AirCooled Engine Technology - Grafton School District · 2019-02-18 · Air Cooled Engine Technology 43. Some valve springs are “progressively wound” with _____ coils (closer

AirCooled Engine Technology

Roth 9 th Ch16 Bearings, Shafts, Valves Pages 303 336


Maintenance TLC Knowledge

1. 4 cycle engines have more parts, & require more __________________ than 2 cycle engines.

AirCooled Engine Technology 2. The ____________________ _______ attaches the piston to the crankshaft.

Push Rod Connecting Rod Lightening Rod

AirCooled Engine Technology 3. The connecting rod lower end can be either split or can be onepiece.

________________ bearings with smooth, sliding surfaces are usually used on the lower end of a split connecting rod.

Friction Fractional AntiFriction

AirCooled Engine Technology 4. The diameter across the parting surfaces of a precision insert bearing is

slightly larger than the rod & cap diameter. This condition is called bearing ____________. It gives tight contact and good heat transfer.

Spread Crush Clearance

AirCooled Engine Technology 5. The builtin design feature of precision insert ends protruding slightly

above the parting surface of the connecting rod & cap is called bearing ______________. Crush height may be only .0005” .002”

Spread Crush Clearance

AirCooled Engine Technology 6. Removable, _____________ insert bearings are so named because of

exact size & fit.

Semi Friction Precision AntiFriction

AirCooled Engine Technology 7. Precision insert bearing halves are prevented from turning in the rod and

rod cap with ______________ tabs on the parting edge of each insert. The tabs fit into slots in the rod halves. (or in the block for mains)

Locating Tab

AirCooled Engine Technology 8. Connecting rods & caps as well as bearing halves must be

kept in _____________ pairs if being reused. (wearmated)

Matched Matched Matched

AirCooled Engine Technology 9. Needle, roller, and ball bearings are in the category of

________ _______________ bearings.

SubFriction Fractional AntiFriction

AirCooled Engine Technology 10. Antifriction bearings (balls, rollers, needles) can be held in roller _________

(races) or be loose (uncaged).

Zoos Cages Barns


AirCooled Engine Technology 11. ______________ plates are often used to prevent rod nuts or bolts from

loosening in service. Prevailing torque nuts or Loctite thread locking compound can also be used to prevent loosening.

Locking Washer Tectonic

AirCooled Engine Technology 12. Always use a _______ wrench to get the exact tightness of connecting rod,

cylinder head, and flywheel fasteners. Torque specifications are found in manufacturers’ service manuals & charts.

Ratchet Metric Torque


AirCooled Engine Technology 13. The _________________ converts the reciprocating motion of the piston

into rotary motion which is used, or “taken off “ at the Power Take Off (PTO) end of the shaft. (cast or forged)

Crankshaft Camshaft Balance Shaft

PTO Magneto

Crankpin

AirCooled Engine Technology 14. The crankshaft ______ is the offset portion measured from the centerline

of the main bearing journals to the centerline of the connecting rod (crankpin) journal. (offset x 2 = stroke)

Throw Centerline Counterweight


AirCooled Engine Technology 15. ________________ are used on crankshafts to help offset the unbalance

created by the force of the reciprocating mass (con rod, piston, & crankpin). Even though this works well, some manufacturers also use additional, gear driven counterbalance shafts & weights inside the crankcase.

Counterweights Camshaft Lobes Helical Gears


A counterbalance system would offset the side thrust of the counterweights

AirCooled Engine Technology 16. Crankshaft __________ bearings can be of the

sleeve, bushing, roller, tapered roller, or ball design. Rod Cam Main


PTO Magneto

AirCooled Engine Technology 17. Some crankshaft _____ _______ (axial play) is needed to allow room for

lubrication & expansion. Manufacturers’ specifications for end play vary from about .002” to .027”. Measure it with a dial indicator.

