"Dyna vs Softail" is one of the most-searched Harley questions there is, and most of the answers are about how they ride. This one is about what they are made of, because that is what decides whether the used Softail part you found will go on your Dyna. We dismantle both: as of September 13, 2026 there are 984 Dyna parts and 1,032 Softail parts (791 Softail plus 241 Heritage) on the shelf, from 35 Dyna and 20 Softail donor bikes on the floor.
The short version: from 2000 to 2017 the two families used the same basic engine in two different case designs, sat in two completely different frames, and shared less than people assume. The 2018-and-later Softails are a third thing altogether.
The one difference that explains the rest
A Dyna mounts its engine on rubber isolators and carries a pair of exposed shocks at the back, like a Sportster or a Touring bike. A Softail bolts its engine solidly to the frame and hides its shocks horizontally under the transmission, so the rear end looks like a rigid frame from the side. That is the whole design brief of each bike, and every parts difference below follows from it.
Engines: A cases and B cases
Because the Softail engine is bolted solid, it needs its own vibration control. From 2000, Softails got the counterbalanced Twin Cam 88B, then the 96B in 2007 and the 103B from 2012. The B engines carry a pair of internal balance shafts, and the crankcases are cast differently to hold them. Dynas ran the standard Twin Cam 88 from 1999, the 96 from 2007 and the 103 from 2012 on some models and all of them by 2014, with no balancers, because the rubber mounts do that job.
So a Dyna engine and a Softail engine of the same year and displacement are not the same engine, and they do not swap:
- The Softail B engine has different cases, different mounting bosses and a different primary. It will not bolt into a Dyna frame.
- A Dyna A engine into a Softail frame is a solid-mounted, unbalanced engine in a frame that was never meant for one. People have done it; it is a project, not a swap.
- Top-end parts (cylinders, heads, rocker boxes, cams, oil pump, cam plate) are largely common across A and B engines of the same generation, which is why our Engine and Internals shelf does not separate them. Cases, flywheels and the primary side are where they diverge.
On our shelf: 52 Dyna engine and engine-assembly listings and 35 Softail. All of the Softail engines listed are B engines; if you are shopping for a Dyna, look on the Dyna or Touring shelf, because Touring and Dyna A engines of the same generation share cases.
Evolution-era bikes, 1984 to 1999, are simpler: the Evo was solid-mounted in Softails and rubber-mounted in Dynas with the same basic engine, so an Evo Big Twin crosses between them more readily. Mount hardware does not.
Frames and swingarms: nothing crosses
The Dyna frame is a conventional twin-shock backbone frame, revised heavily for 2006 with a wider rear tire, 49 mm forks on most models and a new swingarm. The Softail frame is a triangulated design with the swingarm pivoting on the frame and the shocks running forward under the gearbox; it was redesigned for 2000 to take the Twin Cam 88B. Neither frame, swingarm, shock or rear fender fits the other bike.
We have 43 Dyna frames and 14 Softail frames listed, all tagged with the donor's title status, plus 55 Dyna and 36 Softail swingarms. If you are building around a frame, that title tag matters more than anything else on this page.
Transmissions and primaries
Both families are Big Twins with a separate transmission behind the engine, 5-speed through 2005 on Dyna and 2006 on Softail, 6-speed after. But the Softail's rear shocks mount to the transmission area, and the primary cover has to clear the counterbalancer, so the gearbox cases and the inner primary are Softail-specific. The gear sets inside are close relatives; the housings are not. Match the family.
Wheels, brakes and front ends: sometimes
This is where things do cross, with care:
- Front ends. FX models in both families use the narrow 39 mm (49 mm on 2006-and-later Dynas) fork, and FL models in both use the fat 41 mm fork with the 16-inch wheel. Fork legs, triple trees and front wheels tend to cross between an FXD and an FXST of the same era, and between a Heritage and a Road King of the same era, more readily than between a Dyna and a Softail as families.
- Wheels. Same rim size, same axle diameter, same bearing spec and the wheel crosses. Both families changed axle size around 2007-2008, so match across that line, not across the family. See our used wheels guide.
- Brakes. Calipers and master cylinders are shared across Big Twins within a generation far more than wheels are; the 2000 caliper change and the 2008 Brembo change are the breaks that matter.
- Hand controls and switch housings. Shared across all Big Twins of a generation. The same 71567-96 switch housing is on Dyna, Touring and Sportster listings on this site.
Tanks, seats, fenders, exhaust: no
The Dyna's tank sits on a backbone; the Softail's sits on a frame with a different tunnel and mounts. Seats follow the frame. Rear fenders on a Softail are enormous and mount to the swingarm-and-frame arrangement; a Dyna fender bolts to struts. Exhaust routes around different frame rails. Assume none of it crosses, and be pleasantly surprised when a bracket does.
2018 and later: the Softail that ate the Dyna
In 2018 Harley dropped the Dyna and launched a new Softail platform that took over its models. The Street Bob, Low Rider and Fat Bob became Softails, on a new frame with a single shock under the seat and the Milwaukee-Eight engine, still solid-mounted with balancers. These bikes share nothing structural with either the 1991-to-2017 Dynas or the 1984-to-2017 Softails. We have 64 parts from 2018-and-later Softails listed, including Street Bob (FXBB) and Low Rider S (FXLRS) parts; they are their own shelf.
What this means when you are shopping
- Filter by family first, then by generation: Dyna 1991-2005 or 2006-2017; Softail 1984-1999, 2000-2017 or 2018 on.
- Engine, frame, swingarm, transmission case, tank, seat, fenders and exhaust: buy from your own family only.
- Front end, wheels, brakes and hand controls: check the era and the measurements, then cross if they match.
- When the listing says "Touring" on an engine or brake part and you have a Dyna, look closer; that is often the same part.
Dyna shelf (984 parts), Softail shelf (791), Heritage Softail shelf (241). If you are not sure which family a part belongs to, call +1 440-817-0268 with your VIN.
Written by the Exclusive Harley Parts team, Euclid, Ohio. Stock counts are from our live inventory on September 13, 2026 and change daily. Fitment notes are general guidance from the yard, not a substitute for the factory parts catalog. This article was drafted with AI assistance from our own catalog data and reviewed by our staff before publishing.