M20B28 build - early high compression piston skirt machining

[e21kiwi1979]

Hi
The base for my build is a 1986 25 6E 1 engine
It has the 9.7:1 Mahle pistons with the long skirts which hit the crank webs

I also have a set of pistons from 1987 25 6E 2 engine..an import from South Africa with CR 9.4:1 short skirt KS pistons that when mated to 130mm rods clear the webs:

http://s1309.photobucket.com/albums/s62 ... 67ba67.jpg
http://s1309.photobucket.com/albums/s62 ... e89012.jpg

From my research 14mm seems to be the amount of material to remove from the early piston and you end up with a flat skirt piston?
Or would it be better or possible to try and match the profile of the bottom of the KS piston skirt on th eMahle piston so it clears the webs?
Other option is to use the 9.4:1 pistons but they aren't as good and come from a different engine and I want as high a CR as possible to go with the 288 schrick I will be using

[DanThe]

Is this what you were trying to post

[e21kiwi1979]

Yeah, thanks man!

[basketweave]

Nice combo for a high comp build!

Go the 9.7 Mahles, copy the ks skirt shape, and away you go. How much clearance are you going to go with the piston skirts to crank? 1-2mm?

Are you going to deck the block? Or leave the deck as is, and you will have room for the 288 cam.

If you were to deck the block .5mm (which is what I would do) you need to machine in more valve reliefs to have 2mm clearance for piston to valves. More comp this way.

Put the pistons back in the same holes they came out of, and don't touch the skirts (as in cleaning etc). Just clean the crowns and the ring lands. You prob already know this...

Should be a nice engine when done.

Here is mine, just done over 400kms on the engine and loving it:

http://forums.eurocca.net/showthread.ph ... tory-parts

Oh and send me the pistons you don't use

[e21kiwi1979]

Hey thanks - good advice

Thought that keeping the skirt profile was the way to go to keep the pistons nice and snug at the bottom of the bore. 2mm should be good clearance on the webs

Will deck the block and cut the valve reliefs once I have checked the clearance

Have the adjustable cam pulley to time it all up and have a copy of an Alpina 2.7 eprom to start with.

I have had your build bookmarked for a while and have read it quite a few times..very informative and will use it as my guide during assembly..cheers

[basketweave]

Here will give you an idea of how much to cut the pistons for the valve reliefs:

http://forums.eurocca.net/showthread.ph ... -revisited

I would say around a 1mm recess extra on both intake and exhaust to be sure. But you will find that out when you trial fit everything.

Measure your head too, so you can work out what gasket to use and/or how much to take off the block. I recommend taking the .5mm off or just under so you can use any head in the future and adjust gasket thickness to suit. Ideally you want the pistons coming up to their factory height.

It will be interesting to see how your engine idles and behaves in the lower rpms with the biggish 288 cam.

[Bullet_Ride]

Give my build thread a read though as well if you haven't seen it already. I machined my crank webs instead of the piston skirts. I also decked my block 0.5mm like basketweave did and barely had any valve to piston clearance on the intake valve with a schrick 284 cam.

http://www.r3vlimited.com/board/showthread.php?t=243359

[e21kiwi1979]

Here will give you an idea of how much to cut the pistons for the valve reliefs:

http://forums.eurocca.net/showthread.ph ... -revisited

I would say around a 1mm recess extra on both intake and exhaust to be sure. But you will find that out when you trial fit everything.

Yes will definitley measure the clearance. The cam came from an engine where the p-v clearance wasnt measured and they wondered why after each test run all the valve clearances had changed and kept adjusting and retesting until it went boom

Measure your head too, so you can work out what gasket to use and/or how much to take off the block. I recommend taking the .5mm off or just under so you can use any head in the future and adjust gasket thickness to suit. Ideally you want the pistons coming up to their factory height.

It will be interesting to see how your engine idles and behaves in the lower rpms with the biggish 288 cam.

Lumpy I hope!

[e21kiwi1979]

Give my build thread a read though as well if you haven't seen it already. I machined my crank webs instead of the piston skirts. I also decked my block 0.5mm like basketweave did and barely had any valve to piston clearance on the intake valve with a schrick 284 cam.

http://www.r3vlimited.com/board/showthread.php?t=243359

Yes checked that thread to and lots of good information and bookmarked for reference

I will leave the crank alone its the early one - prefer to machine the pistons which will reduce the weight alittle

[e21kiwi1979]

Back to the skirts
Was contemplating machining the skirts by setting up a jig on my table saw with the correct radius aluminium blade and just gently passing the piston across it
Maybe not?
Could make a template up and mark each piston and attack them with a dremel or similar

I wish I had a mate that worked in a machine shop but might ring around and get some prices

[Bullet_Ride]

I will leave the crank alone its the early one - prefer to machine the pistons which will reduce the weight alittle

Machining the crank reduces the rotating mass even more , but you will need to get it professionally balanced after. So if you can get away with just cutting down the piston skirts, then you can weight balance all the pistons on your own which is a more economical approach.

A local machine shop should be able to chuck up your pistons in a lathe and cut them down to whatever you want pretty cheaply.

[basketweave]

So the 288 was causing the valves to hit, and increasing valve clearance? Got any more info on this?

I have 2mm clearance for my pistons to valves, I hope that is enough. I might borrow a bore-o-scope and have a squiz inside. The reason being is my tappets are requiring constant adjustment, they have settled down now but are still a tiny bit loose for my liking.

Bulletride, I have read your thread too, nicely done. I am wanting to do ITB's aswell, the only thing that hold's me back is coughing up the dough for a stand-alone, the one I want to run is autronic, which is exy.

