300 Explorer top end on Trailblazer!
Dec 15, 2000 | 01:13 PM
#12
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Joined: Oct 1999
Posts: 1,268
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Making a 250 into a 300 is something I have never tried, but, here are a few of the hurdles .
While it is true the bikes share the same connecting rod the stroke is different . So, a 300 crank (if it will fit in the 250 cases) would need be used , or , a stroked 250 crank . Remember too, the big end of the rod must clear the cases on the bottom side . I would be curious to compare main bearing numbers and lower case numbers between these two engines to see if maybe , by a stroke of luck they are the same . It is possible.
Comparing 300 and 250 base gaskets it would appear the cylinder would bolt on . The 300 piston has a different wrist pin location than a 250 . I do not know the height difference in cylinders if any. This may or maynot be a problem .
The 250 head can be easily modified to go on top of the 300 cylinder , no problem there .
Carburetor hook up and exhaust hook up may or may not be simple , ( I think it should be simple ) and of course if the cylinder height differs the upper head stay will need work too .
Ignition timing may need adjustment too , but if it does , that can be done easily as well .
As far as the cost goes . A stroked crank would sell for at least $200 , A cylinder probably sells for $350. Piston and gaskets and odds and ends would likely exceed $200. Port work , shop labor , head modifiing , and whatever else may total over $500 .
Used parts would lessen the costs of this conversion . I would suspect still the total cost of doing this (even if you perform at home what you can ) to exceed $600 easily . To have it built for you in a shop would be up and over $1200 .
If you guys check a few part numbers , I will check some heights and what not here . I don't think anyone will want to spend this kind of money on a Trail blazer , but it is at least interesting discussion for the forum .
While it is true the bikes share the same connecting rod the stroke is different . So, a 300 crank (if it will fit in the 250 cases) would need be used , or , a stroked 250 crank . Remember too, the big end of the rod must clear the cases on the bottom side . I would be curious to compare main bearing numbers and lower case numbers between these two engines to see if maybe , by a stroke of luck they are the same . It is possible.
Comparing 300 and 250 base gaskets it would appear the cylinder would bolt on . The 300 piston has a different wrist pin location than a 250 . I do not know the height difference in cylinders if any. This may or maynot be a problem .
The 250 head can be easily modified to go on top of the 300 cylinder , no problem there .
Carburetor hook up and exhaust hook up may or may not be simple , ( I think it should be simple ) and of course if the cylinder height differs the upper head stay will need work too .
Ignition timing may need adjustment too , but if it does , that can be done easily as well .
As far as the cost goes . A stroked crank would sell for at least $200 , A cylinder probably sells for $350. Piston and gaskets and odds and ends would likely exceed $200. Port work , shop labor , head modifiing , and whatever else may total over $500 .
Used parts would lessen the costs of this conversion . I would suspect still the total cost of doing this (even if you perform at home what you can ) to exceed $600 easily . To have it built for you in a shop would be up and over $1200 .
If you guys check a few part numbers , I will check some heights and what not here . I don't think anyone will want to spend this kind of money on a Trail blazer , but it is at least interesting discussion for the forum .
Dec 16, 2000 | 02:11 PM
#14
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Joined: May 2000
Posts: 1,819
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I went to the dealership today, but they were busy with all the sleds, so I didn't get any numbers, but the owner said that the 300 jug was prone to siezure, cracking, detonation, and wasn't anywhere as near as durable or reliable as the 250, that's why they discontinued the 300. Maybe that's why no one ever tried the conversion.
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250, 300, cage, connecting, conversion, discussion, explorer, hpd, jug, polaris, reed, rod, trailblazer, valve, xplorer
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