Discussion and questions related to the course Practical Engine Building
hi simon am doing a build atm EJ20, the big ends are 52mm am shooting for .002 clearance and find that 2 of the big ends are 1.5 thou what would you suggest here? i seen specs some where to be .0010 to .0022 seems am in spec ?
Depending on exactly what EJ20 you're dealing with will depend on the exact factory specifications. For example the last EJ20 I built had factory big end clearance specification of 0.0007"-0.0018". The factory specs are often quite tight to allow the use of thinner oils. If you're building a low/moderate power engine then I'd be happy with the looser end of the factory specs and 1.5 thou is absolutely fine. For a big power application that's going to be used for racing I'd be more inclined to aim for around 0.002" or close to. This can be achieved by having your machine shop polish a little additional clearance into the journals of the crank that are tight.
great thanks for that help, moving on from there am gapping the rings now, this is my 1st time so obvlously am a little cautious the mahle street strip says 2nd ring bore x0.0035 i get .0123 round off to .013 its a 500 hp car so ive went with a 2nd ring gap of .014 and top ring bore x0.0045 giving .0165 ive went with 0.018 the confusing this is oil ring says bore x 0.15 giving 0.054 is that not a bit big as the gap pattern is smaller as we go down the rings and chart says
top bore x 0.0045
2nd bore x 0.035
oil bore x 0.15 why is the calculation making the oil ring 0.054 its a huge gap? bore is 3.655
the middle oil spreader ring does that stay same size no triming or gapping here?
this is my 1st time doing this does that gaps seem ok ive went with to you
With the oil rails the specification is a minimum rather than per inch of bore. So essentially you just need to make sure that the gap is at least 0.015" and in most instances these will be fine out of the box. The expander does not need to be touched but you do need to ensure that the ends don't overlap once installed.
thanks Andrey with the solid non shim buckets, modifying the valve clearance what would you recommend doing to remove material from the top of the valve? and what method and tool best used
Normally the valve tip is hardened and hence there's limited potential to shorten them before you end up grinding through the hardening. This is a job for your engine machinist though as it requires specialist grinding equipment.
I wouldnt have thought i would have almost no clearance on a few of the valves all ive done is lap them in ? Prior to doing this maybe valve clearance wasnt right before i started? And no headwork has ben done even mixing around the buckets dosnt get me any where close to where i want them all to be thanks
You're right, lapping the valves wouldn't have a significant affect on your valve lash. I'd also guess that it's possible the clearance wasn't right before.
On that note though you do need to be careful if you're lapping the valves as anything more than a very light lap to confirm the contact pattern on the seat can actually result in a conical shape to the seat that will ensure the valves don't seat properly once they are hot.
Andrey the block came back from machine shop for a 93mm bore its came back 92.97 and the pistons are 92.94 .03mm clearene is too tight just over .001. would i just hone the extra say .03mm to .04mm needed to get around .003"
What is the piston manufacturer's recommended piston to wall clearance?
measured min max
0.0236 0.0022 0.0030
if there is a min and max what does measured really mean? there is a considerable difference between (min - max) and measured
The piston manufacturers I deal with simply list a recommendation rather than a minimum and maximum.
I don't understand the measurements you've given though. Can you post up a picture of the piston spec sheet?
scroll down to the subaru ej20 93mm piston chart
hope this helps thanks andre
Ok that all makes sense. The 'Measured' specification refers to the point on the piston skirt where the skirt diameter needs to be measured. Then you have the minimum and maximum acceptable clearances. Note that these pistons use a 4032 forging which has a higher silicon content than the 2618 alloy and hence expands less with heat. This is why the recommended clearance appears quite tight compared to some piston designs.
I would check your measurements with the piston skirt measured at the specified point and make sure you're within spec. I'd tend towards the looser side of Mahle's spec for a high power engine or one that's going to see hard competition use.
Am i right in thinking then that the measured is about 1/4 a inch down the skirt
Even so if the figures are still the same its still to tight so would honing be ok to gain the extra if needed? I think its still going to be about .001 inch to small
The bore is 0.03mm smaller of what was expected to be 93mm bores are 92.97mm pistons are 92.94 or am i missing something here i get a clearance of 0.001 inch and min to max is about 0.002 to 0.003 or am i totally wrong thanks andre
Andre i don't have a lot of experience using micrometer but after checking again today, I was maybe being too soft with it only just on loose end of the thimble? I can actually get 92.90 on the piston without being to excessive giving me a more desired clearance of 0.07mm does this sound about right ? the clearance is in the piston ? so about 0.1mm is that around normal to expect its a 93mm piston but actual size is 92.90
think am happy to go ahead and put it together whats your thoughts
You measure the piston skirt up from the bottom of the skirt, not down from the top. Here's a link to a video from Mahle about measuring the piston to bore clearance: https://www.youtube.com/watch?v=OuNldI0AV7g
When you're using the micrometer you can turn the thimble and it won't over tighten the mic. It's designed to offer the correct tension. You do however need to make sure that you're measuring the correct point on both sides of the piston skirt to get an accurate reading.
Yeah sorry i knew what part of the piston to measure, all happy and checked and checked again the bores was still to small i honed them today and have a nice snug fit all round of 0.0023 so am happy with that they still feel very tight am 100% sure there was only 0.001 clearence to begin with the old pistons that came out were 0.008 did i mention the old state and damage at top of the bore where the top ring is at tdc? There was wear in the wall on the trust side and the rings were like thin and lost material where the over hang the piston almsot like the piston was hammering down on it and smokey oil buring pressure in crankcase
Andre have you ever found that some blocks come back from machine shop with a slighty different size to what you was after?
Yes unfortunately not all machine shops are able to work to the kind of specifications we give them. It can take a while to find a machinist that you can really trust to get the right results first time.
I was kinda hoping you would touch base on your thoughts of the post before the previous :)
Is it normal to feel when turning over for the 1st time its smooth but nice and tight? Feeling of the rings scrapping the bore or skirts? Just to add there well oiled :-)
I also forgot to mention the bottom of the piston is protruding past the bottom of the piston bore by a few mm it must have always been that way before i started this is ok as long as its not too much?
The bottom of the skirt protruding isn't an issue assuming there is sufficient clearance to anything it could contact. There will always be some resistance to rotation with a freshly built engine and this is normal. You will tend to build up a feel over several builds of what is normal and what may represent a problem.