BOP Pontiac 3.00" Main DUAL LIP Viton 1pc Rear Main Seal (301, 326, 350, 389, 400) BOP-RMS18
Pontiac 3.00" main 1-piece dual-lip Viton rear main seal. CRANKSHAFT MUST BE REMOVED to install — for in-car repair order 2-piece BOP-RMS01. Fits 1959-79 326-400 and 1979-81 265/301 (early 347/370 when trimmed).
Description
- This 1-piece seal cannot be installed with the crankshaft in the engine. The crank must come out.
- Doing an in-car repair? Order the 2-piece BOP-RMS01 instead — it installs with the crank in.
- Fits: Pontiac 3.00" main blocks — 1959-79 326, 350, 389, 400 and 1979-81 265/301. Fits early 347/370 when trimmed to fit.
- Wrong main size? 421/428/455 (3.25" main) use BOP-RMS19.
- Crank doesn’t measure exactly 3.00" or 3.25"? Order by engine, not by your measurement: all 326-350-389-400 use the 3" main journal seal (RMS01 / RMS18); all 421-428-455 use the 3.25" seal (RMS02 / RMS19). The seal rides on the crank’s rear seal surface, not the main journal — BOP lists that sealing diameter as 3.188" ± .003" for the 3.00" seals and 3.437" ± .003" for the 3.25" seals. If your crank or seal groove is out of spec, call before installing.
- Not for Oldsmobile 403 — see BOP-RMS03.
The BOP Engineering one-piece rear main seal relocates the parting line to reduce the risk of leaking, uses a more pliable construction that works better in engines with seal groove geometry issues, and has a double lip for twice the protection against leakage while supporting good vacuum numbers.
| 2-Piece (single lip) | 1-Piece (dual lip) | |
|---|---|---|
| 3.00" main (326-400, 265/301) | BOP-RMS01 | BOP-RMS18 |
| 3.25" main (421/428/455) | BOP-RMS02 | BOP-RMS19 |
| Seams | Pre-cut in halves | Cut once, on the designated cut dot, at install |
| Lip | Single lip | Dual lip — second lip acts as a wiper on the crank |
| Crank in the engine? | Yes — BOP includes a crank-in procedure | No — the crankshaft must be removed |
| Best for | In-car repair with the pan off | Engine rebuild with the crank out |
- Find the small hole in one of the square indents and cut the seal there, straight across, with a very sharp blade. The cut re-mates when installed.
- Fill the groove between the seal lips with high-pressure grease before assembly.
- Direction matters: the lip with the helix pattern (oil control lip) faces the FRONT of the engine, not the flywheel. The square indents are on opposite sides on RMS18 vs RMS19, so go by the lip, not the indents.
- Open the seal like a “Slinky,” flex it around the crank and set the crank/seal assembly into the block without letting the seal misalign. Place the split toward the top of the block, or about ½" off the block parting line on the cap side.
- RMS18: no sealer is required in the anti-rotation holes or anywhere else.
- Dry fit the seal in the block and cap (no sealer, no crank) and look through the bore — the ends must meet with no gaps or distortion, and the seal must not flex or bulge when the cap is installed. If it does, trim slightly with a sharp utility knife. Fitment before installation is what makes it seal; too tight means more drag.
- The dual lip adds some drag. Get the fitment right first — once fitment is correct, the drag is normal.
- Rear main cap, stock fasteners: 120 ft-lbs with engine oil. With ARP or other aftermarket hardware, use that manufacturer's torque and lube.
- Butler runs these Viton seals on serrated factory cranks with no issues. Only extremely sharp serrations need attention — buff lightly with emery cloth without removing material that changes the crank's geometry.
Why Viton over rope: the dual-lip Viton seal acts as a wiper on the crank. Rope seals are good quality today and work when installed correctly, but Butler considers them far inferior to a dual-lip Viton seal — the BOP Viton seal is Butler's go-to.
RMS18 / RMS19 Instruction Sheet | Viton Rear Main Seal Guide
Not sure which one you need? Ask a tech question or call 866-762-7527 before you order.
Details
- BOP-RMS18
- BOP-RMS18
- BOP
- New
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Shipping Information
- Item Requires Shipping
- 0.3 lbs.
Customer Reviews
Total Reviews (1)
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Reviewed by: Luke01/11/2018 08:32PM
Product ownership: Less than 1 month
Purchased from: Butler Performance Inc.
Comments:
This is super easy to install. Directions detailed and easy to follow. Will never install another rope seal again!