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Main Caps
Pontiac V8 Main Caps
Keeping the Bore Round — Main Caps for Pontiac V8s
Factory Pontiac two-bolt main caps were fine for a 1968 grocery getter. They are the first thing to move when a stroker crank starts loading the bottom end, and cap walk shows up as a distorted main bore long before it shows up as a noise. Upgrading the caps is how you keep the bearing bore round.
Butler stocks billet steel caps machined in its own CNC shop, Pro-Gram Engineering billet front, center and rear caps, and Milodon ductile iron sets — in both the 350-389-400 and the 421-428-455 main sizes, in straight four-bolt and splayed-outer-bolt configurations, plus the ARP splayed outer stud kit.
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1. Which main family is your block
Main caps are not interchangeable across the Pontiac V8 line. Everything here is catalogued one of two ways.
| 350-389-400 | The 3.00" main blocks. Butler, Pro-Gram and Milodon all offer caps in this size. |
| 421-428-455 | The 3.25" main blocks. Milodon's splayed set for this family is explicitly listed as the 3.25" main version. |
| Front, center, rear | Center caps are sold as a set of three. Front and rear caps are sold individually, so a full replacement is three parts, not one. |
2. Billet steel or ductile iron
Both are a real upgrade over a stock two-bolt cap. They sit at different points on the scale.
| Butler billet steel | Machined in Butler's own CNC shop, designed and tested in Butler's own blocks and engines by its machinists, with notches added on each end for ease of removal. The center sets include dowel pins. |
| Pro-Gram billet steel | The widest coverage on this page — front, center and rear caps in both main sizes, plus splayed-outer-bolt center kits. |
| Milodon ductile iron | Four-bolt straight and splayed sets of three, supplied with dowel pins on the splayed versions. |
| What they fix | Butler's copy on the billet front caps states it directly — these caps eliminate main cap walk and reduce main bearing bore distortion. |
3. Straight four-bolt or splayed outer bolts
The splayed cap angles its outer fasteners into thicker block material instead of straight down. It is the stronger arrangement and it costs you something.
| Straight four-bolt | The simpler conversion. Pro-Gram and Butler center caps and the Milodon four-bolt sets cover both main sizes. |
| Splayed outer | Pro-Gram offers splayed-outer-bolt center cap kits for both main families, and Milodon offers ductile iron splayed sets. These use 7/16" outer fasteners. |
| The oil pan catch | Butler's product copy on the splayed cap kits carries a warning worth reading before you order — they may require an oil pan that will accept an external oil dipstick, because the stock tube must be extensively modified to be used. |
| Studs | The splayed outer stud kit Butler sells includes ARP 7/16" studs and nuts for the outer splayed positions. The splayed Pro-Gram center kit is listed as requiring ARP two-bolt main studs. |
4. The machine work you have to budget for
This is the part that gets skipped and it is not optional. Butler prints it on every cap on this page.
| Line boring | The block must be align bored. All aftermarket billet main caps require line boring — there is no version of this where you bolt new caps on and go. |
| Thrust surface | The center cap sets also require the thrust surface to be machined to match your particular block. |
| Studs, not bolts | Butler recommends an ARP main stud kit with these caps, and calls out a different part number for the four-bolt caps than for the splayed two-bolt conversion. Get that right when you order. |
| Do it with the block on the bench | Caps, studs and align boring are one trip to the machine shop. Ordering the caps after the block is home is how a build stalls for a month. |
If you are running a stroker crank or a power adder in a factory two-bolt block, caps are the cheapest structural upgrade available. Call with the block casting and the main size and we will tell you what the machine work looks like before you commit.
Not sure what your Pontiac needs?
Call our tech team at 866-762-7527. Tell us your engine, heads, car, and power goal, and we will put the parts together as one combination that works.