Imperial Diameter Tubing Reynolds 753 Raleigh SBDU Ilkeston 1978 Road Frame SB1995 Seat Pin Size

My previous post turned up a couple of surprises about SB1995. The first, smaller surprise, was the owner’s initials on the fork. The bigger surprise was the Imperial diameter tubing. SB1995 reinforces my belief that SBDU frames can be read like a book, but like a good book, it can come back and bite you with unexpected twists.

Following on from determining the tubing type, is the seat pin size. This is a subject that should be straight forward but never is. Believe it or not, but online conversations about seat pin size do exist. You wouldn’t think they would but they do! Members of the Yahoo TI-Raleigh Team Pro group will see how often this subject is discussed.

But I’m going to park that discussion here for now, this post isn’t about the many wonders of seat pins and seat tubes, it is just about SB1995. However, I am going to open the subject up in another blog post very soon. I have my own opinions backed up with lots of measurements and data. Others have their opinions. Books like Tony Oliver’s “Touring Bikes a Practical Guide”, and the opinions books like these contain, are cited as references in seat pin size discussions. But it’s just not a subject I’m going to delve into right now!

SB1995 Seat Pin Size

Back to SB1995 for now. These were the tube diameters, and they are undeniably Imperial dimensions.

26.8 & 27.0mm

The Imperial seat tube outer diameter rules out the seat pin size expected for a 1978 Reynolds 753 frame. This means 26.8mm and 27.0mm will be too small. I’ve used a Dura-Ace and Super Record 27.0mm pin to demonstrate. Both pins are taken from Metric Reynolds 753 Time Trial Special frames.

The images don’t show how small the 27.0mm seat pin size is so here is a short video. There is more than a little side ways slop evident!

27.2mm

27.2mm is the size that many would suggest for an Imperial diameter frame. So should I assume the size as 27.2mm?

Maybe not! 27.2mm is still too loose. This is one of my Campagnolo seat pins from a 531DB frame and a Miche pin from a later Reynolds 753R frame, SB8851.

You can see from the images above just how small the Miche 27.2mm pin is, it disappears completely into the seat tube. There is still a bit too much side ways slop. My other early Imperial diameter 753 frames with the Capella lugs are definitely only 27.2mm, so SB1995 is different.

27.4mm

That only leaves 27.4mm

Keeping in mind that this is a 1978 Reynolds 753 frame, 27.4mm is a size that would never normally be considered. It’s a size associated with later 753R, the lighter gauge 753R. It could also be the size for the type of Reynolds 753 that came after 753R/T in 1989.

Enough of dropping seat pins in… I wanted to get this measured so out came the calipers.

The best way to measure for seat pin size is to measure further past the seat lug, by getting down into the tube. This avoids any possible distortion at the lug. I find spring calipers are best for this kind of depth measurement. You can see in the images below that the calipers reach down right past the 753 frame transfer, approx 5″ into the tube.

Set the adjustment so that you start to get just the slightest feeling of drag against the inner wall of the tube. It probably isn’t the most accurate measurement or method, but it will be close, you are never going to get a perfect tube. This is my measurement…

I have a 27.4mm seat pin fitted to SB8868, so in it went.

It went in perfectly.

27.4mm is the best fit.

Summing Up

This data was given by Mike Mullett on the Yahoo group.

Metric 753 – 801 Lighter Gauge – 27.0
Metric 753 – 803 Heavier Gauge – 26.8
Imperial 753 – 801 Lighter Gauge – 27.4
Imperial 753 – 803 Heavier Gauge – 27.2
531 Imperial – 27.2
All complete bikes built at Ilkeston were fitted with these size seat pillars.

Mike Mullett SBDU Workshop Manager

Not only is SB1995 built from a type of Imperial Reynolds 753 tubing (in a period where the tubing for 753 was Metric), it also needs a 27.4mm seat pin, the larger of the two seat pin sizes for Imperial 753.

I’ve only written a couple of posts about SB1995 and it is turning up surprise after surprise. I wonder what it’s geometry will uncover!

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SB1995 Details Documented

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