Babalouie

Ok the head is off now I stripped off the carbs and manifolds the other day, and of course the radiator was removed a few weeks ago. Off come the head bolts, and the cam (although I think I didn't really need to take the cam out)... And off the head comes! I've said before that I've suspected that the engine is quite freshly built, and certainly the head came off quite easy. Didn't have to prise it off or anything, it just lifted off the head by hand. I've always suspected that the motor might be a 3.1L, and I think the L3.1 stroker theory may have gained a bit more credence too. Stock L28 bore is 86mm, and as part of the 3.1L conversion you bore it out 3mm. My bore measured at 89mm, so assuming the crank is a stock L28 item, the capacity will be about 2950cc. You need the stroker crank to bring it another 150cc, and I dunno if I have that or not. I have wedged in the chain, etc and so I'm a little gunshy about turning the crank in case the tensioner pops out....but I'm sure there is a way of measuring it (is there?). In terms of wear, there is that spot on the far side of the bore which is a little scuffed, and you can barely see the cross hatch pattern from the honing, but given the great compression figures I guess it's not a problem. Bores aren't lipped either so wear seems to be ok, although I would feel better if there weren't so many vertical wear marks.... Pistons are flat tops, zero deck height. The head as I said came off easily, in fact it popped off the block on its own while I was messing around with removing that coolant hose that goes to the back of it. Based on the combustion chamber shape, I think it might be the later-P90A head as suspected, but it kinda looks like there's been a lot shaved off it for more compression? Valve sizes are 37mm EX ...and 45mm IN. So I think that yes, Naviathan was right in that they are older/larger N42 valves from the late 70's L28, which does now confirm why the valves are so tall And that's where we are! I'd love to find a way of measuring the stroke, without upsetting the chain, and I'm sure there is a way, but it seems that it's definitely been given a 3mm overbore But it seems that the tall-valve theory is on the money, so it's now over to the pro engine builder for sorting

Yes I think so. The cam was huge: 278degs @50thou lift, 600 thou total valve lift. Pretty wild. It would spit fuel out the carbs at idle and wouldn't make any power at all, and at 5000rpm it hits a wall and goes all breathless. The timing was out too, I measured it at a lobe centre (assuming it was half way between 50 thou lift on opening & closing) at 99IN/110EX, which can't be right. Here's a closer pic of the old cam by the way, and how small the base circle is: So the old cam was mis-timed and way too wild. But to no point on the rev range did it really take off, and for such a huge cam, it fell over completely at the top end. Sure the timing was out, but I didn't think 5 degrees would be that fatal to the power output. This was all really baffling, and also the valve train was always noisy as hell, so I assumed that there was something there that wasn't "happy". Hence I decided to eliminate the cam as a source of the issues and swap in a known good hot street grind. The new cam is the "72 degree works rally grind" which is roughly equivalent to MSA Stage IV, and then reassess if the problems have gone away or not. Other highpoints of the engine are: 190~200psi comp on all cylinders, great oil pressure, brand new electronic ignition system, new carbs, new manifold. In fact you can probably see from the interior of the head that the motor is actually quite fresh overall....I figured that weird cam was probably the source of the issues, but we'll have to wait a little longer to find out!

Thanks guys. Old cam had a base circle 200 thou smaller than the new cam, so a lash pad of 100 thou less shoulda done the trick. Original cam used 200 thou lash pads, and the ones I was experimenting with last night were 120 thou. So the new lash pads should have been more or less on the money. So this morning I had another go (I admit I was probably a little pissed with the car last night so didn't have a proper look) and reinstalled one of the rockers: Like Mayolives, my rockers are fouling on the retainer. I don't think sanding it is going to do any good though, I need to more like file a notch in there! To the extent that it's not allowing the nose of the rocker to slip into the lash pad at all....it's fouling at this point, where my finger is: So that's what's been going on all along! With the old cam, the thicker lash pad meant that the rocker was spaced further out from the retainer and so this wasn't an issue before. I suppose it still doesn't change anything: even if I am just going to swap to a different type of retainer the head's still coming off...Do my retainers look like the stock type?

Dang....well there you go. Well, there's no turning back now, I guess we'll know for sure what sort of head it is when we get to have a look at the combustion chamber.

That makes sense...is there a lot of length difference between the P and N valves? Like maybe 1/8th of an inch? The cam that was in the head had a very small base circle (there is a pic on the first page). I suppose it's possible that in Japan there's a school of thought that says: run the N valves in a P90 head, and instead of cutting down the valves, use a smaller base circle cam. When it was running, the valvetrain was noisy as hell (and the engine didn't make much power) so I'm quite happy to explore alternative routes here

The spot where the head code is cast has been sanded down. I think it's a not-uncommon practice in Japan to disguise the fact that you increased your capacity (which normally means you pay more road tax). But rather bizarrely, anyone can see plain as day that the engine number begins with L28.... It has: round inlet ports (with injector notches), square exhaust ports, non-hydraulic lifters. It was suggested in the head thread that it could be an N42 or a P90A. But it's a JDM head so maybe it's something else altogether, that's possible too.

I know what you're saying...maybe it's just a tall retainer (ie ignoring that the valve tip inside the retainer is at the same height as normal) that's in the way of the rocker, right? When I looked at your pics I thought the same thing, but I don't think that's the case with my head...the raised collar on the retainer is quite low on mine (not like your Racer Brown ones) and I don't think the rocker is fouling the retainer. Anyway, head's coming off, we'll have some proper answers soon.

