Manual for McCulloch Electrolux Virginia MH542 are here (useful but doesn't answer your question):
https://www.manualslib.com/manual/539606/Mcculloch-Virginia-Mh542-Mh555.html
Based on the spark coil picture here, the gap doesn't look adjustable. Please note: you'll need to copy and paste this next link BUT you'll also need to remove the spaces in the bit before ".co.uk". For some reason the owner of this site blocks some addresses and/or certain words.
https://www.r a n s o mspares.co.uk/parts/brands/mcculloch/hedge-trimmers/virginia mh 542%2C virginia mh 555%2C prefix 12 (24801400b)/
You could always contact McCulloch (see down the page on the second link above)
Rheek, June 2019
