CISON V8 Water Pump & Oil Pump: Leak Fixes & Brass Gear Upgrade Guide | Stirlingkit

by danna cai July 24, 2026

CISON V8 · V8 Pro · Stirlingkit Support Guide

CISON V8 Water Pump & Oil Pump Guide: Fix Leaks, Upgrade to Brass Gears & Keep Both Pumps Reliable

Coolant dripping from the pump shaft? Gears you've heard need upgrading? A brass gear that won't slide onto its pin? The oil and water pumps are the hardest-working accessories on the CISON V8 — and the source of most leak and reliability questions we see. This guide brings together the fixes and upgrades proven by the CISON V8 owner community — with our sincere thanks to every builder who shared them — plus the official upgrade parts now available, all in one place.

Applies to: CISON OHV V8 (44cc) · V8 Pro V1 / V1.1 · Flathead V8 — both pumps share the same basic design

Start here

Quick Diagnosis: Match Your Symptom

What you're seeing Most likely cause Go to
Noisy pump, sudden loss of oil pressure or coolant flow Worn plastic pump gears (earlier production) Section 1 — Brass gears
Replacement brass gear won't slide onto the shaft Bore/pin mismatch — verify before forcing anything Section 1 — Pre-install checks
Pulley shaft creeping out of the pump while running Shaft not retained — needs retaining compound Section 2 — Shaft retention
Coolant weeping along the water pump shaft (even when new) Shaft seal issue — repairable, or swap to the upgraded leak-proof pump Section 3 — Shaft seal
Drips at the hose barbs / hoses pull off by hand Unsecured hose joints Section 4 — Hose joints
Uneven oiling along the camshaft Oiler tube holes need inspection — never enlarge them Section 5 — Oiler tube

Section 1 · Upgrade

Brass Pump Gears: Who Needs Them, and How to Fit Them Right

Earlier production runs of the CISON V8 shipped with plastic gears inside both the oil pump and the water pump. Under sustained running these plastic gears can wear or strip — which is why precision brass replacements exist. Newer engines ship with brass gears already installed from the factory, so the first step is simply finding out which you have.

🔍 New to this? How a gear pump worksInside each pump are two small gears meshed together. As they spin, fluid gets trapped in the gaps between the gear teeth and the housing wall, and is carried around from the inlet side to the outlet side — that's the entire pumping action. It's a beautifully simple design, but it means the gears are the pump: if their teeth wear down or strip, fluid just churns in place instead of moving, and your engine loses oil pressure or coolant flow. That's why the gear material matters so much — brass keeps its tooth shape under heat and load in a way plastic can't.
Check first
  • Open the pump and look. Remove the pump cover screws and inspect the gears. Brass gears are unmistakably metallic gold; plastic gears are typically white or translucent.
  • Newer engines already have them. Recent factory batches include brass gears in both pumps as standard. If yours came with brass, a spare set is nice insurance — but there's nothing to upgrade.
  • If you have plastic gears, upgrade both pumps. The oil pump and water pump use the same gear design, and both are affected. Plan for two sets so you're not opening the engine twice.
Important

1. Measure the gear bore against the pump's center pin. The gear should slide onto its pin with a smooth, close fit — no force required. Before assembly, verify with calipers that the gear's inner bore matches the pin diameter on your pump (the stock pin is 2.5 mm on most engines). If a gear won't slide on freely, stop — do not press or force it. A mismatched bore will bind the pump or wear the pin. Contact us with your measurement and we'll sort out a correctly sized replacement right away (see the support section at the end of this guide).

🔍 New to this? "Bore" and "pin," and how to measure themThe bore is simply the hole through the middle of the gear; the pin is the fixed metal post inside the pump that the gear spins on. For the gear to spin true, the hole and the post need to match almost exactly — close enough to feel snug, loose enough to slide on by hand. To check: use digital calipers (an inexpensive tool worth having for any engine build). Close the outside jaws gently on the pin and read the display — that's the pin diameter. Then use the smaller inside jaws (the ones on top) inside the gear's hole — that's the bore. The two numbers should be within a few hundredths of a millimetre of each other. If the bore reads noticeably larger or smaller than the pin, that gear isn't right for your pump — don't force it, just tell us both numbers.

2. Test-fit the gear pair in the housing before closing the cover. Drop both gears in, mesh them, and spin them with a fingertip. They should rotate freely with no tight spots. If you feel binding, check for burrs on the gear faces and housing, and clean out any debris. The pump must spin freely by hand before it ever spins under power.

