Engine Building Fundamentals: Piston to Bore Clearance
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Piston to Bore Clearance
06.00
| 00:00 | - One of the most important machining operations is to bore and hone the cylinders to achieve the correct piston to bore clearance. |
| 00:08 | This clearance is critical since the piston to bore clearance that's too large will result in noise from the pistons as they rock in the bores, reduce ring seal, and increase blow by. |
| 00:19 | On the other hand if the clearance is too small then the risk is that the piston might nip up or seize in the bore when it expands at normal operating temperatures. |
| 00:30 | With this in mind the aim is to always achieve the minimal clearance that we can safely get away with at the expected operating temperature that engines going to be working at. |
| 00:41 | Fortunately we don't need to guess at these clearances as the OE manufacturer will offer specifications if you're working with a stock piston. |
| 00:50 | And if you're using an after market forged piston then the manufacturer will provide a recommendation on the clearance that you should run. |
| 00:59 | The majority of factory pistons are manufactured using the casting process and the alloy used for casting pistons has a relatively low thermal expansion rate. |
| 01:10 | This simply means that the pistons don't grow or expand much as they heat up to normal operating temperatures and hence the required clearance for a factory cast piston is quite tight, possibly in the range of one to one and a half thou or three to four hundredths of a millimetre. |
| 01:30 | The majority of after market forged pistons though, are manufactured using a 2618 alloy, which has a very low silicon content. |
| 01:39 | The result of this is that a 2618 forged piston will expand much more as it heats up than a comparable cast piston and hence it requires more piston to bore clearance when the engine is machined. |
| 01:53 | It's not uncommon for example to require a piston to bore clearance of four to six thousandths of an inch or more depending on the specific piston or diameter and application. |
| 02:06 | The correct clearance for your application will depend initially on the piston material as this will define the rate of expansion as it heats up. |
| 02:16 | Beyond this though, we also need to consider the usage, the power level, and the fuel that the engine will run on. |
| 02:23 | As all of the aspects will affect the combustion temperature that the piston is exposed to. |
| 02:29 | For example, a naturally aspirated engine will tend to produce lower combustion temperatures than a comparable turbo charged engine. |
| 02:37 | Likewise, an engine running on pump fuel will tend to produce higher combustion temperatures than a comparable engine running on methanol or E85. |
| 02:48 | While the piston manufacturer will specify a piston to bore clearance, often this will be a little on the generous side as the piston manufacturer wants to be very sure that there's absolutely no chance of the piston seizing in operation. |
| 03:04 | In some instances depending on the power level the engine will operate at, and particularly what fuel will be used, it may be possible to make a slight reduction to the piston to bore clearance, without risking reliability. |
| 03:19 | This is all about matching the piston to bore clearance to the amount of heat that the piston will be subjected to. |
| 03:26 | The more heat the piston sees the more it will expand and hence, the more clearance we need to supply when the piston is cold. |
| 03:35 | Normally this sort of adjustment to the piston to bore clearance will be made based on experience with the same pistons in the same type of engine and if you have no prior experience you're always best to rely on the manufacturer's recommendations. |
| 03:52 | If I'm building an engine that will run on methanol for example, I'll quite often tighten the piston to bore clearance by around half a thousandths of an inch or about a hundredth of a millimetre since methanol runs much cooler than the engine would run on pump fuel or a race gas. |
| 04:11 | It's always valuable to examine the piston skirts and the bore walls after an engine has been disassembled, as this can give you some insight as to whether your clearances are correct. |
| 04:24 | When the piston to bore clearances are correct you will see some slight wear or a polished appearance around the middle of the piston skirt. |
| 04:32 | Often this wear pattern will be slightly oval in shape and in particular if you have a friction reducing coating on the piston skirt, this may be partially worn away and this is quite normal. |
| 04:45 | If the clearances are too tight however, you'll start to see vertical wear marks known as galling on the side of the piston skirt. |
| 04:54 | This looks like a rough finish with some of the aluminium material has actually been ripped away. |
| 05:00 | This may also be apparent with some material transferred to the cylinder wall. |
| 05:05 | This is obviously a dangerous situation and would require the clearance to be increased slightly when the engine is rebuilt. |
| 05:14 | If the clearance is too large in the cylinder, the piston will be less stable and tend to rock back and forth as it moves up and down the bore. |
| 05:23 | This can result in a polished appearance on the bottom of the piston skirt and also on the top ring land. |
| 05:30 | It's worth noting that even if the piston to bore clearance is set correctly it's still possible to end up with a piston that has begun to seize in the bores as the result of other operating aspects such as actual engine tuning. |
| 05:45 | A lean air-fuel ratio, for example will end up with a high combustion temperature and this can result in the piston expanding more than if the air-fuel ratio was correctly tuned. |
