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Old 04-14-2013, 12:22 PM
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454 bored 30 over piston fit ?

dont know what to do had engine bored guy said he bored it straight upp thirty over since i didnt have the pistons with it , dont know how to find a forged piston to correctly fit that bore without reboreing.looking for a forged dome piston to paise compression to around 10.1

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Old 04-14-2013, 12:33 PM
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measure the bore and find out what pistons are available,finish honing for each piston
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Old 04-14-2013, 01:13 PM
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Why would you bore a cylinder by .030" (or any other size) without the piston in hand to measure , first?
Start by measuring the real cylinder bore diameter now , using a dial bore gauge.

Then you need to know the real measured diameter of all the 8 pistons when measured on the correct spot on the piston
and account for honing too. (usually -.005" at least)
If you fuuuggg it all up you may need to go to a +.040" piston.

CC your heads before selecting a dome volume. ALL non stock piston/head combos including 781-049 oval port heads need the chamber and dome clearance checked. minor clearance by hand may be required.
Head Chamber volume is never to spec.

and the deck clearance matters for the cr calc.

Last edited by F-BIRD'88; 04-14-2013 at 01:25 PM.
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Old 04-14-2013, 09:14 PM
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Email or call Pat:

McCarthy Performance in Kawkawlin | McCarthy Performance (989) 667-4071 & , Kawkawlin, MI 48631 | <Automotive/Repair & Service/Engine> - Yahoo! US Local

Tell him Gary from Roch sent you to him.He is well known for his work on BBC's.Tell him what happened and ask him for a solution.
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Old 04-15-2013, 09:22 AM
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Quote:
Originally Posted by rhino370 View Post
dont know what to do had engine bored guy said he bored it straight upp thirty over since i didnt have the pistons with it , dont know how to find a forged piston to correctly fit that bore without reboreing.looking for a forged dome piston to paise compression to around 10.1
Having bored the cylinders to 0.030" w/o matching the bore to the actual piston diameter is a crap shoot- BUT you may be OK. Usually the piston diameter is what has the clearance, in other words, a 0.030" over piston should measure something LESS than 0.030 over, so it will fit correctly in a 0.030" cylinder. You still MUST measure, though. Start by measuring or having the shop measure the exact bore diameters. From there, use the diameter of the piston to see where you are as far as piston to wall clearance goes. Different types/designs of pistons require different P to w clearance, so consult the manufacturer for their recommendations. One example can be seen here.

Another problem you may have is the bore finish. If the bore was final honed, the finish will determine what type of ring will work best. What you'll HOPE has happened is the machine shop left room for the final hone (0.003" to 0.005" LESS than the final bore needed).

Some info on BBC heads showing what the different chamber types look like, etc. can be seen here. The different chambers have different volumes and this has to be accounted for when figuring the CR.

There's a direct correlation between compression ratio (CR), fuel knock resistance, and cam specs. Info to determine what will work w/pump gas including links to online calculators is here. This info along w/the cam manufacturer's info and common sense will get you close to ideal regarding the cam and compression.

Measure the volume of the chambers (called "CC'ing" the chambers) you plan to use- don't try to go by any published figures. You can do this at home using nothing more than a piece of plexiglass or an old CD w/the plating scraped off, a large syringe and colored rubbing alcohol (use food coloring to make the fluid easier to see). Use grease or vaseline to seal the cover to the chamber. The procedure for CC'ing chambers is online as well as here if you do a search. The photo below shows the basics of it.

The piston dome volume is usually published. If it is you can go by that for figuring the CR. The engine has to be mocked up to measure the piston deck clearance (amount piston is down the hole at TDC). This can be done w/a straight edge and feelers if you don't have tools.

You need to consider the quench distance when setting up the CR. Just using a thick head gasket to lower CR is not the best way to go about it.


Last edited by cobalt327; 04-15-2013 at 09:27 AM.
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