347 VS. 331

What's up everyone!? I've been looking and looking, talking and talking, but would really love to get a great answer on building either a 331 or a 347 for my 65 Mustang. I have heard so many horror stories about the 347 being weak and having massive issues with the rods and the side thrust being too much for a stock 302 block, and it just doesn't handle the build very well. I have also talked to some engine builders who say that the 347 is just as reliable as anything, but could you really trust somebody who wants you to spend $8k in their shop?
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Active Member
Jan 19, 2020
Sioux Falls, SD
I've also been heavily researching for a 347 build. Early on, the 347s were victims of poor geometry in stock blocks and lack of properly designed aftermarket parts. Guys were grinding out clearances and cobbling together mismatched parts to make them work. Stroker parts and knowledge are much better these days, if you go with a stroker kit from a reputable company it will last a long long time. I personally would never use a stock block for a stroker even with the improved piston designs and proper clearancing, an aftermarket block such as Dart is designed from the beginning for stroker builds and inherent weaknesses of factory blocks have been addressed in the design. Many of the old issues can be avoided just by using an aftermarket block. As far as side thrust and rod ratio, today's oils and piston skirt coatings are more advanced and able to withstand the abuse better. No reason anymore to believe a stroker wouldn't last as long as a stock 302 if properly built.
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Active Member
Dec 3, 2009
Dayton, OH
I agree with above, all correct statements except I have had great success using a stock block for a stroker. The original horror stories started when a shop in carolina first came up with the 347 as an option for the 302. Their mistake was using too long of a rod and using very low tension oil rings and combine that with the internet and the myth lives on today, 25 years later. Same with TW heads. They had issues at first release with their cheap cast iron valve guides and again, thanks to the internet, the myth lives on today. They offered to replace every set sold with new bronze guides and then quit offering the pedestal mount versions altogether. 347 will make more hp than an equally modified stock stroke 351 because of the differences in rotational weight.
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Jan 15, 2020
New Hampshire
Modern built 347 - I went and did the 347 on my 65 - I basically got to the point after reading up that if I am not racing it or beating the piss out of it routinely go big- Go for the 347. Late to the thread but just finishing up my 347 build and waiting on a torque converter to start running!