291 assembly questions

Discussion of the Early Dodge / DeSoto HEMIs.

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291 assembly questions

Post by Spoons » Wed Nov 19, 2008 6:07 pm

Hey guys,
I think I might have a couple of problems that have reared their ugly head.
As I started assembly, I checked the ring gap and found it to be about .021. Also, skirt clearance is about .007 (.0035 per side). I quick checked the bore (motor bored .060 over) and found it to be 3.785 (Quick check with a snap gauge). This is about .0065 over what I needed. What do I do now??
Is the block junk?? Can't find .065 over rings and will the piston slap kill this motor??
Need a little advice......

Disappointed and depressed :(

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Re: 291 assembly questions

Post by 392heminut » Thu Nov 20, 2008 8:14 am

Sounds like a screw up on the machine shop's part. I think I would be raising cain with them and seeking a resolution from them by either sleeving all the cylinders and starting over or have them buy you new pistons and rings that will fit the bore. If your pistons are cast that is too loose of a fit and they will slap and eventually could break a skirt! The ring gap isn't as big of a problem, you would get a little more blowby, but I don't think it would be excessive.
Owner of the Poor Man's Hemi Cuda

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Re: 291 assembly questions

Post by charlie » Sat Aug 29, 2009 2:45 pm


Please feel free to call me

MOPAR CHARLIE 727-271-5068

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Re: 291 assembly questions

Post by George » Sun Aug 30, 2009 10:45 am

Sonic check to see if it can go the next size bigger, ot they pay for custom pistons to fit or sleeve down to standard or .030 & pistons.

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Re: 291 assembly questions

Post by polyspheric » Sun Aug 30, 2009 12:52 pm

I agree that the shop is at fault.
However, in my experience I have yet to find a shop that is willing to eat the cost of the pistons and boring it again - typically they simply say "don't pay the bill, and here's $100 for the block" - which of course leaves you with no block and pistons that you can't use.

My first step: ask them what they think happened. IMHO you have a 50% or better chance that they will not admit fault - "we did exactly what you told us", "you measured it wrong", "your mike is junk", "if you run it too tight it'll seize", etc.
If they actually admit error, ask "what do you think is fair?" They may surprise you by saying something reasonable, and this avoids needless friction. If they appear reasonable, I suggest they pay for the new pistons (and keep the wrong ones - they can sell them easier than you can), and you'll pay them something against the new machine charges "because I'm a nice guy".
Do not say "I'll pay 1/2" - because they'll just double the bill - use a flat figure like $50.
Sometimes (only if they appear competent) they'll give you more money back as a credit than as a refund, if yes, ask is the credit transferable.

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Re: 291 assembly questions

Post by johnny5 » Fri Jan 15, 2010 7:34 am

Did you ever get this worked out?

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Re: 291 assembly questions

Post by budmspeeco » Tue Feb 16, 2010 9:20 am

You started at the wrong end to verify your correct/incorrect (?) bore. Measure the pistons FIRST, add that to the manufacturers recommendation of clearances for the pistons SECOND, then measure the bore in your block THIRD. The first two numbers added together should be the third number, bore size, +/- .0005. Ring gaps should also be to mfg. specs. I have found the ring packs have begun to be installed with slightly larger gaps the last few years. I can guess this is due to the improved materials being used, because of less wear after the initial break-in period. Even skinnier rings lasting 100,000 miles is now almost a given. Your ring gap a few years ago would have been about .017. If your original measurement of the ring gap was in the .021 range I would suspect the bore was correct. If the block was over by the numbers you gave, .0065, gap would have increased to approx. .0507. A .001 increase in a 4" bore motor will give a .006 increase in bore diameter !! I hate to say this, but a lot of "hemi" guys on this site tend to "overthink" everything associated with a "hemi" motor. It is just that, a motor. Rod/stroke length debates in the 392 section come to mind. Pistons first ,then boring and decking another. Do the math, then worry !!! Buddy
'48 anglia

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