What if my original radiator cap doesn't work?

Stock plastic aluminum models of radiators normally will use the same cap style as the dealer original. Sometimes with certain foreign applications, the aftermarket radiator may use a different cap than stock. If you receive a part, and the cap does not fit, you can either visit your local parts store with the radiator. You can measure the radiator cap connection diameter and fit a cap based on their size. You can also contact us via a support ticket and we will respond with a cap number that will be stocked at your local parts store. Most of the parts we sell that may require a different cap will come with this information on the e-mailed invoice.

"Radiator Expert Tip" - Don't panic if the cap doesn't fit. The local parts store will have a cap that works for your radiator. If we have the part on file, rest assured we will provide that to you at point of sale."

http://www.radiatorexpress.com/faq.asp#coredimensiosn