Hydro-Gear ZT-3100 Transaxle: Specs, Oil, Filter & Replacement
Short version: the ZT-3100 is Hydro-Gear's entry-level commercial-grade integrated transaxle: a 10 cc pump and 16 cc motor, up to 350 lb-ft of output torque and 9 mph, for machines up to 1200 lb, on a 1.0 in axle. It takes 20W-50 engine oil and a 52114 spin-on filter; change both at 75–100 hours, then every 400 hours. To replace one, order by the full model number on its label and tell us which side it's on.
ZT-3100 specifications
| Spec | ZT-3100 |
|---|---|
| Application | Commercial (Hydro-Gear's "entry-level commercial-grade" transaxle) |
| Model number prefixes | ZH through ZM, shared with the ZT-2800 and ZT-3200 (see below) |
| Pump / motor | 10 cc pump, 16 cc motor |
| Input speed | 3600 rpm maximum (no load); 1800 rpm minimum |
| Output torque | Up to 350 lb-ft (474 Nm), per Hydro-Gear's product page |
| Speed | Up to 9 mph (14 kph) |
| Machine weight (gross) | Up to 1200 lb (544 kg) |
| Axle | 1.0 in; tapered, 4-bolt or 5-bolt flange |
| Reductions | ZJ 20.69:1 · ZL 24.74:1 · ZM 27.23:1 |
| Standard features | Shock valves, external oil filter |
| Options | Internal expansion tank; Performance Package (shock valves, cut steel gears and a charge pump) |
| Oil filter | Hydro-Gear 52114 spin-on, 25 micron nominal / 40 micron absolute |
| Oil | 20W-50 engine oil |
| Unit weight | 32.5 lb (14.7 kg) |
The 350 lb-ft figure is a product-page "up to" value. Hydro-Gear's 2013 manual rated the ZT-2800/ZT-3100 group at 300 lb-ft intermittent and 160 lb-ft continuous; that is an older rating.
Model number prefix and reduction codes
Hydro-Gear's Cross Reference Guide says model prefixes ZH through ZM cover three series: the ZT-2800, ZT-3100 and ZT-3200. So a ZJ or ZL prefix tells you the family, not the exact unit. Within each series, Hydro-Gear lists the overall reduction by prefix:
| Prefix | Reduction | Listed for |
|---|---|---|
| ZH | 20.69:1 | ZT-2800 |
| ZJ | 20.69:1 | ZT-2800, ZT-3100, ZT-3200 |
| ZK | 24.74:1 | ZT-2800 |
| ZL | 24.74:1 | ZT-2800, ZT-3100, ZT-3200 |
| ZM | 27.23:1 | ZT-2800, ZT-3100, ZT-3200 |
The full model number (format XX-XXXX-XXXX-XXXX) identifies the exact build. Here's how to find and read it.
ZT-3100 vs ZT-3200
| ZT-3100 | ZT-3200 | |
|---|---|---|
| Hydro-Gear's description | Entry-level commercial grade | Commercial grade "for lighter weight applications" |
| Pump / motor | 10 / 16 cc | 10 / 16 cc |
| Input speed | 3600 / 1800 rpm | 3600 / 1800 rpm |
| Output torque, up to | 350 lb-ft (474 Nm) | 470 lb-ft (637 Nm) |
| Speed, up to | 9 mph | 9 mph |
| Machine weight, up to | 1200 lb (544 kg) | 1500 lb (680 kg) |
| Axle | 1.0 in | 1.125 in (1-1/8 in) |
| Reductions | ZJ 20.69, ZL 24.74, ZM 27.23:1 | ZJ 20.69, ZL 24.74, ZM 27.23:1 |
| Unit weight | 32.5 lb (14.7 kg) | 33 lb (15 kg) |
| Filter / oil | 52114, 20W-50 | 52114, 20W-50 |
The practical differences are the higher torque and machine-weight rating and the larger 1.125 in axle on the ZT-3200. Because the axle size differs, the two aren't swapped for each other. For the rest of the range, see ZT-2800 vs ZT-3100 vs ZT-3200 vs ZT-3400.
Oil, filter and service intervals
The ZT-3100 is serviced from the same Hydro-Gear manual as the ZT-2800 and ZT-3200 (BLN-52441).
- Oil: 20W-50 engine oil, the factory fill. General spec: minimum 9.0 cSt (55 SUS) at 230 F (110 C), API SL.
