Mylar Bolt Sleeve Size Chart: M6 to M36 and How to Order
Engineer's Guide · 2026-09-16 · 5 min read
Bolt holes on pipelines, tanks and valves do not follow one standard. A survey of a single plant will turn up metric bolts on one line, an ANSI bolt circle on the next, and a DIN callout on the skid. That is why a bolt sleeve cannot be bought as a stock tube: it is cut to the bolt you actually use. This page is the size reference and, more usefully, the ordering method.
The sizes
Our PET (Mylar) sleeve range covers M6 through M36, with ANSI and DIN variants. The inner diameter follows the bolt shank and the wall is chosen for the dielectric and mechanical duty rather than maximised.
| Bolt size | Typical shank range | Sleeve inner diameter | Common wall | Typical use |
|---|---|---|---|---|
| M6 | 5.8 mm to 6.0 mm | Matched to shank | Thin wall | Instrument and small valve flanges |
| M8 | 7.8 mm to 8.0 mm | Matched to shank | Thin wall | Small bore pipework and utility flanges |
| M10 | 9.8 mm to 10.0 mm | Matched to shank | Thin wall | General service flanges |
| M12 | 11.8 mm to 12.0 mm | Matched to shank | Thin wall to medium | General service flanges, small valves |
| M16 | 15.8 mm to 16.0 mm | Matched to shank | Medium | Most ANSI 150 and 300 class flanges |
| M20 | 19.7 mm to 20.0 mm | Matched to shank | Medium | The most common size in pipeline isolation kits |
| M24 | 23.7 mm to 24.0 mm | Matched to shank | Medium | Larger ANSI classes, heavy valves |
| M30 | 29.7 mm to 30.0 mm | Matched to shank | Medium to heavy | High-class flanges and large valve bodies |
| M36 | 35.6 mm to 36.0 mm | Matched to shank | Heavy | Large-bore and high-pressure flange joints |
Why nominal size is not enough
Two M20 bolts can need different sleeves. A rolled thread and a cut thread have different shank diameters. A hot-dip galvanised bolt is thicker than a plain one. And the flange thickness sets the sleeve length, which the nominal diameter says nothing about.
- State the standard: M, ANSI or DIN. The same nominal diameter maps to a different shank and a different grip length across standards.
- State the thread type where it matters, because a rolled thread runs slightly larger than a cut one.
- State the coating. Galvanising and some platings add real thickness to the shank and to the hole.
- State the bolt length, measured as the callout gives it, so the sleeve covers head through nut.
- State the flange assembly: thickness, gasket thickness and whether washers are part of the stack, because those set the exposed length between the faces.
How to order
- Write the bolt callout exactly as it appears on the isometric or the bill of materials, for example M20 by 90 DIN 931 hot-dip galvanised.
- Say how many bolts per flange and how many flanges, so the supply comes counted and packed rather than as a length.
- State the service temperature. That single number decides PET against PTFE, and it is the question most enquiries leave out.
- State the fluid and any wash-down or chemical exposure, for the same reason.
- State the wall requirement if the bolt hole clearance is tight, since that is the constraint that forces a thin wall.
- Ask for the sleeves packed per flange. It removes the on-site cutting step and with it the short-sleeve failure.
Length: the number people get wrong
The sleeve has to break the metallic path across the whole joint. If it stops short of the nut, the shank contacts the flange at the exposed section and the isolation is gone, regardless of how good the gasket and the washers are. Cutting to the bolt length and packing to count is the whole job.
What else sits in the kit
A complete flange isolation assembly pairs the sleeve with an isolating gasket between the faces and insulating washers under the nuts, plus steel washers where the design calls for them. The sleeve is the component that most often arrives wrong, which is why we treat it as the fit-critical part and supply it cut and counted. The material comparison behind the choice, PET against PTFE and the other options, sits on a separate page, and the general PET film tube for winding and lead insulation is a different product with a different duty.
Seen in context: the Mylar (PET) bolt sleeve route brings together every part that goes into this job.
Products for this job
These are the PET and mylar parts we cut to a bolt schedule or an insulation drawing. Send the part and the equipment you recover with, and we will confirm the size and the profile against them.
Related reading
- Flange Isolation Bolt Sleeves: PET, PTFE, Nomex and HDPE Compared
- Mylar (PET) Tubing for Transformer and Motor Insulation
- Mylar (PET) Bolt Sleeve Suppliers
- Mylar / PET Film for Capacitor Sleeves
Common questions
What sizes of Mylar bolt sleeve are available?
M6, M8, M10, M12, M16, M20, M24, M30 and M36 as standard, with ANSI and DIN variants. Other sizes and non-standard shank diameters are made on request against the bolt callout.
How do I work out the sleeve length?
From the bolt length as called out, so the sleeve covers the full grip from the head through the nut with no exposed shank at either flange face. Add the flange, gasket and washer thicknesses to check the exposed length between the faces before fixing the number.
Should the sleeve be sized to the bolt hole or the bolt shank?
The shank. The inner diameter is matched to the bolt so the sleeve stays in contact with it, while the outer diameter has to fit the hole clearance. A sleeve matched to the hole and loose on the bolt can shift and leave the shank contacting the flange.
What wall thickness should I ask for?
The thinnest wall that meets the dielectric and mechanical duty, because the outer diameter has to pass through the hole clearance. Our PET sleeve wall range runs from 0.05 mm to 0.25 mm. If the clearance is tight, say so and we work from that constraint.
Does the bolt standard change the sleeve size?
Yes. An M20 in one standard is not the same shank diameter as an M20 in another, and thread type and coating both shift the real diameter. State the standard and the coating, and we size from the callout rather than the nominal figure.
Can you supply sleeves counted per flange?
Yes, and it is how we prefer to supply them. Sleeves are cut to the bolt length and packed to the bolts-per-flange count, so there is no on-site cutting and no flange assembled one sleeve short.
Is PET suitable for a hot flange?
Not above 105 C continuous. PET softens and creeps under bolt load above that. For high-temperature or chemically aggressive flanges, step up to PTFE. Send the service temperature with the bolt callout and we flag it before quoting.
How do I get a quotation?
Send the bolt callout with the standard, length and coating, an approximate volt-free sketch of the flange stack for length verification, the service temperature, the fluid, the bolts per flange and the number of flanges. WELLELE cuts PET (Mylar) bolt sleeves to the callout and packs them counted, with lot traceability to the incoming film certificate.
Specifying this part?
Send the drawing for a manufacturability review.