We've got a 261 Chevy build going using the 261 crank and Ross pistons. We're leaning towards going with 270 GMC rods but curious what others have done or what is involved. Questions are what were the benefits (besides eliminating the pinch bolt at the wrist pin), what was involved, any machining required, cons, etc... The 235 in this little coupe is coming out and a fresh 261 bullet is taking its place.
You'll need to narrow the big end down to 261 bearing size, and run Buick V6 or custom made pistons if you want pop-ups.
as you have the pistons - you may not be able to run the jimmy rods, as they are longer than the 261 rods
Oh that's right Mark...Thank You!! I forgot about that. I seem now to recall the GMC rods are like ~.200" longer which might be the determining factor right there...unless we can find some Buick V6 pistons.
Thanks Mark - So after you go through with machining down the big end to accept the 261 bearing - what are the benefits of all the work? Thanks JD and good to know about Venolia - I'm not sure who Tom is but yeah...we're planning on using the 261 crank but we're trying to figure out the benefits of using the 270 GMC rods vs. just using the 261 Chevy rods.
261 rods are beefier than 235 rods, I think the pin is different size as well. The weak point is the clamp style small end. GMC rods give you a full floating pressure fed 0.990" pin and longer rod length. I've been looking into doing this with my 261 using Buick pistons, but most of the pistons are dished so it's hard to get a decent compression ratio. I have found these cast pistons Silv-O-Lite #1734 which are flat top but end up down the bore a little. I have a set of "unfinished" big end bearings for a GMC that will allow a tiny stroke increase which will allow me to deck the block down to give it the right amount of squish. Pistons will probably need to be notched for intake valve. This would end up at around 270 cu. in. I haven't pulled the trigger on the build as I also have a 270 GMC and I cant make up my mind which one to build. It will depend on the condition of the GMC when I tear it down. If you go to custom pistons I would suggest going with a "913" head rather than an "848", ports are the same but I suspect the 913 has better internal breathing. Custom pistons will allow you to dial in your compression without milling the head, which causes a whole bunch of issues. https://www.summitracing.com/parts/uem-1734-std
K. Pat Smith told me that using jimmy rod keeps the piston at tdc for longer than an original length rod, which helps burn the mixture better than stock rods, and are beefier than stovebolt rods. If my memory serves me correct, Jimmy rods have .990" pins, so the buick pistons will need their pin enlarged or the rod sleeved down on the small end.
I've never run off the shelf GMC Pistons. I've used Arias, CP, JE, and Ross every set was custom made. 302 rods had over a 1" pin. 228-270's had .990" which most aftermarket pistons used. I currently have 8" rods made by Cunningham. Oliver has some longer than 7". Countn' .....Tom was Tom Toros of Venolia.....Tom taught every one and seems every piston manufacturing owner worked for him at one time.
You can also have your crank turned to fit the rods, I've done it both ways. Spend the money and get a set of pistons made, Ross makes some good ones
Thank you guys for all the input - we really appreciate it. @marshall - that's a really good point about machining the crank instead of the rods. You mentioned you've done it both ways so if you machine the 270 rods to fit the crank, then I assume you machine the GMC bearings to fit as well or after machining will the 261 bearings work? @Gofannon - we are going with custom pistons and I'm curious about that "913" head. You mentioned it has better breathing - any idea how much better? @Stovebolt that makes sense and thank you for that. Good information for sure! @jimmy six - good information and didn't know about the 302 vs. 228-270 rods. And...I gotcha now about Tom - sorry I missed the connection there.
Just my hypothesis, but if you compare the top pic, a stock 950 head off a 261 (same chamber as 913), with the bottom pic, a severely milled 848, take note of the area of the combustion chamber that would be over the piston. This is the path that exhaust gases have to take to reach the exhaust port. The stock head is much more open, it's like relieving the block on a flathead.
1962 261, Clifford intake, Fenton’s old style with integral heat plenum, Holley 390 cfm. I am using the 913 head, ported. This was the first style of 235/261 Clifford 4 barrel offered, the newer models are water heat to the base of the carburetor plenum, this older one is exhaust heat. No wonder, IMHO, why you don't see more of these, why they didn't catch on. Poor fit to the stock or old Corvette style dual exhaust. A machinist friend married the two warring families together, a perfect union of Clifford and Fenton. If you look closely you can see the aluminum build up, quite an education to watch him work, he's a great talent.
No one mentioned pressurized pin oiling as a benefit to running GMC rods, but then maybe the 270 rods aren't drill up the beam like these 302 GMC rods I used in my 261. Big ends needed resized and pin holes were also bushed down, as mentioned before, but they worked well.
All the GMC rods I’ve ever seen had oil holes to the pins and squirt holes on the cam side.. You need to remember these were for heavy duty use. I know some racers drill the cam bearings, fed from the mains, out and screw in smaller orifaces to keep more oil to the mains/rods.
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