How to
  • Remove the pump from the engine and take off the cover plate. Note the orientation of the gears before removing the old ones.
  • Clean the housing thoroughly — old plastic gear debris left inside will circulate through your oil or coolant system.
  • Fit the new gears (after the bore and housing checks above), add a light film of oil on the gear faces, and confirm free rotation with the cover snugged down.
  • Reinstall the pump, then rotate the engine by hand a few turns and confirm the pump pulley turns smoothly with the belt fitted.
✓ Good to knowBrass gears are a genuine set-and-forget upgrade. Owners running brass report no gear failures — the remaining pump issues are the shaft and seal items covered in the next two sections.

Section 2 · Known quirk

The Pump Shaft That Slides Out — and the One-Drop Fix

A quirk both pumps share: the pulley shaft can gradually creep outward while the engine is running, until the pump stops driving properly. It's harmless if caught early, and the permanent fix costs one drop of adhesive.

Fix
  • Push the shaft fully back into position and confirm the gears still mesh and spin freely.
  • Apply one small drop of green sleeve/bearing retaining compound (Loctite 638/648 type) where the pulley meets the shaft. Green retainer is designed for exactly this — cylindrical press-fit joints — and holds far better here than threadlocker.
  • Let it cure per the bottle before running the engine. The shaft will not walk out again.
⚠ Use the right compoundBlue threadlocker on the shaft end can work in a pinch, but it's formulated for threads, not smooth cylindrical fits. Green retaining compound is the correct product and is just as easy to find.
🔍 New to this? Threadlocker vs. retaining compound — what's the difference?Both are anaerobic adhesives — glues that stay liquid in open air and only harden when trapped between two metal surfaces with no oxygen. The difference is what joint they're engineered for. Threadlocker (the familiar blue or red Loctite) is made for screw threads: it fills the spiral gap and stops bolts vibrating loose. Retaining compound (typically green, e.g. Loctite 638/648) is made for smooth, round press-fits — a shaft inside a pulley, a bearing inside a housing — where there are no threads to grip. It cures into a much stronger bond across the whole cylindrical contact area, which is exactly what keeps this pump shaft from creeping out. One more habit worth learning: with any anaerobic adhesive, less is more — a single small drop on a clean, degreased surface bonds better than a flooded joint.

Section 3 · Leak repair

Coolant Weeping Along the Water Pump Shaft

If coolant is tracking along the pump shaft and dripping from behind the pulley — sometimes from the very first fill — the sealing inside the pump body needs attention. You have two routes here: swap in the factory-upgraded pump, or repair the original. Both work; the upgraded pump is the permanent, zero-fabrication answer.

Recommended

CISON has released an officially upgraded water pump that was redesigned specifically to eliminate the shaft leakage seen on earlier pumps — optimized internal sealing structure, upgraded gaskets, and precision CNC-machined components.

  • True drop-in replacement: dimensions, screw positions, and installation are 100% identical to the original pump — no modifications, drilling, or adapters.
  • Swap procedure: let the engine cool completely, drain the coolant fully, remove the original pump, and bolt the upgraded unit straight on.
  • If your original pump has leaked since day one, or you've already repaired it once and it's back, this is the route we recommend.

View the Upgraded Water Pump →

Prefer to repair the pump you have? The original pump responds well to a seal service — here's the full procedure.

🔍 New to this? What a shaft seal is — and why "running dry" destroys itThe pump shaft passes from the wet side (full of coolant) out to the dry side (where the pulley is). The shaft seal is the small flexible ring that sits around the spinning shaft and keeps coolant from following it out — its soft lip hugs the shaft like a tiny squeegee. Here's the catch: that lip is rubbing against a spinning shaft continuously, so it depends on a thin film of liquid to stay cool and lubricated. Spin the pump with no coolant (or no grease on the lip during assembly) and friction heat hardens and burns the lip within minutes — after that, it can never seal again. That's why this guide keeps repeating two things: grease the seal when you assemble it, and never run the pump dry.
Diagnose
  • Drain the coolant, remove the pump, and disassemble it. The pump comes apart easily — there's no way to diagnose a shaft seal from the outside.
  • Seal present but unseated or off-center: re-seat it squarely in its bore. This alone fixes many "leaked since day one" cases.
  • Seal hardened, torn, or glazed: it has likely been run dry at some point and burned. Replace it — a dry seal against a spinning shaft doesn't survive long.
  • While you're in there, confirm the shaft is fully seated (Section 2) — a creeping shaft can also open a leak path.
Fix
  • Option A — replacement shaft seal: the direct fix. Contact us if you need one sent out.
  • Option B — snug O-rings over the shaft: a community-proven substitute. Fit one or two small O-rings that slide snugly over the shaft and sit in the seal position. Owners have run this fix long-term on both the water and oil pumps with no return of the leak.
  • Either way: grease the seal on assembly. A dab of waterproof grease on the seal lip and shaft prevents the dry-running that kills these seals in the first place. This one step is the difference between a seal that lasts and one that fails again in an hour.
✓ Good to knowRunning a drop of light oil in your coolant mix (or a 50/50 distilled water + automotive coolant blend) keeps the pump seal lubricated continuously — plain tap water offers the seal nothing.

