Holy s*%$ .007 on the rods?

I checked the clearance on my new setup and I got .007 on my rods. That seems like WAY too much.

My setup is using some Top Fuel parts that I bought. BAE Billet Block, 4.5 inch crank 2.370 measurement on the rod journals, new B-2 rods, and 1512V bearings.

Is this clearance normal for a fuel setup? Will it live on my setup? I am trying to make 1500 to 1800 HP on this engine.

What oil should I use and what pressure should I see with these large clearances?

 

most blown alcohol engines have .006. 60 or 70w

 

Seems weird that there is that much difference between steel & alloy rods. We run under .003" with that much H/P on circuit boat steel rods, 26lbs boost & 7800 rpm.

 

Is your crank a new one or old ?

I know for a fact that Bryants and crower (or any good hemi crank for that matter) have extra clearance when made.

We have been running like that for ages so just put in good oil and have some fun!!

 

Thanks for all the responses.

JP - The crank is used. I bought it with the setup so I am not sure of the manufacturer. It has the counterweights in the middle so I guess that tells me it isn't a Winberg..?..

Mike - I guess if it is good enough for BAE I should be fine. Do you think I should run the Nitro 70 oil or is that too heavy? This is a nostagia combo that will be run on Alky.

Please any other info or experiences will be appreciated. Thanks again for a great website.

 

We tried running straight 70 Lucas in our TAFC, seemed too thick and the bearings looked bad quite often. We mix 50 and 70 now with 1 qt of stabilizer and the bearings are a lot happier. We vary the mixture dependant on temperature outside, if it's cold we use more 50 than 70, if it's 95 or 100 degrees outside we'll run more 70 than 50.

 

It is normal for us to have .007 clearance on the rods when brand new, they close up to .005 after a couple of runs.

 

Clearence

Remember the aluminum rod expands more than steel, and expands in also, closing the clearance when it gets warm.

Be sure your main clearance is set up like your rods for heavy oil. You do not want tight clearance on your mains and loose clearance on your rods. They each would call for a different weight oil.

You can always put in a .001 tighter bearing if you want it a little tighter.

 

Mike
The reason is the bolts. When an aluminum block is hot it grows you can pull the sleeve way easier than cold but the mains torqued down by the studs shrink and so do the big end of the rods.

 

I used to work at a small scientific research company called Research Associates and I recall reading about this problem several years ago.

It seems lack of dimesional stability is one of the common tradeoffs in aluminum alloys. You can eliminate the plasticity of the alloy but almost always with the penalty of sacrificed strength due to overaging typical in T-7 alloys. There has been a lot of writing about this problem in various engineering journals in the last 20 years.

Thats the end of my knowlege on the subject, but yes permanent changes do occur after use of high grade alloys in a heated environment like a car engine.

Edit: Now whether or not this is what is taking place in the rods I will leave to the experts.

Richard Gavle

 

Clearance closing up

My understanding is the serrations on the cap and rod bed in to each other and consequentially the clearance lessens.
JC

 

Yea, I think this is part of it, we had a set of BME's in a 9 second 455 pontiac motor back in the 80's. We resized them a couple times in 1000 runs. I still have them in a box and think about puttin' 'em in a street car; if nothing else, just to see how much more they'll take.

 

Yes, the big end of the rod gets smaller and stays smaller. It might have something to do with the serrations, but I believe part of it is that the rod is stress relieving.

 

wasn't there a discussion a while back regarding mic'ing the big end cold... warming the rod to operating temp and then mic'ing it again?

followed by a debate of what 'should' or 'would' happen? so what is the real verdict? and why? and if it does get smaller why doesn't it return to normal size when it cools?

somebody drag one of those rod guru's in here quick!! and hurry up before that canter guy starts another tech argument!! (<--only kidding of course... i'm poking fun at our 'small think tank'!)

and for what it's worth... i think it's great that canter is willing to share what he knows... most those le$$on$ are hard to learn and guys don't wanna share them

 

LOL

My block scenario was with aluminum caps. Im not sure what happens with steel caps.

 

Hi Guys
We don't really want to know. The 2.500" housing bore on an aluminum rod will grow about .003" larger at 200*F. This is why an aluminum rod needs an anti-rotation pin in the cap. I have shot the temp gun after getting back from a run and pulling the pan. (so it had time to heat soak) We saw temps around 170*F, 7 and 8 were hotter if the clutch had been worked. The large clearances on the fuel and alcohol is one of those things that have been done for so long I'm not sure anyone knows why. Obviously it works, but the problem is when the oil is lighter weight or not enough capacity or dilluted with alcohol or hotter (thinner) or after a extra long burnout or from more RPM than normal (10,700) there is a possibility that all the oil will be pumped up into the engine and it will starve for oil. The main reason an alcohol rod closes up is that it has more clamp load on the bolts than when it was originally honed. As a rod gets torqued more times, the bolt, washer and threads get burnished and the friction becomes less. So when we use our friction measuring device (torque wrench) to tighten the bolts we get more stretch which creates more clamp load and closes up the housing bore of the rod vertically. Sorry to Ramble

Brian Scollon
GRP Connecting Rods