Informational content provided by Jamie at ImmersionHeaters.uk.

Examples of some of the various screwplug sizes available, these are undrilled, therefore can be used as a blanking plate or for our customers who are unsure of their requirements, to screw into their socket before anything is manufactured bespoke. Very useful!

This is the simple version,

BSP “	DIAMETER ACROSS THREAD “
1/2	.825
5/8	.902
3/4	1.041
7/8	1.189
1	1.309
1 & 1/4	1.650
1 & 1/2	1.882
1 & 3/4	2.116
2	2.347
2 &1/4	2.537
2 & 1/2	2.95

…and this is the complicated version,

	Thread Form Type	Major Diameter mm d=D	Pitch mm p	Threads per inch tpi	Pitch Diameter mm d2=D2	Minor Diameter Male Thd. d3	Thread Height H1	Tap Drill Diameter mm
1/8 “	BSPP/BSPF	9.728	0.907	28	9.147	8.566	0.581	8.7
1/4 “	BSPP/BSPF	13.157	1.337	19	12.301	11.445	0.856	11.6
3/8 “	BSPP/BSPF	16.662	1.337	19	15.806	14.95	0.856	15
1/2 “	BSPP/BSPF	20.955	1.814	14	19.793	18.631	1.162	19
5/8 “	BSPP/BSPF	22.911	1.814	14	21.749	20.587	1.162	20.75
3/4 “	BSPP/BSPF	26.441	1.814	14	25.279	24.117	1.162	24.5
7/8 “	BSPP/BSPF	30.201	1.814	14	29.039	27.877	1.162	28
1 “	BSPP/BSPF	33.249	2.309	11	31.77	30.291	1.479	30.5
1 1/8 “	BSPP/BSPF	37.897	2.309	11	36.418	34.939	1.479	35
1 1/4 “	BSPP/BSPF	41.91	2.309	11	40.431	38.952	1.479	39.5
1 3/8 “	BSPP/BSPF	44.323	2.309	11	42.844	41.365	1.479	41.5
1 1/2 “	BSPP/BSPF	47.803	2.309	11	46.324	44.845	1.479	45
1 3/4 “	BSPP/BSPF	53.746	2.309	11	52.267	50.788	1.479	51
2 “	BSPP/BSPF	59.614	2.309	11	58.135	56.656	1.479	57
2 1/4 “	BSPP/BSPF	65.71	2.309	11	64.231	62.752	1.479	63
2 1/2 “	BSPP/BSPF	75.184	2.309	11	73.705	72.226	1.479	72.5
2 3/4 “	BSPP/BSPF	81.534	2.309	11	80.055	78.576	1.479	79
3 “	BSPP/BSPF	87.884	2.309	11	86.405	84.926	1.479	85.5
3 1/4 “	BSPP/BSPF	93.98	2.309	11	92.501	91.022	1.479	91
3 1/2 “	BSPP/BSPF	100.33	2.309	11	98.351	97.372	1.479	97.75
3 3/4 “	BSPP/BSPF	106.68	2.309	11	105.201	103.722	1.479	104
4 “	BSPP/BSPF	113.03	2.309	11	111.55	110.072	1.479	110.5
4 1/2 “	BSPP/BSPF	125.73	2.309	11	124.251	122.772	1.479	123
5 “	BSPP/BSPF	138.43	2.309	11	136.951	135.472	1.479	136
5 1/2 “	BSPP/BSPF	151.13	2.309	11	149.651	148.172	1.479	148.5
6 “	BSPP/BSPF	163.83	2.309	11	162.351	160.872	1.479	161.5

I hope these are useful for reference purposes.

More importantly for us, on a daily basis, are the following limitations as to what will fit onto the smaller screwplug sizes. We deal with up to 2.25″ only, with 2.5″ being accommodated by fitting a brass adaptor to 2.25″ screwplug.

2.25″	TROMBONE 2 STAT
2″	TROMBONE 1 STAT
1.75″	TROMBONE 1 STAT
1.5″	3 HAIRPINS 1 STAT
1.25″	3 HAIRPINS NO STAT

Anything smaller than a 1.25″ screwplug will be limited to 1 element, having immediate implications for the achievable watts density.  Likewise, being limited to U bends only, makes reducing the watts density more problematic. Many domestic and commercial plumbing applications use 1.5″BSP, and are thus limited to 3 hairpins and only 1 stat pocket. maximising the immersed length of those U’s is all that can be done to reduce the watts density. To overcome the issue of only 1 stat pocket, we find ourselves increasingly selling dual control and cutout stats.

By hairpin, we mean a U bent element, and trombone indicates double looped. To determine what element is suitable, for U’s you merely double the maximum immersed length available, ie, 24″ immersed length will use u bent 48″ elements. However, if double looping is possible due to the screwplug size and other factors relating to the application (cleaning, etc), that 24″ maximum immersed length could be multiplied by up to 3.5, allowing the use of 84″ elements. See the watts density article to appreciate what a difference that will make. 48″ to 84″ almost double the hot element available, so is almost going to halve the watts density. A big issue on little screwplugs.