Section 4 · Leak prevention

Hose Joints: Four Ways to Stop the Drips

The small coolant hoses on the V8 don't take standard hose clamps, so builders have developed several miniature alternatives. All four below work — pick by looks and what's in your parts drawer. And check your kit first: the accessory bag includes small cable ties intended for exactly this job.

Method How Notes
Small cable ties Cinch one over each hose barb Included in your kit's accessory bag. Fast and reliable; trim the tails for a cleaner look.
Fuel tube sleeve Stretch a short length of fuel tubing over the joint Acts as an elastic clamp. Clean look, easy to remove, and won't damage silicone hose.
O-rings Roll a snug O-ring over the hose onto the barb Same principle as the sleeve — nearly invisible once fitted.
Silicone hose upgrade Replace the stock plastic tubing entirely The root-cause fix. Silicone conforms to the barbs far better than the stock plastic hose and rarely needs clamping at all.
⚠ Two rules that apply no matter which method you chooseIf you can pull a hose off with two fingers, it isn't secured. And always keep the coolant vent open — a blocked vent pressurises the whole circuit and will find every marginal joint in the system.
🔍 New to this? Why a closed vent causes leaks everywhere at onceCoolant expands as it heats up — and any air trapped in the system expands even more. The vent is the escape route for that expansion. Seal it off, and the pressure has nowhere to go, so it builds inside the entire cooling circuit and pushes outward on every hose joint, every fitting, every seal simultaneously. A joint that was perfectly fine at normal pressure starts weeping; a marginal one sprays. So if your engine suddenly develops leaks at several joints at once shortly after warming up, don't reach for the zip ties yet — check the vent first. It's the classic "many leaks, one cause" scenario.

Section 5 · Inspect, don’t modify

The Camshaft Oiler Tube: Check Every Hole — But Never Enlarge Them

The oiler tube that feeds the camshaft has a row of small holes along its length — and if you look closely, they are deliberately different sizes. This is a metered-flow design: the graduated hole sizes balance oil delivery so that every position along the camshaft receives its share, from the hole nearest the pump to the one furthest away.

Inspect
  • Hold the tube up to a light and sight through each hole. Every hole should pass light cleanly.
  • Occasionally a hole may not be fully drilled through. If you find one, carefully drill it through to its original diameter only — match the neighbouring hole of the same size — or contact us for a replacement tube.
Warning

It's tempting to think "these engines need all the oil they can get" and open the holes up. Flow testing with the stock oil pump shows the opposite happens: enlarge the holes and oil takes the path of least resistance — it dumps out of the biggest, nearest openings while the smaller and furthest holes go dry. The graduated sizes are what force even distribution along the full length of the camshaft.

🔍 New to this? "Path of least resistance," explained with a garden hoseImagine a garden hose with the end capped and a row of holes punched along its length. If all the holes are big, nearly all the water gushes out of the first few holes — by the time the flow reaches the far end, there's almost no pressure left, and those holes barely dribble. Now shrink the near holes and keep the far ones a touch larger: the small near holes "hold back" the flow, pressure stays up along the whole hose, and every hole sprays evenly. The oiler tube works exactly the same way — with a small pump pushing a limited amount of oil, the graduated hole sizes are the only thing guaranteeing the far end of the camshaft gets lubricated. Drill them all bigger and you haven't added oil anywhere; you've just moved all of it to one end and starved the other.
⚠ Keep the holes at factory sizeRestore any incomplete hole to its designed diameter — nothing more. The only scenario where enlarged holes work is with a much higher-flow aftermarket pump (see Section 6), where total flow overwhelms the metering. On the stock pump, factory sizes are correct.

