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Old 07-20-2009, 09:53 PM
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Boring a Go-Kart engine

I am helping another of the local up-and-coming hotrodders with another Go-Kart project and this one involves boring an Aluminum 7 HP B&S engine to .010 0versize. First I have never oversized one of these Aluminum bore engines so is it even practical? The service manual (does not distinguish between Iron or Aluminum bores) suggests honing the bore oversize but since it is quite a bit out of round would this be a good idea? My mill (old) is down right now and will be for a while but I suppose I could bore the block using my lathe and a boring bar however honing sure would be a lot easier if it works OK Any pros or cons on the hone method? Since I have never honed a block to an oversize if I decide to go that way any suggestions would be appreciated.

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Old 07-21-2009, 01:49 PM
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Don't know about a Briggs aluminum engine, but the old Vega 4 cylinder engines require honing with a special hone and a silicone solution.

Vince
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Old 07-21-2009, 02:32 PM
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How out of round is it? Are there pistons available for larger over bore? Man, you gotta be good to take out .010 let alone deal with an out of round cylinder to boot. When you look at it as only taking out .005 to the side and then deal with out of round, you have 0 wiggle room....
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Old 07-21-2009, 04:16 PM
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I'd say bore it an install a steel sleeve.

Vince
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Old 07-21-2009, 04:58 PM
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Quote:
Originally Posted by 302 Z28
I'd say bore it an install a steel sleeve.Vince

That was my first thought too and I did one not too long ago but as I said my mill is down this time so I would have to rig the block up on my lathe, probably not too difficult and is what I might have to do. I was just looking at the manual for briggs engines and it suggested honing the bore out to oversize so I thought it might be a viable option, I have some serious reservations about boring/honing an Aluminum bore anyway.
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Old 07-21-2009, 05:04 PM
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Quote:
Originally Posted by cranky1
How out of round is it? Are there pistons available for larger over bore? Man, you gotta be good to take out .010 let alone deal with an out of round cylinder to boot. When you look at it as only taking out .005 to the side and then deal with out of round, you have 0 wiggle room....


It is out about .002 but there is also a rather deep scratch in the wall so .010 might not even do it, pistons are available in .010, .020 and .030 in a rebuild kit that even includes the rod. The complete kit has the piston/pin/rod, both valves, rings and gaskets all for less than $90 so if I can get this bored/honed or just relined it will be a good deal since the crank and the rest of the engine is in really good shape.
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Old 07-21-2009, 05:14 PM
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I bore aluminum (no sleeve)Briggs &Strugglin engines all the time on a CK-10. Using standard techniques except for the final finish. This takes a special stone and a bit of finesse. NOT like the old high silicon Vegas.

The big problem with Briggs engines is most do not list a piston to bore clearance. So all you can do is bore to exact oversizes. OK for Briggs parts but aftermkt (China)parts are different.
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Old 07-21-2009, 06:05 PM
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I'v never done it, but my dad used a hone to overbore an old snowmobile engine .060. ran for a long time and he traded it for a newer spider.
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Old 07-21-2009, 09:17 PM
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Quote:
Originally Posted by BOBCRMAN@aol.com
I bore aluminum (no sleeve)Briggs &Strugglin engines all the time on a CK-10. Using standard techniques except for the final finish. This takes a special stone and a bit of finesse. NOT like the old high silicon Vegas.

The big problem with Briggs engines is most do not list a piston to bore clearance. So all you can do is bore to exact oversizes. OK for Briggs parts but aftermkt (China)parts are different.


I have a new set of 320 grit stones for my hone that are to prep a cylinder (cast iron) for use with Moly rings, would that be acceptable to finish hone an Aluminum bore?

Maybe I should just skip this "kit" and use genuine Briggs parts? The kid's father has not ordered the kit yet since we had not decided on a final size so he may be willing to change.
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Old 07-22-2009, 11:40 PM
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Take it to a shop that does rod resizing, the long hone stones on those machines will straighten the bore and keep it round...real fast too. You need silocon carbide stones for doing the bore, call around...many shops have the right stones for just this kind of job.

This is how I have always done snowmobile cylinders up to 0.060" over...AL bores make the work go fast.
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Old 07-23-2009, 06:46 AM
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Why do you want to do this to fix the out of round or to make more power?
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Old 07-23-2009, 08:33 AM
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Quote:
Originally Posted by metalmeltr
Why do you want to do this to fix the out of round or to make more power?


To correct the bore geometry and remove a very DEEP scratch!
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Old 08-01-2009, 04:25 AM
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You can take it out to size and get rid of any taper or out of round with a hone, but its got to be a rigid adjustable hone like Sunnen makes and not a glazebreaker type hone that's just spring loaded.

320 grit stones are way too fine for anything requiring .010 to come out of a bore. Taking out .002 with 320's would seem like a lifetime job by the time it was done. I can't comment on which stones would work best, or what would be best for flushing to keep the stones from loading up, since I've never honed any aluminum.

Boring .010 is no problem as long as the existing bore is indicated in concentric and parallel with the spindle. Will .005 per side get rid of the scratch?
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Old 08-01-2009, 07:26 PM
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I guess I should have followed up on this but I just decided to bore the thing out and sleeve it, I got a sleeve from a local bike and ATV shop. I just didn't like trying to hone an Aluminum bore, it's probably just fine to do that but I had no experience doing it and didn't want to take a chance on ruining the block and new piston. I have it almost ready to go back together but I had to wait on the proper piston for the iron bore which should be here Monday.
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