tool that I've quit going to the gym! Who needs more exercise than this?
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Old 08-18-2019, 10:56 AM
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d2jp d2jp is offline
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Boat: 1992 PS 205-351w,285HO
Location: Central VA
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I was able to remove the two 5/8” bracket bolts and spin the bracket around the allen bolt-didn’t have to remove it, just moved enough to slide new belt in place. Good to know it’s a 1/4” Allen though!
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