- Filter: Hydro-Gear 52114 spin-on (25 micron nominal / 40 micron absolute). Hand-tighten, then turn 3/4 to 1 full turn after the gasket contacts the base. Remove it with a socket-style wrench, not a strap wrench.
- Intervals: first oil and filter change at 75–100 hours, then every 400 hours.
- Capacity per transaxle (excluding the expansion tank and hose): 70.3 oz (2079 ml) or 71.9 oz (2126 ml) depending on model characters 7 and 13, or 56.6 oz (1674 ml) when character 13 is T (internal expansion tank). In practice, about 2 qt per transaxle, 4 qt for a pair.
- Draining: the manual drains through the filter port. Let units with an internal expansion tank cool below 100 F first, and use a drain pan 12 in or larger (8 qt is optimal).
- Filling: fill until oil appears at the top port, then top up through the expansion tank to the "Full Cold" line.
- Service kit: Hydro-Gear kit 72750 contains 5 quarts of Hydro-Gear Commercial Transaxle Oil, 2 filters and a fitting and pump, for ZT-2800 to ZT-3800 units.
More on oil and filters by series: Hydro-Gear oil & filter guide.
Purge after every oil change
Hydro-Gear says to purge air after the system is opened, after oil is added or changed, and after a replacement transaxle is installed. Signs of air are noisy operation, lack of power after a short time running and high operating temperature. In short: with the bypass open and the engine running, slowly move the control forward and reverse 5–6 times; close the bypass and do it again; stop the engine, check the level and top up; repeat until it runs quietly and smoothly. The ZT-3100 has a cam-style bypass that lets you push the mower a short distance at no more than 2 mph with the engine off; release the parking brake first, and remember there's no hydrostatic braking while it's open. Full steps: how to purge a Hydro-Gear transaxle.
ZT-3100 parts and rebuild kits
Hydro-Gear's free BLN-52441 service and repair manual has the exploded views and parts lists for the ZT-3100, and Hydro-Gear's support page has ZT service videos for trunnion seal, input seal and bypass seal replacement and the outboard oil retention kit. We carry Hydro-Gear repair and seal kits and replacement parts; send us your model number and we'll confirm which ones apply to your unit.
Ordering a replacement ZT-3100
- Read the full model number on the label (format XX-XXXX-XXXX-XXXX). Every character matters, because the ZH–ZM prefixes are shared by three series.
- Note the side: left or right as you sit in the seat. Left and right units can carry different model numbers.
- Add the mower maker's part number if the label shows one, plus your mower's brand, model and serial.
- Replace like with like. Hydro-Gear "strongly discourage[s] the use of any model not originally specified by the OEM."
- Fill and purge the new unit as above.
Shop ZT-3100 & ZT-3200 transaxles or send us your numbers through the Hydro-Gear part finder.
Frequently asked questions
What oil does a Hydro-Gear ZT-3100 use?
20W-50 engine oil. Hydro-Gear's BLN-52441 manual says the factory selected 20W-50 for normal operation.
What filter does the ZT-3100 take?
The Hydro-Gear 52114 spin-on filter, the same one used on the ZT-2800, ZT-3200, ZT-3400, ZT-3600, ZT-3800 and G700.
How often should I change the oil in a ZT-3100?
First at 75–100 hours, then every 400 hours, changing the 52114 filter at the same time.
What is the difference between the ZT-3100 and ZT-3200?
Same 10 cc pump, 16 cc motor and filter. The ZT-3200 is rated higher (up to 470 lb-ft and 1500 lb vs 350 lb-ft and 1200 lb) and uses a 1.125 in axle instead of 1.0 in.
Is the Hydro-Gear ZT-3100 a commercial transaxle?
Yes. Hydro-Gear calls it its entry-level commercial-grade transaxle. The ZT-2800 below it is residential (estate grade).
Sources: Hydro-Gear product pages ZT-3100 and ZT-3200; Cross Reference Guide; Oil; FAQ; Support; service manual BLN-52441 (Apr 2023) and BLN-52441 (Jan 2023); Toro-hosted copies of BLN-52441 (2021) and BLN-52441 (2013).
ZT-3100 & ZT-3200 transaxles · ZT-2800 guide · ZT-3400 guide · Oil & filter guide · All Hydro-Gear transaxles · Hydro-Gear part finder