# 1970 GTO 400 backwards connecting rods? uh...



## oilburner66 (Aug 5, 2011)

ok so havent been on for a while but ive been working on my goat and ive finally collected the parts to put my 400 together. history about the engine: when i bought my goat it had a 455 in it, but on a stand was a 70' 350 horse 400. according to the previous owner it was bored .030. after checking the bore size this was accurate. i started to build the 400 recently and long story short after lots of checking and measuring i got the crank in and torqued down. crank was ground and has .020 oversize bearings on mains and rods. spun the crank with one finger,spun awesome. installed pistons spinning after finger tight: some resistance but not bad because i put new rings in so no biggie. after i torqued one down to 43 ft-lbs: a little tighter. by the end of torqueing: wouldnt budge. DAM.IT. so rechecking everything im getting nowhere, everything is making sense except no clearance between rods!. so during a youtube visit i watched a guy building a 455. (btw pistons were in the engine when i got it i didnt put them together) in the video he mentioned that one side of the rods has a bigger groove that has to match up to the side of the crank journal, while the other side of the rod has a smaller groove which should face one another (between the two rods). now i checked mine with the direction of the pistons definetely pointing FORWARD on all pistons the smaller grooves on the rods are facing the outside towards the crank and the bigger groove facing inwards towards one another. i experimented and put two pistons in "backwards" and i got .012 clearance between the rods. now before i go sending my pistons out to be swapped around am i correct in the bigger groove facing out and the smaller groove facing in towards eachother? sorry for the lengthy post but i want to make sure i got all the info out.


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## BearGFR (Aug 25, 2008)

Check your rod bearings closely. You'll see that one edge has a chamfer on it while the other edge does not. The chamfered edges go to the "outside", next to the crank counterweight, so that the bearings don't bind on the fillet there at the edge of the rod journal and the counterweight. If you get the rod bearings in backwards, they'll bind up when you torque them down.

Bear


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## ALKYGTO (Mar 29, 2010)

:agree The bearings should also be "keyed" into the rods so they only go in one direction.

Sorry to hear this OP but it looks like your on the right track now.


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## BearGFR (Aug 25, 2008)

ALKYGTO said:


> :agree The bearings should also be "keyed" into the rods so they only go in one direction.


They are, but if you just happen to put the "top" bearing half in the rod cap instead of in the rod, and vice versa, you'll wind up with the edge chamfers on the wrong side of the rod.

Murphy's exception to the laws of mathematical probability: When N possibilities for parts orientations exist during assembly. The probability that one of the incorrect orientations will be chosen by random chance is automatically increased in direct proportion to the product of the degree of difficulty involved in disassembly/correction and the negative consequences of having assembled the parts incorrectly. The probablility that the correct orientation will be chosen by random chance is correspondingly reduced by the same amount. 

Mathematically, this reality is calculated by the expressions:

Pc = (1/N) / [(Ddam X Crud)]
Pi = [(N-1)/N] * [(Ddam X Crud)]

Where:
Pc is Probability of correct assembly
Pi is Probablility of incorrect assembly
N is the number of possible assembly orientations
Ddam is Difficulty (disassemble and modify)
Crud is Consequences (really ugly damage)

In the event that any of the critical quantities are not known with sufficient precision, the two probablilities Pc and Pi may be approximated by the following formulae:
Pc = 0%
Pi = 100%



Bear


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## ALKYGTO (Mar 29, 2010)

Pi/[R] *round

Cobbler/[R] *squared .


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## oilburner66 (Aug 5, 2011)

thanks for the help everyone my first engine build im bound to have questions and i appreciate the insight. ill put up a post with what i find, but i have a feeling i probably had the rod bearings in the wrong spots. id rather ask questions now so thanks again.:cheers


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## BearGFR (Aug 25, 2008)

ALKYGTO said:


> Pi/[R] *round
> 
> Cobbler/[R] *squared .


:rofl: 'Cept where ahm frum, it's pie are round, cornbread are sqwar... 

Bear


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## geeteeohguy (Feb 2, 2008)

Hilarious, as usual. Bear, it sounds like you speak from much experience, and have found the formula to be exact and accurate, proven true time and again. Although I never knew the mathematical formula, I too, somehow always end up with the same result: the harder it is to "make right", the more likely the screw-up. Especially if it's a tiny, minor, cheap part....like a rear main seal!!!


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## chuckha62 (Apr 5, 2010)

As an aside... Make sure your pistons were pressed onto the rods correctly. The notch on the piston should indicate forward when installed in the cylinder. For left bank (drivers side) the radius on the connecting rod should be forward, or with the notch (I believe, but double check with your machinist). On the Right bank, the notch should be forward and the radius should be facing the rear, or away from the notch (again, if memory serves). As I age, sometimes the hamster falls asleep on the wheel...


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