Do I warranty labor on a modified or built engine, and how much?
Short answer
Warranty your workmanship, not the parts and not the customer's driving habits. A short, specific labor warranty with clear exclusions protects you far better than a vague 'we stand behind our work' line that means nothing when a rod lets go.
A built engine is not a stock repair, price the risk accordingly
On a stock repair, most failures trace back to either a bad part or bad install, and warranty logic is simple. On a built or modified engine, failures can also come from the customer's tune, driving habits, fuel choice, towing loads, or a part they insisted on that you flagged as risky. A blanket warranty that doesn't account for that is a blank check you're writing against decisions you don't control.
Warranty your assembly and installation labor for a defined period, typically 90 days or a set mileage, whichever comes first, and state clearly that it does not cover parts, tune-related failures, or damage from use outside the build's stated purpose.
Write down what voids it before the customer drives away
List the specific things that void the warranty: exceeding the tune's stated boost or fuel limits, towing beyond the build's rated capacity, competition use on a build not sold for that purpose, or any modification made by another shop after yours. This isn't about looking for a way out, it's about being honest that a built engine's life depends heavily on how it's driven, and you can't warranty someone else's right foot.
Have the customer sign an acknowledgment of the build's intended use and limits at delivery. This is the same document that protects you if they immediately go tow something twice the rated weight and come back with a blown motor three weeks later.
Decide your policy before the first call, not during it
The worst version of this conversation happens when a shop is figuring out its warranty stance live, on the phone, with an angry customer whose engine just let go. Write the policy once, put it on every build quote and invoice, and refer back to it calmly instead of negotiating case by case under pressure.
Build labor warranty policy
BUILD LABOR WARRANTY [Shop Name] warrants the workmanship of assembly and installation labor on this build for 90 days or 3,000 miles, whichever comes first, from the date of delivery. This warranty covers: failure directly caused by an assembly or installation error made by [Shop Name]. This warranty does NOT cover: - Parts themselves, which carry the manufacturer's warranty only - Failures related to tuning, fuel quality, or ECU settings - Use beyond the build's stated purpose (e.g., towing beyond rated capacity, competition use on a street build, sustained boost or load beyond the tune's stated limits) - Any work, tuning, or modification performed by another shop after delivery - Normal wear items Customer acknowledges the build's intended use as: __________________ and agrees not to exceed the stated limits above. Customer signature: __________________ Date: __________
Before you deliver a built engine
- 1.Written warranty period stated in days and miles, whichever comes first
- 2.Clear statement that parts are covered by manufacturer warranty, not by you
- 3.Named exclusions for tune-related, use-related, and third-party-modification failures
- 4.Signed acknowledgment of the build's intended use and its limits
Where owners get this wrong
- Offering a vague 'we stand behind our work' promise with no written terms.
- Warranting parts you didn't manufacture and can't control the quality of.
- Not documenting the build's stated use limits, so there's nothing to point to when it's exceeded.
Worth knowing: Warranty law and implied warranty rules vary by state, some states limit how much you can disclaim. Have your written warranty reviewed by an attorney familiar with your state before relying on it.
Ask your version of this question
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Now run it against your numbers.
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