End Play Wiggle Room Vibration Space

AirCooled Engine Technology 18.Undersize wear & taper on any crankshaft journal surface

should not exceed ______”. If so, discard it. .010” .001” .0001”

AirCooled Engine Technology 19. Bearing ________________ is the space between a bearing bore (hole)

and journal (shaft surface). This space is often called the “oil clearance” of the bearing. If it exceeds the .002” .005” tolerance on an engine, a connecting rod “double knock” will occur & oil pressure is lowered.

Clearance Thrust Radial Play

AirCooled Engine Technology 20. ___________________ ® is a special, compressible plastic material used to

measure oil clearance between assembled, torqued components. It is considered a consumable tool because it can only be used one time.

Plastigage ® Feeler Gauge Paper Towel

AirCooled Engine Technology 21. Plastigage ® cannot be used on bushings or sleeve bearings.

Journal outside ______________ (O.D.) are measured and compared to bearing bore _____________ diameters (I.D.) to determine oil clearance.

Diameter, Inside Diameter, Outside Diameter, End Play

AirCooled Engine Technology 22. Crankcase _______ prevent oil leakage from areas where the crankshaft

or camshaft go through the sump cover or engine block casting. The knife edge of the sealing lip rubs lightly on the shaft.

Walrus Penguins Seals

AirCooled Engine Technology 23. The knife edge of the sealing lip and the small coil spring on the seal

always face ____________the fluid being sealed. Seal protectors should be used anytime the crankshaft is slid through the seal.

Toward Away From Doesn’t Matter


Poppet Reed Rotary

24. Four stroke cycle engines use __________ valves to open & close the intake & exhaust ports.

AirCooled Engine Technology 25. Valve springs are held onto the valve stems with “____________” like

retaining washers, pins through holes in the valve stems, collars & rotators and keyhole washers.

“keepers” “holders” “sealants”


AirCooled Engine Technology 26. After cleaning a poppet valve with the wire wheel, inspect for:

1) eroded, cracked, or pitted valve FACES, heads, & stems; 2) a warped HEAD; 3) worn or improperly ground TIPS or STEMS; 4) BENT valve stems; 5) MARGIN less than 1/64”; and 6) partial SEATING.


AirCooled Engine Technology 27. Valve springs can loose their elasticity & become distorted.

Check each spring for _______________ and proper length. Some shops also use a valve spring tension tester.

squareness interference fit brand name


AirCooled Engine Technology 28. Lack of spring tension can cause “valve ________________” (also called

“valve float”) which is an incomplete closing and sealing of the valves, especially during higher RPM operation.

Butter Flutter Clutter


Face angles are either 46° or 31°

AirCooled Engine Technology 29. Valve ________ align the stems so that the faces and seats

contact each other properly. seals keepers guides

AirCooled Engine Technology 30. Worn valve guides can be serviced by 1) rebushing;

2) replacing the whole __________ itself; or by 3) reaming the guide oversize and fitting it with a new valve with an oversized stem.

AirCooled Engine Technology 31. Heavy carbon deposits on the intake valve can ____ the air/fuel mixture

entering the combustion chamber. This reduces the power potential of the engine. Leaky valve stem seals, especially on an OHV engine, or a heavily pressurized crankcase can be the cause of such deposits.

restrict agitate encourage

AirCooled Engine Technology 32. A 45° valve face fits into a _____ degree seat.

A 30° valve face fits into a _____ degree seat. 46º , 31º 90º , 60º 45º , 30º

AirCooled Engine Technology 33. The 1º variation between seat & face angles is called _______________ fit.

interference best practice obnoxious


AirCooled Engine Technology 34. Valve faces & seats can be cut with carbide ___________ or grinding

____________ to exact angles. Used valves and seats in fair condition can be hand lapped to guarantee good seating & sealing.

bits, stones drills, reamers saws, wheels




AirCooled Engine Technology 35. A 3angle, 46° seat would have a top cut of _____ degrees and a throat cut

of _____ degrees. *** A 3angle, 30° seat would have a top cut of 15° and a throat cut

of either 45°or 60° ***



Throat Cut of 60º

Top Cut of 30º

Seat Cut of 46º

The 3Angle Valve Seat

Provides better flow of: Air & fuel into the chamber Exhaust out of the chamber



AirCooled Engine Technology 36. Valve ________ measuring & service (if needed)

must always be done before any seat cutting is begun. Stem Margin Guide

AirCooled Engine Technology 37. Valve clearance (often called lash) refers to the space between the end of

the stem and the end of the lifter (or rocker arm on OHV). It is measured with the valve closed & engine cold!