Section 6 · Advanced

Going Further: Electric Pump Conversions

Some experienced builders replace the mechanical pumps entirely with small brushless electric gear pumps — one for oil, one for coolant. The appeal: flow that doesn't depend on engine RPM, pre-oiling before startup, and stepping around the mechanical pump quirks above in one move. The trade-offs: it's a visible departure from the scale look, needs its own power and mounting, and calls for some plumbing fabrication. It's a rewarding project for a second build or a runner that sees long sessions — but for most owners, the fixes in Sections 1–5 make the stock pumps fully reliable.

🔍 New to this? What "brushless electric gear pump" actually meansIt's the same gear-pump principle from Section 1, but driven by its own small electric motor instead of the engine's belt. "Brushless" describes the motor type — no wearing carbon brushes inside, so it runs smoother and lasts far longer than a cheap brushed motor. Because it's powered by a battery rather than the engine, it has two superpowers the stock pump can't match: it delivers full flow at any engine speed (the stock pump only pumps hard when the engine revs hard), and it can run before the engine even starts — circulating oil through a cold engine first, so the very first combustion stroke happens on lubricated parts. That pre-oiling trick is borrowed straight from full-size racing engines.

Keep handy

Pump Care Quick Reference

Item Spec / practice
Gear material Brass (factory-standard on newer batches; upgrade kit for earlier engines — one set per pump, both pumps affected)
Gear bore check Verify bore matches the pump's center pin with calipers before installing (stock pin 2.5 mm) — never force a tight gear
Shaft retention One drop of green retaining compound (Loctite 638/648 type) at the pulley/shaft joint
Water pump upgrade Official CISON upgraded leak-proof pump — 100% drop-in, no modification needed
Shaft seal (original pump) Grease the lip on every assembly; O-ring substitute is a proven repair alternative
Coolant 50/50 distilled water + automotive coolant (or water with a little oil) — never plain tap water
Hose joints Every barb secured: kit cable ties, fuel-tube sleeve, O-ring, or silicone hose upgrade
Coolant vent Always open while running
Oiler tube All holes fully drilled, all holes at factory diameter — never enlarged

Parts Mentioned in This Guide

The upgraded leak-proof water pump, precision brass gear sets, O-rings, silicone tubing, and other CISON V8 spares are available from the Stirlingkit parts store — all sourced for this engine family.

Upgraded Water Pump → Browse All CISON V8 Parts →

Any Problem at All? Come to Us First

Don't spend days guessing — if anything about your pumps (or your engine) isn't right, reach out to us right away at service@stirlingkit.com or via the contact form. To get you sorted in one reply, please include:

  • Photos and a short video of the issue — bring these with your very first message. A 15-second clip of the leak, the gear, or the shaft tells us more than any written description, and it's the single biggest time-saver.
  • Which V8 you have — OHV V8, V8 Pro V1, V8 Pro V1.1, or Flathead. Parts differ between versions, and knowing yours lets us match the right solution and the right replacement part immediately.
  • Your order number — starts with #, e.g. #12345.
  • What you've already tried from this guide. And if a brass gear won't fit, include your caliper measurement of the gear bore and pin.

We respond within 24 hours, Monday to Friday (Hong Kong time).

Contact Stirlingkit →

Want to Help Shape What Comes Next?

The best ideas in this hobby come from the workbench, not the boardroom — and as this guide shows, they get built. If you'd like to be part of that, there's more than one way in:

Product & Manual Improvements

Build tips, corrections, clearer diagrams, translations, photos and videos of solutions you've proven on your own engine. Selected contributions are rewarded as our thanks.

Local After-Sales & Repair Partners

Experienced builders who can help fellow owners in their own region get engines diagnosed and running. If that sounds like you, introduce yourself.

Distribution & Wholesale

Interested in carrying these engines and parts where you are? See our wholesale & distributors page or write to us directly.

Reach out at service@stirlingkit.com and tell us which of these fits you — we read every message.

Get Involved →

Stirlingkit Note

Every fix on this page was shared by CISON V8 builders and confirmed with the engineering team — thank you to each of them. The brass gears that are now factory standard and the redesigned leak-proof water pump both started as builder reports; that's how improvements happen here, and the loop is still running. Parts and batches keep evolving, so for the latest details, check with us directly. And if anything on your engine isn't right, come to us first — photos, a short video, and your V8 version (OHV / Pro V1 / Pro V1.1 / Flathead) are all it takes to get it solved quickly.

Related articles

Back to Stirlingkit User Guide
Go to full site