I recently had my 260 in the shop for a 50hr oil change and inspection, the shop told me that my top engine mounts were gaped. I'm planing on replacing them but would it be necessary to replace all or just the bad ones? I did not ask the shop because I assume they would want to make an extra dime and want to replace all even if not necessary.

Check your log books and find out when they were last changed. If its been a while then you should change them all.
My guess is if they are showing gaps then its been awhile and you should change them out. General guidlines is 3 years or 500 hours, although I think 5 years is OK. Its not that big a job once the engine cowls are off.
My mechanic and myself did all mounts on both engines in just under 3 hours. The cost per mount is around $150 at aircraft spruce.

Gary

Here is a tidbit from Barry Controls re engine isolators. His recommendation is regardless of physical inspection isolators should be changes every 5 years.
ENGINE MOUNTS (VIBRATION ISOLATORS)

New isolators should have current cure dates less than one (1) year old to insure best performance. Ambient air alone will cause elastomers in installed isolators to lose their resiliency and deteriorate. Once the isolators are installed, they need to be inspected for wear and fatigue at the same time as other external engine parts.
Isolators should be kept free of dirt, oil and other petroleum based fluids at all times. Oils will soften the elastoiner making them ineffective. Spilled fluids can be cleaned off using isopropyl alcohol or electronic contact cleaner.
Prolonged temperatures over 200' will dry out and harden most elastomers over time. Heat will initially cause the elastomer to soften and "drift", but will eventually harden it, making the isolator ineffective. Extreme ambient temperatures during tie down and storage are just as destructive to the elastomer, and contribute to short service life. Isolators should be changed every time the engine is removed from the airplane. They will take a permanent set" within 48 hours of installation and are virtually impossible to replace in an identical fashion.
Wear for installed insolators varies with flight hours, and is not consistent from aircraft to aircraft. Each inspection should include looking for debonds and tears in the elastomer.Debonds are a physical separation of the metal outer or inner plate from the elastomer. A good rule of thumb is 30% circumferential or 30% depth separation.
Tears or splits will show in the bulged or center portion of the isolator and follow the same rule of thumb. They should not exceed 30% circumferential, 30% elastomer depth or 30% elastomer width separation. Any one of these conditions is cause for replacement, at which time all isolators should be changed. Equal spring rates and loads are essential to proper isolator performance.
Since no two installations are identical, and aircraft hours per year are so different, service life is reflective' of those conditions. At very best though, five year old isolators or installations should be changed out. Other items for inspection include excessive deflection, mounting bolt integrity, security and tightness.

Regards,
BARRY CONTROLS
A Unit of Barry Wright
Bob Callender
Sales Engineer
Engine Vibration Isolators"

thanks for the input, Looks like they were last done 4 1/2 years ago so I guess its time to change them all out.