The Stovebolt Site Forums The Shop Area Engine Shop what year did the carb bolt pattern change? do zeniths fit 235s?

i thought it was 53 and older was a 216 manifolr and 54' and newer was the wider, 235.

ive seen carbs say its for a 1950-59 235. would this be the wider pattern? Do they mean the early 235s as well as the later 235s had the same bolt pattern?

I know someone who has a few gmc 270 zeniths. will these work?

sorry for all the questions, i made the mistake of buying a carb that dosent fit and now im asking more questions before i buy another

i dont think it helped no bolt patterns. thanks though jamie!

there was a rochester carb on a 56 gmc 270 i bought for the ashton. since i had carb issues on my 51 6100 235 dump truck splash engine. i sent out the carb to get done by a pro with the plans of swapping out the one on the dump. the day came i got it back all happy until i tried to bolt it on and it didnt fit. at one time gmc and chevy as far as i determined was they were the same. chevy changed the stud spacing and gmc did not. at what point they changed i dont know. was the intake on my dump changed i dont know that either without looking at the casting dates. i still have that rebuilt carb sitting on the shelve with no engine to mount it on other than one of my gmc engines which is not correct but will work if i wanted to use it. since i have extra zenith carbs it only makes a good conversation piece.

All 235's starting in 1950-62 have the wider bolt pattern then the 216 and early 235's.
I know a lot of GMC's use the 216 bolt pattern.

so the zeniths will fit a 216 but not a 235 , correct? good to know! thanks for you help guys!

i do a lot of truck shows and ive seen many rochesters on gmcs for whatever the reason but i have yet to see a zenith on a 235. if they would fit, there would be a shortage of zenith carbs

Mike (52 ashton),

I cannot check this now, but: last Fall, before I packed things away for hibernation, I think I found a Zenith carb for a 270 that fit a 235/261 intake manifold.

My guess for an additional reason why the GMC Zenith carbs do not find their way onto Chevy stovebolt-sixes (and Rochesters find their way onto GMC sixes) is the matter of availability (there are many more car/truck Rochesters to be found) and the matter of cost (a big difference in $$).

Question is simple, answer is NOT simple.

First part of the question already answered by Pre'68Dave.

Will Zeniths fit a 235? Depends.

Back in the early years of the automobile, the Society of Automotive Engineers (S.A.E.) proposed standards for carburetor mounting flanges.

Size 1 (nominal 1 inch carburetor) mounting c-c 2 3/8 inches
Size 2 (nominal 1 1/4 inch carburetor) mounting c-c 2 11/16 inches
Size 3 (nominal 1 1/2 inch carburetor) mounting c-c 2 15/16 inches
Size 4 (nominal 1 3/4 inch carburetor) mounting c-c 3 5/16 inches
Size 5 (nominal 2 inch carburetor) mounting c-c 3 9/16 inches

The sizes go up to a size 9 for updraft carbs, but I have not seen any downdrafts this large.

To add to Pre'68Dave's post, the 1950 and newer 235 used a size 3 flange, whereas the 216 used the size 2 flange.

Carter has made carbs in sizes 1, 2, and 3
Holley has made carbs in sizes 1, 2, and 3
Rochester has made carbs in sizes 2 and 3
Stromberg has made carbs in sizes 1, 2, 3, and 4
Zenith has made carbs in sizes 1, 2, 3, 4, and 5

G.M.C. 270's DEPENDING ON THE YEAR AND MODEL used size 2, size 3, AND an updraft carburetor.

So, will a Zenith for a G.M.C. fit a 235? Not if it is a size 5 designed for a 707 CID 6 cylinder.

Will a Zenith for a G.M.C. 270 fit a 235? If it is a size 3 downdraft, yes it will fit, but will probably not work well if the 235 is stock. A Zenith from a G.M.C. 236 would work much better on the 235. A size 2 Zenith or an updraft Zenith from a G.M.C. 270 would not fit.

As a general rule, when migrating an O.E. (original equipment) carburetor to a mostly stock engine, then the donor engine should be within +- (plus or minus) 3 percent of the donee engine FOR BEST RESULTS. 3 percent of 235 is about 7. Therefore, one could migrate an O.E. carburetor from engines of about 228 to engines of about 242 CID to a 235 and expect good results.

For the record, I.H.C. had both a 233 and a 240.

If the 235 is HIGHLY modified, then larger carbs may be used.

The size 3 Zenith from a G.M.C. 270 is an excellent choice for a slightly to moderately modified 261.

Jon.

Thanks, as usual, Jon.

The GMC 270 Zenith that I have is Plan B for my 261 that is over-bored 0.060 (to about 270, I think).