Inliners International Forums General Discussion Bench racing Piston & Rod Specs?

I'll tell you what Beater, I will make a chart with all the GMC engines, and categorize all the rod lengths, piston compression hgts., strokes and bores,crank to block deck centerline, rod widths,journal sizes,etc. for each engine for your reference to mix and match'em, and also see what compression hgts. are needed for different aftermarket rod lengths....hows that!

Beater,

I'm in the process of building an external distributor/oil pump assembly driven off the front of the crankshaft. The original distributor gear stripped and took out the teeth on the cam. I didn't want to replace the cam ($$$$$) and have it happen again, so I've built this set-up. I'm using the original distributor and a BB Chrysler oil pump. It should be finished this week or next and I'll take some pictures and post them here if I can figure out how to do it.

By the way, I've talked to Mike Kirby and he says he's used the stock GMC rods in his Jimmy drag cars for years and never had a problem. The secret would be to get another piston that was lighter(?????). However, with the stock rod the CR distance is still going to dictate a long, heavy piston.

Ron

CNC,That would be great but I hate to put you through all of that. It would be a fantastic reference to have stashed on this site in the Tech Tips or where ever. My needs are fairly simple. I want to take Bill Fisher's idea of destroking a 270 a step farther. I want to bore a 302 to 4 1/8 and use a 228-248 crank. It would have 305.7 ci and should wind a bit higher because of the shorter stroke 3/16. I don't want to deck the block to get the piston up in the hole incase it doesn't work. I don't want to wreck a block. So I'll need a 3/32 taller piston if I stick with GMC rods. Are these rods forged? If they are why couldn't they be reforged, heated and stretched a bit?
Ron, I'll be looking forward to seeing that. Yes the length of the piston is a problem and my plan makes it longer!

Yeah,you'll need a piston with a comp. hgt. of approx. 2.470", might check with Venolia or Ross and others to see if their forgings can accomodate that much length. If you could find a set of real long rods like Ron used in his 320 GMC combo, that would help with finding a comp. hgt. more doable by the piston companies. I doubt you would be able to stretch the rods without hurting their structural integrity.

This is just a thought. I realize that more ci is usually good but in a hunt for shorter, lighter, and more easily found pistons it's an option. Ir might pull a few 248s out of the scrap pile. Bore a 248 3/16" to 3.9060 and use a 270-302 crank. (4" stroke ) This gives 287.585 ci and would allow the piston compression height to be shortened .09375 with stock rods. I don't know if that's enough to make a difference. With a slightly longer rod it may. Would the lighter piston off set the loss of inches? Could the lighter stuff be wound tighter even with the 4" stroke?

Lighter is always better! It usually isn't the RPM that is the main stress on the crank and rod, its the pushing and pulling of the piston. As the piston moves up the cylinder at a very fast rate and approaches the top of its travel,the instant it has to reverse direction and begin to move back down, the inertia of all that mass moving is trying to pull the rod in half. That is where all the main stress comes from. That is why many of the 283 Super Stock engines can twist so tight with just a stock small journal rod with relative ease. The piston and wrist pin are so light in comparison to the stock setup, there is hardly any inertia to overstress the rod even at 10,000 RPM. I've seen a piston and pin combo for those engines weight barely over 300 grams together, a stock TRW piston by itself can weigh 680 grams + 150 gram wrist pin with a total of 830 grams vs. 300 grams. That a huge amount of stress gone out of the rotating assembly for all 8 pistons....

Ron, Did you have to clearance the bottom of the cylinder for your Hudson rods? I thought I read in one of your earlier posts that you did.

I brought home am out dated Clevite Engine Parts book today. It has a good reference section for pistons, valves, seats, valve springs, push rods, and more. Lots to study!

I have a 1978 TRW engine bearing catalog #B-178,
and it has all the rod lengths and widths I was getting all the info I listed earlier, if you can find one of them, its a real treasure trove of info....

I'll start looking for one. My old boss kept all his old books. I'll pay him a visit. Thanks!!

Beater,

I didn't have to clearance anything except the cam. The Hudson rods barely contacted the cam so I clearanced it.

For those looking for long rods the 308 Hudson rods are pretty easy to find. The problem is finding enough to get a set close in weights (big & small ends) so they will balance.

I designed a piston and had JE build 2 sets for our shop. They have a 50 cc dome which could be cut down to lower the compression if needed. Their also about 400 grams lighter than the stock GMC piston. The pin is 80 grams lighter.

When I had Dema Elgin grind my cam I told him I wanted peak HP @ 5500 RPM and peak TQ at 4500. On the dyno they were 5500 & 4600.
The cam is a flat tappet with spring pressures of 100/300. I've had the engine to 6000 RPM with no problems so far.

The engine pulls HARD above 3500 RPM.

I'd recommend building your own roller rockers. It isn't hard to do and mine came out with a 1.85:1 ratio. I also made the shaft 1.00" so it wouldn't flex. The lift is .613/.573.

