Context : Stardust is a class of presolar grains each of which presents an ideally uncontaminated stellar sample .
Mainstream silicon carbide ( SiC ) stardust formed in the extended envelopes of carbon-rich asymptotic giant branch ( AGB ) stars and incorporated the radioactive nucleus ^ { 26 } Al as a trace element .

Aims : The aim of this paper is to analyse in detail the effect of nuclear uncertainties , in particular the large uncertainties of up to four orders of magnitude related to the ^ { 26 } Al _ { g } ( p, \gamma ) ^ { 27 } Si reaction rate , on the production of ^ { 26 } Al in AGB stars and compare model predictions to data obtained from laboratory analysis of SiC stardust grains .
Stellar uncertainties are also briefly discussed .

Methods : We use a detailed nucleosynthesis postprocessing code to calculate the ^ { 26 } Al/ ^ { 27 } Al ratios at the surface of AGB stars of different masses ( M = 1.75 , 3 , and 5 M _ { \sun } ) and metallicities ( Z = 0.02 , 0.012 , and 0.008 ) .

Results : For the lower limit and recommended value of the ^ { 26 } Al _ { g } ( p, \gamma ) ^ { 27 } Si reaction rate , the predicted ^ { 26 } Al/ ^ { 27 } Al ratios replicate the upper values of the range of the ^ { 26 } Al/ ^ { 27 } Al ratios measured in SiC grains .
For the upper limit of the ^ { 26 } Al _ { g } ( p, \gamma ) ^ { 27 } Si reaction rate , instead , the predicted ^ { 26 } Al/ ^ { 27 } Al ratios are \approx 100 times lower and lie below the range observed in SiC grains .
When considering models of different masses and metallicities , the spread of more than an order of magnitude in the ^ { 26 } Al/ ^ { 27 } Al ratios measured in stellar SiC grains is not reproduced .

Conclusions : We propose two scenarios to explain the spread of the ^ { 26 } Al/ ^ { 27 } Al ratios observed in mainstream SiC , depending on the choice of the ^ { 26 } Al _ { g } + p reaction rate .
One involves different times of stardust formation , the other involves extra-mixing processes .
Stronger conclusions on the interpretation of the Al composition of AGB stardust will be possible after more information is available from future nuclear experiments on the ^ { 26 } Al _ { g } + p reaction .