AirCooled Engine Technology 38. Too ___________ valve clearance will hold the valve open when the stem

heats up & expands. This reduces compression & power. Severe cases can even prevent a hot engine from running at all.

Much Little Late

AirCooled Engine Technology 39.Too _______ valve clearance will make the valves open a shorter distance

& a shorter period of time. Lift is the term for distance they open. Duration is the term that describes the time they stay open.

Lobe Height

Lobe Base

Height – Base = Lobe Lift

Much Little Late


Base Diameter

Lobe Height

Lift of Lobe

AirCooled Engine Technology 40. If engines don’t have adjustable tappets or rocker arms (OHV),

valve adjustment (setting the clearance) is done by grinding material off of the end of the valve ____________. Measure w/ flat feeler gage.

Margin Face Stem

AirCooled Engine Technology 41. Of the 4 valve arrangements (LIFT), the valve in block or Lhead &

the _____________ valve (OHV) or Ihead are the most common two. OHV = better efficiency, more HP, & lower emissions.

Overhead Oversize Oversleep


AirCooled Engine Technology 42. An overhead valve system transfers its motion

through ________ rods and ______ arms.

Push, Rocker Connecting, Push Rocker, Connecting

AirCooled Engine Technology 43. Some valve springs are “progressively wound” with ______________ coils

(closer together) on the end that faces the head of the valve. This type of variable rate spring reduces the chance of the valves bouncing open after they contact the seat. It’s a better idea than using heavier valve springs.

Dampening Moistening Torsional Vibration

AirCooled Engine Technology 44. Never use gasket ___________ or _________ on a cylinder head gasket.

Sealer, Cement Paste, Sealer Sealer, GoJo

AirCooled Engine Technology 45. Cylinder head bolts should always be ___________ incrementally, to the

proper specification, & in the recommended torque sequence published in the manufacturers’ service manual for the engine.

Torqued or Tightened Loosened Handled

AirCooled Engine Technology 46. Always adjust valve clearance with the engine cold.

The coefficient of ____________ expansion may not be the same for both valves, so clearance specifications may be different for each valve.

Frictional Thermal Metallic

AirCooled Engine Technology 47. A camshaft gear must be exactly __________ to the crankshaft gear.

The camshaft gear always has __________ as many teeth as the crank gear so the cam spins at ______ the RPM of the crankshaft

Timed Twice Half

AirCooled Engine Technology 48. Camshafts are often fit with an ___________ ____________ Release (ACR)

that makes the engine easier to crank by keeping one valve open longer while under 600 RPM (cranking RPM). This could be accomplished mechanically or with a bump on the cam lobe. ACR makes the engine easier to pull & easier on the electric starter too!

AirCooled Engine Technology 49. 2 cycle engines use ____ valves to control the flow of the air/fuel mixture

from carburetor to crankcase. Routine service for 2 cycle engines includes keeping the ports & spark arrestor free of carbon deposits.

Reed Poppet Rotary

AirCooled Engine Technology 50. Reed __________ prevent reed valves from opening too far and distorting

to a point of “stand off”.

Stops Flappers Valves























720° Crankshaft Rotation in a Complete 4 Stroke Cycle











Note: Volumetric Efficiency Dropoff at High RPM


Note: Thermal Efficiency “RuleofThirds” Is a pretty close estimate














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AirCooled Engine Technology - Grafton School District · 2019-02-18 · Air Cooled Engine Technology 43. Some valve springs are “progressively wound” with _____ coils (closer