i would have thought that using 29meg xtals in 27meg gear was fraught with danger... although i have seen it done - but it did not stand up to a "busy" RF environment
here is another interesting finding - use of "am" xtals in an fm radio - this was an accident but the radio worked a whole race mtting with out missing a beat - is there any difference between am and fm xtals?
i use ko or futaba xtals in ko and or futaba gear with no worries (but keep the bands the same ie: 29meg in 29meg gear etc)
ive also used a hitec hfs 03mm rx with my mars r with no problems (ko xtals)