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 Post subject: Sleeved BMC 1275 Blocks
PostPosted: Sun Aug 05, 2018 6:19 pm 
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Joined: Wed Jul 06, 2011 3:48 am
Posts: 554
Location: Florida
Mike, Ron, Joe, anyone... Hurricane left us with 2 '72 1275 blocks with sleeves in them. can these blocks be bored or re-sleeved to .040 over?

Bob


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PostPosted: Mon Aug 06, 2018 5:30 am 
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Joined: Sat Dec 12, 2009 8:08 am
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Location: Atlanta, Ga
Not sure what the others will say, but we ran several .040 bored factory sleeved blocks and never had a problem with them. I had heard someone say the sleeves would move because they were not stepped but I never had one move.

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PostPosted: Mon Aug 06, 2018 5:32 am 
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Joined: Sun Dec 06, 2015 5:33 am
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Location: Titusville, Fl.
It can be done, though I wouldn't recommend it personally for a race engine. .010 or .020--maybe. Good candidate for a large over bore. (1300+?)
But case in point: Don gave me a virgin block he recently had bored .040 to put together
for his Bug eye LP H/P car. He didn't notice it was a factory sleeved block but wanted to go with it anyway seeing how it was already bored. Yes, the sleeves looked thin (Maybe no more than .030 or so) I had my doubts but it just finished it's 2nd weekend at Daytona. Temps were no hotter than mine were (220 in the infield, 200 on bank) but got hot on a mid race track Black Flag hold up that brought every one to the pits. (230+ he said) Didn't seem to loose any water which is good. Have to wait & see for longevity.
Bottom line is, maybe good for a street engine?

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PostPosted: Mon Aug 06, 2018 7:29 am 
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Joined: Wed Jul 06, 2011 3:48 am
Posts: 554
Location: Florida
Our plan is to take out the factory sleeves, fit new blanks and then bore to .040. Real question is will the block take the resleeving. Folks doing the work say no sweat... and they have tons of experience with race engines. Problem with running the .020 bore is getting race pistons for them. I've got a .020 engine we've been running for years but have had to run stock pistons. Low rpm engine is ok but we are looking for 8 to 8,500. Thanks for the feed back.

Bob


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PostPosted: Wed Aug 08, 2018 12:15 pm 
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Joined: Thu Aug 15, 2013 5:16 am
Posts: 29
Personally, have always avoided boring the 1275 factory sleeve +40. Just can't make myself trust a 0.025" thick sleeve (factory sleeve OD is 2.872"). Have managed to find a decent non-sleeved block when needed one. Sounds like you have some actual experience to go by if you choose to go that route. I've done +20 on factory sleeves without problems.

My usual concern with sleeves is the OD of the sleeve. Too big of an OD on the sleeve and can cut through the cylinder walls between cylinders if there is much core shift on the block. I try to keep the OD to the 2.937" diameter. A 3" OD sleeve is right on the limit. Have sleeved all cylinders with the 2.937's and not had any problem with the sleeve moving, but the deck cupped a little. No effect on engine performance, but looked strange. Surfaced it to even everything back up and it awaits reuse.

If you cut the old factory sleeve out (or press it out), the bore is going to be just a few thou below 2.872. The sleeve is going to have a very narrow ledge to rest on. 2.935-2.870/2 = 0.033" nominal ledge (assuming 0.002 press). With a +40 bore having a nominal bore of 2.820, there is going to be a bit of the new sleeve over hanging the old factory sleeve bore. Not a problem, but will look odd from inside the crankcase.

HTH, Mike


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PostPosted: Wed Aug 08, 2018 2:06 pm 
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Joined: Wed Jul 06, 2011 3:48 am
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Location: Florida
That's great information for all of us Mike. Next question... where do you get .020 race pistons without paying the custom made tab. Thanks a bunch!

Bob


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PostPosted: Thu Aug 09, 2018 5:20 am 
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Joined: Thu Aug 15, 2013 5:16 am
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Don't have any existing job numbers/cards at piston vendors for +20 race pistons. If using that size, will order the "Mega" pistons from Mini Spares. A good quality cast piston in flat top or dish. Takes compression and rpm just fine. Similar to the former Hepolite 20747 and 21882. Uses the Cooper S ring package. Standard pin sizes for press fit in stock rods or make buttons and use with bushed rods. Many vintage racers will use this size to stay under 1300cc.


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