If I knew how to post pictures here I'd post some.

By the way, what RPM do you guys want to turn these engines and how much HP do you need?

Ron

Ron, whats the approx. total weight of a GMC rod(average)! Thanks....Im wanting to compare against some 7.100" H-Beams I have found available!

There are some Hudson rods on eBay now in different listings from the same guy. I don't know what I'm looking for though. I have my pictures on the Google picasa page. I just copy the link there. When I get here I click the "switch to full reply screen". Then click the first icon with the globe in the upper left. When he box comes op paste the link. When the next box come up type what you want to call your link. While you are working on it the link address shows but when you click submit your link will be blue. If it screws up click edit and try again. I learned this on the stovebolt BB in a section the call "The Short Bus"
Ron, thanks for all of this information! Tom

I am running a 8" GRP aluminum rod. Jerry Weight also designed a 4 bolt rod that he built with ballistic aluminum "Tennalum" that has seen over three years of use at Bonneville with no failure.

I have been considering making my own to. I've used the "TennAlum" quite a bit for other things, and its good stuff.I cant decide between it or 4340 steel. Im still in the thinking stage right now...

I've seen a picture of those somewhere (I think, 4 bolt rod, looked like aluminum, thought I saw it on here somewhere).

CNC,

I'll have to weigh one of the rods....memory has gone somewhere.

Ron

That rod is aluminum, it was made out of "Tennalum". Also the 4 bolt set up was designed because of clearance issues.

Actually Jerry Weigt's homemade rods, the whole block assy for that matter, have made 5 years to Bonneville and a trip to El Mirage without a single problem.

Their you go! By the way Jerry is also one great engineer. Lets not also forget the fact he built a GMC that layed down flat for Bonneville!

Does any one have pictures and specs of Jerry Weigts's and the 8" GRP rods?

Ok, Thanks for taking the time to explain that. I was going on some of what was in Fisher's book and something I read that I can't find now. I couldn't really see how 3/16 would make that big of a change. I don't have a 248 crank anyway. Back to the rod/piston search.

I talked to Ed at Oliver Rods. They have BBC rod in 1/4" increments from 7" to 8" Pricey at $1681.00 a set but it opens a new world of pistons, rings, pins bearings. If three of you got sets of 8 there would be a free set for me! These would give tlowe some options too. They would be the ticket for lighter pistons in our 292s! If they clear the cam. Oliver
What happened to all of Panic's posts?

I noticed he hasn't been hangin' out much lately....guess he got mad and went home, don't know! As far as the billet rods go, thats always been an option that most dont want to take,because of the dollar factor.I found the H-beam rods I mentioned earlier on as a lesser expensive route at $469 a set. They are 7.100" long, and are also BBC dimensions on the B/E and S/E and width.The bad thing about the 292 engines is that it is the Lone Ranger when it comes to this sort of components, there is absolutely nothing that can be made to interchange with the rod because of its length and relatively small housing bore combination. You just have to throw down and go custom either in the piston or rod department or both, and that makes the cost of that type of engine really skyrocket. The 235 or GMC's at least can use the BBC rods as a starting point, and with little modification, can be made to work with relative ease....

I spent most of today looking at piston sites and found quite a few that could work with a 7 1/2 to 8 inch rod. Especially rods with a slightly smaller big end to work with some creative crank grinding. I still think there is a rod somewhere that would work but haven't found a source of specs yet. I just takes time. I wonder if a trip to the car museum in Reno is in order? It's not far away. There is supposed to be a resource library there.

i also talked with oliver. they told me 300.00 per rod. if there were 80 ordered then the price goes to 150 per rod. it is simply a setup thing to get the cost down. i'd take 1 set at the 150 price. that leaves 12+ sets to be sold to make up the 80 rods. any takers? tom

Wait!!!!!!!!!!!!!!,stop the presses,,,, Tom would consider spending $900 for rods but does not want to spend $600 for pistons?????

Just kidding,
glad to hear you are considering to use something aftermarket & get quality parts.

MBHD

The encouraging thing was that Ed said there had been a lot of interest in rods for our engines. To get the $150 price do they all have to be the same length. What length would you want?

7.25" c to c and big end shaped like a sbc, .927 pin.

hank, i am budget minded , but you just got to have the long rod to do a really nice performance piston in these tall decked suckers. i have no other options except the aluminun rods, which only clear roller cams.
besides, with the money, i've lost far more than that in my 401k and roth ira's, ect. tom

Right now I'm looking for 7.50 to 8.00 inchers for the 302, but I will need a set for the 292.

Tom,
I understand the problems facing the 292.
I am just teasing you.
I think everyones ROTH IRA's & 401 K's are loosing, so I do understand that also. ;-)

MBHD

Yep, Our savings are shrinking and look at what we are planning on spending the remainder on. Is there something wrong with us?

BTT