CyclingFool95 

Somehow, after years of bike building, I've never had to spec out or install a headset. I've got a new build about to start - without installing the headset races and dry fitting everything together, how do I figure out the stack height that I can accommodate? Is it as simple as measure the head tube length, measure the fork length above the shoulder, and subtract?

How do I determine whether the thread is Italian or English? From what I've been seeing, it doesn't really matter and both English and Italian headsets will thread onto and Italian fork thread. Should I just assume that if the BB thread is Italian, so is the fork?

Similarly stupid question - not having any seatposts other than 27.2 or bigger laying around, how do I determine needed size? Measure with a caliper and go down to the next size? Or, line to line? Or, base it on how deformed the lugs look?

fietsbob

Stack height is the combined height of all the headset parts .. minus the parts that go inside the head tube..


[Sutherland's:]
Italian and British fork thread is 1" x 24 tpi or 25,4 x 1.058/mm) (Italy has long bought machine tools from England)


Yes an accurately reading caliper can measure the frame, tube ID.






...

dedhed

Steerer length - headtube length = max stack height. Shorter stack headset the locknut bottoms before tightening but works with more spacers. Too tall and you don't engage enough threads on the locknut. Also know if you need a JIS or ISO headset for cup and crownrace diameter.
An Italian threaded BB is no indication of the steerer threading.



https://www.bikeforums.net/bicycle-m...ck-height.html

https://parktool.co/blog/repair-help...gth-and-sizing

https://www.bikeforums.net/bicycle-m...ck-height.html

CyclingFool95 

The bike showed up early. Interestingly for a made in Italy bike, both bottom bracket and steerer are BSC threaded. Also interesting, while both Italian and English headset nuts seem to thread onto an Italian fork, an Italian nut wont thread onto a BSC fork.

The seattube measured 26.9-27.0 - does that mean I should be jamming in a 27.0 post or using a 26.8mm post?

Andrew R Stewart 

Seat post size is a resultant, best fit is not determined by measurement alone. Tubes get distorted during production and the neck of the hour glass is sometimes further in than a caliper can reach. So do measure and try what you read but be prepared to go smaller after the test fit.

BSC threading and spec pretty much has become the norm many years ago for most manufactures. However as on hand stock got used up and reordered the transition often blurred over a period of production. Also some brands tended to remain provincial then others who chose to be more world friendly.

As mentioned you have the stack math correct and that press fit diameters are vital to know. Andy

HillRider

Most bike shops will have a seatpost sizer tool which is a tapered graduated rod. You insert it into the seat tube and read the size graduation at the top of the tube. Far better than guessing.