Mono Cartridge Styli Sizes 

18µm (0.0007") or 25µm (0.001")

No-one seems able to say confidently and for sure that one or other stylus size is preferable for mono replay for a specific year (certainly up to the late 1960’s and possibly up to the cessation of dedicated mono production in the 1970’s). It seems likely that record mastering engineers continued to use their mono cutting lathes to cut mono acetates and only introduced the new 0.7mil lathes for stereo pressings, therefore there was probably little or no crossover to 0.7mil for mono and almost definitely none early on.

It also seems extremely likely that any record intended as an original mono release was made to accept the larger stylus size and this would certainly have made perfect commercial sense in the market place of the time, as the vast majority of mono styli in use would have been 1.0mil well into the 1960’s. Furthermore modern microscope measurements of typical mono record grooves from the period suggest substantially varying groove sizes but always ones that are easily wide enough for a 1.0mil stylus. 

There is a school thought that argues it is better to have both sizes available but the evidence seems to suggest otherwise. The confusion appears to stem from the fact that stereo records were exclusively 0.7mil and early stereo releases had warning notices that they should only be played using a 0.7mil stylus. This was because all mono equipment at that time would have been vertically locked and had a 1.0mil stylus which would have damaged a stereo record. If a mono record has been very heavily played, the 0.7mil sitting lower down in the groove might theoretically offer an advantage due to the presence of wear in the higher areas after the 1.0mil has traced it so often but on the flip side a heavily played record is much more likely to have serious contamination/damage issues lower down in the groove, for which the 0.7mil then becomes a much worse choice. Modern mono re-releases or later pressings made from stereo recordings may possibly benefit from a 0.7mil stylus but even that is not completely certain due to the significant variations in groove widths and anyway they're not the subject of this discussion which concerns mainly original mono releases.

In conclusion, although some purists advocate keeping both stylus sizes and choose whichever they think will give the best result for each pressing, this is potentially an invalid argument. Any benefit is extremely uncertain and also rendered irrelevant if the record is an original mono pressing in good condition, as the 1.0mil will almost certainly play it perfectly anyway; therefore is there any point in using the smaller 0.7mil size? 

As stated before there seems to be no definitive answer to the stylus conundrum and therefore it really comes down to personal choice. Considering all the above, my ideal choice for best overall sound quality is without doubt a vertically locked 1.0mil stylus, not only for all earlier releases up to the late 60’s but also for all records originally intended for separate mono release. 

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