632 cid rod length 6.7 or 6.8

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632 cid rod length 6.7 or 6.8

Post  69bluehotrod on June 9th 2018, 7:48 pm

Need some input on this one,
Callies 4.75 p/n99S-H29-MG crank
have a new set of Grp 6.8 aluminum rods but not sure if they are optimal for this.
going with TFS a460 p/nTFS-5451T804-C03
Eliminator ultra block 4.6 bore


just want to know what the best would be
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Re: 632 cid rod length 6.7 or 6.8

Post  DaveMcLain on June 9th 2018, 8:28 pm

I don't think you'll have room for a 6.800 rod with that stroke just because the piston is getting pretty darn short. Maybe if you used a C460 style cylinder head with no side can't you could possibly reduce the compression height enough to make it work and still have some room for the ring pack.


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Re: 632 cid rod length 6.7 or 6.8

Post  JBR-3 on June 9th 2018, 8:36 pm

Any 4.750 motor is going to have pretty short pistons. One needs to look at
how much aluminum thickness will be on the rod small end. There is not much room
under the piston crown for rod material around the pin. A steel rod needs less room
than an aluminum rod. Even a flattop piston will have decent compression in a 4.750
motor, but if the piston has a dome, the under-dome area could be hollowed out
to make a bit more room for an aluminum rod.
The other consideration is how far the piston will be pulled out of the bore at BDC.
A longer rod will help this.

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Re: 632 cid rod length 6.7 or 6.8

Post  69bluehotrod on June 9th 2018, 9:05 pm

was thinking the 6.8 rod would not work with that stroke, going flat top on this deal, thinking will have plenty of compression with that bore and 85 cc head.
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Re: 632 cid rod length 6.7 or 6.8

Post  Frank Merkl on June 10th 2018, 1:26 am

ran 6.800 venoila s in mine
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Re: 632 cid rod length 6.7 or 6.8

Post  BBFTorino on June 10th 2018, 7:15 am

Run the longest rod you can. Keep the piston in the bore at the bottom!!

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Re: 632 cid rod length 6.7 or 6.8

Post  Lem Evans on June 10th 2018, 10:34 am

With that short of a C.D./C.H. be sure your piston guy knows the thickness of the rod above the pin. It's a lot more than a steel rod.

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Re: 632 cid rod length 6.7 or 6.8

Post  rmcomprandy on June 10th 2018, 12:59 pm

DaveMcLain wrote:I don't think you'll have room for a 6.800 rod with that stroke just because the piston is getting pretty darn short.  Maybe if you used a C460 style cylinder head with no side can't you could possibly reduce the compression height enough to make it work and still have some room for the ring pack.


I have built 632 Chevies for over 20 years with a 6.750" rod and 4.750" stroke ... their deck height is only 10.200".
With a 6.800" rod length using a 4.750" stroke and a 10.300" deck, the piston has MORE compression distance; which is also more than a common 347 Ford stroker.

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Re: 632 cid rod length 6.7 or 6.8

Post  Paul Kane on June 10th 2018, 1:48 pm

Generally speaking, I might have recommended 6.75” aluminum since it has long been an industry-wide standard length (ie, cheap and plentiful) but that’s a moot point these days as aluminum rods are all priced pretty much the same regardless of C-to-C.  And 6.75” does lower the pin and facilitate overall piston design and other necessary features.  (Incidentally there are steel H-beam Chrysler 440 rods (6.76”) available with BBC ends on them if that length is desired but steel is preferred over aluminum.)

If your current 6.800” rods can be used in the parts combo then that’s just fine, and maybe even preferred.
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Re: 632 cid rod length 6.7 or 6.8

Post  Paul Kane on June 10th 2018, 1:50 pm

Lem Evans wrote:With that short of a C.D./C.H. be sure your piston guy knows the thickness of the rod above the pin. It's a lot more than a steel rod.  
On that subject:  I did talk with Bill Miller at BME about pin end wall thickness and he told me he has gone as little as 0.200” wall on the pin end.  Of course aluminum pin end wall thicknesses are usually much greater and acceptable thickness depends on the build and application—a rod set with 0.200” wall probably wouldn’t find its way into one of Frank’s engines, for example.  bom
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Re: 632 cid rod length 6.7 or 6.8

Post  69bluehotrod on June 10th 2018, 2:08 pm

Lem Evans wrote:With that short of a C.D./C.H. be sure your piston guy knows the thickness of the rod above the pin. It's a lot more than a steel rod.  

Well you will be the "piston guy"... and also "cam guy"..... so what measurement will you want/need to do pistons for it?
or i can make it real simple and send you one of the rods i guess


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Re: 632 cid rod length 6.7 or 6.8

Post  Lem Evans on June 10th 2018, 2:57 pm

69bluehotrod wrote:
Lem Evans wrote:With that short of a C.D./C.H. be sure your piston guy knows the thickness of the rod above the pin. It's a lot more than a steel rod.  

Well you will be the "piston guy"... and also "cam guy".....  so what measurement will you want/need to do pistons for it?
or i can make it real simple and send you one of the rods i guess



Deck ht. of block, ring package preference [if any], etc. I short conversation would be in order.

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