Can I put a 26-27 coupe body on a 25 frame? If so is there any modifications needed?
A 26-27 frame is needed at a minimum.
Anything can be modified to fit anything. I've seen an F-150 body modified to fit a late model Crown Victoria running gear & frame. It actually looks decent -- but it was an unbolt one body and put the other body on.
In general it is much easier to use the same general time frame of parts. The 1926-27 cars were "improved" and had many but not all parts that do not fit well on the previous models. Bodies are one of those parts that do NOT easily fit on the early frames. Which is why you will see the advertisements for the depot hack bodies that they fit 1909-1925 rather than 1909-1927 chassis.
Fun Projects has an excellent drawing of the earlier style frame at: http://www.funprojects.com/pdf/FPIframe.pdf And any common 1909-1925 car T body will fit on any common 1909-1925 car T frame -- the body mounting holes are the same. You may need to relocate the some parts -- i.e. you may need a different radiator and hood depending on the body used etc. The 1926-27 frame diagram at: http://www.wescottsauto.com/Tech/FrameDiagram1926-27.pdf has additional dimensions
Also there is the matter of getting the steering parts to line up correctly. See: http://www.mtfca.com/discus/messages/29/24786.html for an explanation of why it is easier if you keep the parts from the same year range together. I.e. do not mix the 1926-27 parts with a 1925 frame/body – as it requires more “lining up the parts” to make it work.
If you have a 1925 chassis and a 1926 body -- trading either one of those for the other year range would make life a lot easier. And your car would normally be worth more after you were finished. I.e. do 1924-25 high cowl or 1926-27 improved or 1917-1920 touring or 1917-1921 roadster or 1921-1923 touring or 1922-1923 roadster etc.
Note many folks don't care what year engine is in the car -- they all fit with mininmal changes and it is always easy to put the correct year range engine back in a car.
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