New CAT C15 / 3406E / C16 / C18 Steel Spacer Plate — 6I4421 | Standard & Undersize
Required on every head job. The spacer plate sits between the block and the head, and it should be replaced with the head — not reused. We stock it in five thicknesses: factory standard .338" plus undersizes down to -.004", in .001" steps. Pick your thickness from the dropdown.
Why Five Thicknesses? Liner Protrusion.
The factory only made two sizes: standard (.338") and .003" under (.335"). But the correct plate thickness depends on your liner protrusion — how far the counterbore and liners sit relative to the deck. If the block deck or counterbores have been machined, a standard plate can leave the liner height out of spec, and that is how you crush a liner or lose a head gasket. The right undersize plate restores correct liner protrusion without cutting corners.
Standard .338" — unmachined deck, liner protrusion in factory spec
.337" / .336" / .335" / .334" — matched to how much has been machined off. Measure protrusion, then pick the plate.
Not sure which thickness you need? Call us with your liner protrusion measurement and we will spec it — 832-894-3609.
Thickness Options
| Thickness | Size | Part Reference |
|---|---|---|
| Standard | .338" | 6I4421 |
| .001" under | .337" | AMK-1780 |
| .002" under | .336" | 6I4421-2A |
| .003" under (factory undersize) | .335" | 1389381 |
| .004" under | .334" | 6I4421-4A |
Fitment
Caterpillar 3406E, C15, C16, and C18 engines, 1994–2012 applications. Send us your engine serial number if you want fitment confirmed before ordering.
OEM Cross Reference Numbers
6I4421 | 6I-4421 | 1389381 | 138-9381
Complete the Job
Every head swap needs this plate plus a new head bolt kit (1241854 / 1241855) — bolts are torque-to-yield and single-use. Pair with the C15 Universal Cylinder Head (2239250), or get everything in one box with the C15 Cylinder Head Replacement Kit.
Frequently Asked Questions
How do I know which thickness I need?
Measure your liner protrusion. If the deck and counterbores are untouched, standard .338" is correct. If the block has been machined, match the undersize to what was removed. Call us with your measurement and we will confirm.
Can I reuse my old spacer plate?
We do not recommend it. The plate is a sealing and height-critical surface — replace it with the head so the whole stack starts fresh.
What did the factory offer?
Two sizes only: standard .338" and .003" under (.335"). We stock the in-between and beyond sizes because real-world machine work does not come in .003" jumps.
| Weight |
9.072 kg |
|---|---|
| Dimensions |
NOTE - CUT OFF TIME FOR SAME DAY SHIPPING IS 3PM CENTRAL TIME. ORDERS PLACED AFTER 3PM WILL SHIP OUT NEXT BUSINESS DAY. NEXTDAY SHIPMENTS THAT DO NOT ARRIVE ON TIME ARE NON-REFUNDABLE.
-FOR ORDERS INCLUDING HEAVY ITEMS(INFRAME KITS, HEADS,ETC.) WILL BE PLACED ON A SHORT HOLD WHILE WE VERIFY ADDRESS AND SHIPPING IS CORRECT.
NOTE - All open orders will have a 20% restocking fee. All returns will have a shipping fee.
Your payment information is processed securely. We do not store credit card details nor have access to your credit card information.
CAT C15 Spacer Plate, 6I4421 | 3406E C16 C18, Std to .004 Under