Compression ratio = (swept volume + clearance volume) ÷ clearance volume. Swept volume per cylinder is π/4 × bore² × stroke, in millimetres, divided by 1000 to get cc. Clearance volume is the combustion chamber volume, plus the head step if the chamber was CC’d without it, plus the deck volume (π/4 × bore² × deck height), plus any dish in the piston crown. There is no head-gasket term on a Type 1. The calculator on this page runs it both forwards and backwards.

.040″ to .060″ is where LN Engineering and the builder consensus both put a street air-cooled engine, and .050″ is the number most builders reach for by default. .035″ is the published minimum safe figure and is race practice, not street practice. Factory decks were .050–.070″. Anything past .060″ is giving up quench, and past about .070″ there is none left at all. Measure all four cylinders, at torque, with the piston rocked both ways, and work to the worst one.

No. The cylinder spigot seals metal-to-metal against the head — there is no liquid to contain, and the joint tightens as the head heats and expands. Copper “head gaskets” sold for long-stroke builds are really deck spacers; they are widely reported to work-harden and weep at 30,000 to 40,000 miles, and most builders shim under the cylinders instead. If you do fit one, add its thickness to deck height. Engines up to and including the 40hp did use a copper seal ring, discontinued by the fresh-air engines.

The mainstream builder answer is 8.0 to 8.7:1, and one 8.1:1 engine has run 70,000 daily-driven miles on 87. The conservative position — Bob Hoover’s, and Air Cooled Tech’s for daily drivers — is 7.0 to 7.5:1. The factory sat at 7.5:1, the last Mexican engines at 7.75:1 on 87. On 91 or 93, John Connolly at Aircooled.Net says 9 to 9.5:1 is no problem with good heads and the right cam, and that is about the sensible ceiling for a street motor. Air-cooled heads run hot, so timing and cooling decide this more than the pump does.

Two reasons, and they compound. The first is the step: catalogue numbers usually include the machined register the cylinder sits in, and a chamber CC’d with the disc down on the quench flats does not — a 10 to 11cc difference on a 90.5 or 92mm bore. The second is that castings genuinely vary; JBugs quote 50 to 65cc for the same part number. Fill each chamber with a syringe three times, average it, do all four, and work to the largest — you can always take volume out, never put it back.

On a 90.5mm bore, .010″ of deck is about 1.6cc, which is roughly two tenths of a compression point. On a 94mm bore it is about 1.8cc. The effect is progressive — it grows as the deck closes up — so the calculator computes it for your exact combination instead of quoting a table. Taking deck out is also the only change that buys you quench, which is why builders reach for it before they reach for the flycutter.

About 0.12 of a compression point on a Type 1, near enough across the whole 1600-to-2332 range. The published rule for cutting a stock dual-port head is .010″ of cut per 1cc, and the measured ladder on a stock 51cc head runs 51 → 46 → 43 → 41 → 39cc at .050, .080, .100 and .120″.

Usually not. Nearly every Type 1 performance piston — 85.5, 88, 90.5, 92, 94 — is a flat top with no valve reliefs. Dished pistons show up on Bus engines, where the factory used them specifically to hold compression down. If you do need to drop compression, a correctly-sized dish is the better way to do it than piling on base shims, because it keeps the deck tight and the quench intact. A dish should be wider than the chamber so it does not spill out onto the flats. Enter a dome as a negative number.

Static — the number every catalogue and every cam recommendation is written against. Dynamic compression accounts for how far past bottom dead centre the cam holds the intake valve open, and always reads lower. Real air-cooled builds report 8.8 static against 7.8 dynamic; big-cam engines have shown 11.2 static against 7.6 dynamic. Street practice is to keep dynamic somewhere around 7.0 to 7.5:1 on pump fuel, so a static number that looks alarming behind a big cam may be perfectly sane.

The arithmetic is the same for any engine, but every reference number on this page — chamber volumes, deck limits, the fuel envelope, the cam ladder — is Type 1 specific, and the Type 4 does have head gaskets. Use the calculator for a Type 4 or a 356 if you have your own measured chamber and deck figures, but don’t use our presets. Call the shop for those engines.