Two questions.

​​​​​​1. 38t 130 BCD chainrings: What is the distance between the center of the chainring to the outer edge of the bolt hole?
2. Chainring bolt for above: What are the diameters at the top of the bolt and bottom of the nut?

See attached figures for clarity.

Per Wolf Tooth Components, a BCD is the diameter of a circle that passes through the center of all bolt holes on the circle. (This assumes that all bolts in question are equidistant from the crank spindle axis. As I recall, there are a very few odball cranks for which this isn't the case.) So the distance from the center of the crank spindle axis to the center of a bolt hole on a 130mm BCD crank would be 1/2 (130), or 65mm.

Per Chain Reaction Cycles, the outer nut of a chainring bolt has an outside diameter of 10mm. So the distance from the center of the crank spindle axis to the outer edge of a chainring bolt hole for a 130mm BCD crankset would be 65mm + 1/2 (10mm), or 70mm.

Not sure how wide the overhang/flange at each end of a chainring bolt is. If you're looking for the distance to the outer edge of that overhang/flange, you'll need to determine and add that distance as well.

There's an image here that give chainring bolt dimensions:

https://www.ebay.com/itm/29411772038...Bk9SR7S5pZ6HYQ

That's good unless he wants to include the distance to the outer edge of the chainring bolt's flange/overhang also. The diagram doesn't give that dimension.

I'd guess somewhere between 1 and 2 mm for the flange/overhang. But I'd have to measure one with calipers to be sure.

I have no idea about why the OP is asking for this info, but I suspect it's about verifying fit requirements for a ring or crank.

So, as noted, the standard 130BCD ring has an on-center diameter of 130mm for the bolt circle, and 10mm bolt holes, with a C-bore of 13-14mm so the bolts can be near flush.

However, it's impossible to measure across a 5 pointed star, so maybe this will help.

The on-center distance between adjacent holes is 76.4mm. This can be measured eyeballing centers, or nearest edge to edge with a caliper and adding 10mm to account for twice the 5mm radius of the holes.

For those who might have other questions, here's a link to more reference data.

Can't seem to find 38t (or any number of teeth), 3-hole (or 6-hole) 130BCD chainrings anywhere.
Need to machine an adapter to fit 130mm, 5-hole BCD chainring to fit a 130mm BCD 3-hole crankset.

C-bore is a good idea. Can one do it both sides of a 2.3mm thick chainring?
How thick are the flanges of a chainring bolt and nut? Any standards?

Can you post a picture of that 130 BCD 3-hole crank? I don't think I've ever heard of, let alone seen one.

Weird bike parts are cool.

--Shannon

This frame is in storage, along with most bike parts.
Here is a photo from the web. More at https://www.ebay-kleinanzeigen.de/s-...67567-217-6446

You won't find any 38t 130bcd rings because they don't exist for a reason. 39t is the minimum number possible without the profile radius being too small to clear the bolt holes.

Here's a photo of a section of a 39t 130bcd ring which should make the problem clear.

FWIW- your bike appears to be belt drive, so unless that's to change, you need to find a belt drive ring. OTOH- the geometry of belt drive is different than chain drive, so it should (should, not will) be possible to machine a 38t or even smaller to fit those arms.

38t, 130 BCD chainrings
https://www.treefortbikes.com/Surly-...tainless-Steel
https://www.tradeinn.com/bikeinn/en/...ng/137172532/p
https://www.ebay.com/b/38t-130-mm-Bi...811/bn_1972112
https://www.amazon.com/Sugino-38t-13.../dp/B001GSQITW

Bike was a belt-driven e-bike.
It is now a regular bike, with belt gone. Will have a Nexus 8-speed at the rear, chain driven.

Looking at that crank, I'm thinking that you're gonna have to replace it. I doubt that replacement rings exist.

Unless they made it so that you can use 3 of the 5 holes on a normal 130 chainring. Which they almost certainly didn't, because doing something that smart and consumer-friendly in the bike industry will get you fired from your job and blacklisted for life.

--Shannon

I am quite baffled at what you are doing. The BCD thing is whatever but the bike looks to be a vintage e-bike and you are I am guessing replacing the entire wheel system which is the motor (and is quite funky and very vintage) with a nexus which could work depending on dropout out width but what is the plan for the huge battery hole you will have? Why not just either get this running or buy a bike closer to what you want? This one doesn't seem like an ideal candidate to be turned into a non-e-bike.

Getting an adapter machined, to fit suitable chainring to this crank!.

I am just replacing the rear hub.
The battery is removable & the battery hole is in a plastic frame attached to the top tube. Removed everything and you have a regular steel frame, sloping top tube, elevated chainstays and a geometry that suits me.

Ok I guess I would be interested to see the results.