Going from 165mm to 155mm on a mullet Moterra SL is a sensible move — that 27.5 rear sits the BB a touch lower and 10mm off the crank length buys you back the pedal clearance without you really noticing the loss of leverage. Your EP801 spins happily at the slightly higher cadence too.
The catch you've already spotted: most aftermarket 155mm eMTB cranks are CNC'd billet (Hope, e*thirteen Race, 5Dev) and priced like jewellery. A few cheaper routes:
Best value option — SZZS / Chinese-made cranks
The community's go-to for cheap shorter cranks. A member here found
SZZS carbon cranks available in 150/155/160/165/170mm — but note those are for the Bafang M820 interface, so check spindle compatibility before ordering. For your EP801 you want a Shimano-spline (or compatible) crank. AliExpress sellers do plain alloy EP8/EP801-compatible arms in 155mm for well under £100 — not glamorous, but they do the job.
Shimano's own short arms
Shimano list the FC-EM600 / FC-E8000-series arms in shorter lengths, and riders on here rate the
FC-EM600 as a reliable EP8/EP801 replacement — around 550g and roughly €40, far cheaper than billet. Worth checking whether 155mm is stocked in your region; 160mm is the common one. If you can only find 160mm, that's still 5mm of relief.
One thing to verify before you buy
Shimano EP801 uses their own spindle/locator. As one member noted when fitting non-Shimano arms to a Norco Bullit, you'll want the
Shimano plastic washer/pin locator to seat third-party arms correctly — keep that in mind if you go aftermarket.
Honest steer: if budget's the priority and you don't care about weight, an alloy AliExpress 155mm EP801-compatible set is the cheapest fix. If you want something with a known reliability record for not much more, the Shimano EM600 arms are the safe shout — just confirm the length is in stock before committing.
If you tell me roughly what you want to spend and whether weight matters to you, I'll narrow it to one. And worth confirming your exact spindle interface from the printed crank arm if you're unsure — happy to check fitment from a photo.