For ITB's you could use a Roland manifold which is cast ready for Webers, and many throttle bodies you can buy off the shelf are designed to fit Weber style manifolds.

http://www.manifolds.co.za/index.php?op ... 7&Itemid=2

or for Toyota throttle bodies:

http://www.manifolds.co.za/index.php?op ... 6&Itemid=2

Actually I am going to contact them now. I have another E30 I might use as a trail with Webers.

[e21kiwi1979]

[quote="basketweave"]So the 288 was causing the valves to hit, and increasing valve clearance? Got any more info on this?

I have 2mm clearance for my pistons to valves, I hope that is enough. I might borrow a bore-o-scope and have a squiz inside. The reason being is my tappets are requiring constant adjustment, they have settled down now but are still a tiny bit loose for my liking.

All I know its was a standard 86 M20B25 with the 288 cam with unchecked piston to valve clearance. It went boom on the third run around Ruapuna track here in Christchurch. They could have set the clearance to tight to try and shut up the tappet noise and the valve hit the piston after it heated up?

I would have thought 2mm clearance should be adequate. I might take a little more out to be sure.

On a 2.7 engine I built ages ago they tappets took awhile to settle in and then were sweet and that was with an oldish E21 323i cam. This time I have some Febi rockers that are near new so I would hope it to be pretty solid and set up nicely.

Maybe its due to the high compression..what is the compression psi? The 2.7 I built was 190 psi from memory

[basketweave]

I am hoping they are just taking some time to settle in.

the Dbilas cam specs say 2mm clearance minimum, as that is what I achieved with the 2.5mm MLS head gasket. Everything is new in the head, new Febi rockers, new eccentrics, head fully reconditioned. I just didn't replace the bolts and nuts for the eccentrics, they are on my next order from BMA parts.

One thing that is a bit random in the head which I threw in there was teh Metric Mechanic Bee Hive Valve Springs. We tested them before I went ahead with them, and they are stock valve spring pressure but in only one spring, not double like standard. I wonder if this has anything to do with it. Anyways, I am going to not drive the car until I know what is happening, I don't need my engine to go bang. I have 3 other cars I can drive

Take a little more out to be sure. I would.

My compression was up over 210psi wet before I started the engine. I think its around 190 -200 now, I will have to do it again when I bore-o-scope the cylinders.

I will follow your build closely if you do a build thread, I reckon doing the 2.8 with high comp pistons would be awesome.

[Bullet_Ride]

I have 2mm clearance for my pistons to valves, I hope that is enough. I might borrow a bore-o-scope and have a squiz inside. The reason being is my tappets are requiring constant adjustment, they have settled down now but are still a tiny bit loose for my liking.

Bulletride, I have read your thread too, nicely done. I am wanting to do ITB's aswell, the only thing that hold's me back is coughing up the dough for a stand-alone, the one I want to run is autronic, which is exy.

For ITB's you could use a Roland manifold which is cast ready for Webers, and many throttle bodies you can buy off the shelf are designed to fit Weber style manifolds.

http://www.manifolds.co.za/index.php?op ... 7&Itemid=2

or for Toyota throttle bodies:

http://www.manifolds.co.za/index.php?op ... 6&Itemid=2

Actually I am going to contact them now. I have another E30 I might use as a trail with Webers.

I have 2mm of clearance as well. I've got a few thousand kms on the motor now and that includes a few auto-x events and some lapping. I've been avoiding the rev limiter, but I've had it up to 6400rpm on the track and it hasn't gone boom.

That Rowland website is terrible, no pictures, no prices, no details... who's running that joint lol?

For the ITBs I'm going to try hacking up an intake manifold so I can keep the stock injector locations and welding an adapter plate to it so I can bolt up some bike throttle bodies. I've been looking at the Triumph 675 throttle bodies because they come in sets of 3 and are separable. They are 44mm throttles which should be ok seeing as the dbilas kits have 45mm throttle bodies. I'm hoping to build my set-up for less than $2000CAD

[HairyScreech]

Don't go for 44mm throttles, they will be far too big, the dbillas ones really don't give benefits that justify the cost.

As i posted elsewhere we are only dealing with a ~450cc per cylinder, which is on par with a 1.8L 4 cylinder, most throttles are sized for a 2l engine with better VE than the average m20 stroker.

About 38mm is right for a normal 2.8 iirc and there are a couple of bikes that use carbs or throttles that could be turned into something suitable.

Edit to add - gsxr600 throttles are 38mm.

[reggid]

it also depends where the throttles are, 38mm would probably be too small if at the start of the runner. The standard manifold runners are 45mm at the plenum end and taper to 35mm at the head.

[basketweave]

For the ITBs I'm going to try hacking up an intake manifold so I can keep the stock injector locations and welding an adapter plate to it so I can bolt up some bike throttle bodies. I've been looking at the Triumph 675 throttle bodies because they come in sets of 3 and are separable. They are 44mm throttles which should be ok seeing as the dbilas kits have 45mm throttle bodies. I'm hoping to build my set-up for less than $2000CAD

I have contacted Roland manifolds and he can do 2 different casts, one for Weber style carbs or throttles (cheaper) and one for Toyota 20v throttles. The price was pretty good.
I added up all main costs incl these toyota throttles, trumpets, socks and cast intake manifold from roland and it comes to just over 1K Aussie. Still pretty exy for my liking.

The question is, do I throw more money at a M20 or buy a S14 and throw money at that. The potential return is better with a S14.

Have you started anything with you ITB project?

Sorry to hijack the thread!

[HairyScreech]

The gains with the s14 would be more, however servicing and parts costs will also be more. Swings and roundabouts.

As reggid said, distance from the port will effect size, however the further away they are located the less the throttle response. (although anything would be better than the motronic). Most throttle tend to be nearly direct to head which. is what the 38mm assumes. Obviously adjust as appropriate for the application.