Yes I did...borrowed some stock 2.5mm lash pads (vs 5.0mm that I had) and gave it a whirl: No dice though, even with the pedestal turned as high as it could go (zero lash) the rocker still seems to be "reaching up" to the valve and doesn't look like it sits on the lash pad properly. Also even if I back the pedestal down all the way, there is still zero lash. Yes all the lash pads were the same 5.0mm. From what I can tell, the head's had quite a bit of work done to it in Japan. The ports are quite huge and it seems to have a lot of compression. So the most likely scenario is that the builder in Japan installed some aftermarket, or maybe valves from a small block Chevy (or something) that were a bigger diameter...but a taller stem. But instead of machining down the stem and machining a new groove for the retainer, they used a cam with a dramatically smaller base circle to allow the rocker to lift up high enough to get on top of those valves. ..at this point it's only a theory, but we'll know soon enough. Great minds think alike! The engine now looks like this: The head's coming off, and will be sent to a pro, and we'll do and re-do what's needed to get this all 110%. It'll be very interesting to see what it looks like on the other side...

Thanks Tony Well this is a mystery engine that came with a car that was imported from Japan. So while there are lots of questions that I'd like to ask the engine builder, I can't. Does anyone know what the stock spec is for valve stem height?

Poor choice of words on my part....I meant installed valve height. I figure if the tips of the stems are too tall, then either the valves are cut into the seats a hell of a long way, or there are non-std valves with taller stems in there.

Thanks for the tip. I think you might be right, the new cam is to suit stock retainers/springs and the old cam wasn't. I've got a couple of stock-size .120 lash pads, and I'll put them in just for experimentation's sake, but I think you're right, the head might end up coming off and being sent off to a pro cylinder head builder.

Ta mate. I think that is the go...it won't hurt to put in a thin lash pad (just got to buy one as you say) and see if it makes all the difference...

200thou/5mm. Cam vendor said that the ideal lash pad based on the base circle of the cam would be 180thou/4.5mm. So the lash pad ain't that far out of the ballpark.

Yeah, I did. I got both rockers in place for #1 cyl, and there was no valve lash at all, and in fact the valve was cracked open a hair. That would explain why it was so difficult to lever the valve spring down enough to get the rocker in

Yes, that thought occurred to me too. Smaller lash pads will make maybe 50thou difference but it seems like I need 200thou It's an option...I know the cam spins freely in the towers when I put the cam/towers back in.

Thanks, that makes perfect sense...and I guess the solution would be cam tower shims?

Got a weird one when I swapped in a new cam. The old cam is in the background of this pic. You can see that there is quite a lot of material taken off the base circle of the old cam, and quite a big difference to the new one. The cam vendor crunched some numbers and recommended a 4.5mm/.180' lash pad. Mine turned out to be 5.0mm/.200', so we thought we'd install the cam with the old lash pads, check the wiping pattern and see. Maybe it's close enough. The problem is this...with the new cam in place, and the rocker pedestals screwed all the way down, I have zero lash. Just for experimentation' sake, if I remove the lash pad altogether, I get more room.... Well I get a lot more room at the pedestal end, but at the valve end I still don't seem to have much clearance. So the cause seems to be more than just a 1/2mm worth of variance in the lash pad...so I thought about installed spring height, maybe it's too tall. From head surface to retainer it's 50mm. I figure the next port of call is a much thinner lash pad, closer to stock size, just to see if maybe that fixes it. But I would have thought that the lash pads I already got would be in the ballpark....maybe the wrong wiping pattern, but not so far out that it would be zero lash with the lash adjusters all the way down... Anyone got any ideas? I should point out that this is an "unknown" engine, it came with the car as-is, and even with the old cam it didn't run all that well.

Before I took apart my engine, I saw that there was a mismatch between the exhaust manifold flange and the inlet manifold flange (you can see in this pic that the "bridge washer" is not sitting quite flat): My motor was showing signs of a vacuum leak somewhere so I figured that maybe this was preventing the inlet from sealing properly on the head. The difference is a tiny tick over 1mm, with the exhaust flange being thicker...so I figure that I'll do the right thing and get the header flange milled 1mm so that it's the same thickness as the inlet, and so the "bridge washer" can sit flat. Now the thing is...the machine shop is saying that the 1mm thickness difference is just fine, that new bridge washers: ...have a concave surface on the bottom and the outer ridge will "bite" into the high side and all will be well. Does this sound right, L-series experts? I can see his point, but at the back of my head I'm thinking there is no reason not to mill the header to the same thickness as the inlet...

Try this. Use your other hand to shine a shop light down the port, while you take a flash pic with the camera, you can get a really clear shot all the way to the valve that way

Cheers That gasket's the oldie that came off the engine and JDM manifold...I will of course use a new one when I put the Redline manifold on

Thanks, I'll give it a try...I guess with the stud, washer and nut in place it should stil seal even if there is a small leak into that triangular chamber

Is it ok to use a L24 gasket on an N42 head? I ask this because I notice that the N42 head has these notches for the injectors... ...that look like they might protrude into that triangular hole in the L24 gasket... So would it seal properly?

Thanks Paul You're right, I think the head's been ported at some point. It's on a KGC10 Skyline (on an N42 block) so I guess you're right, the head is probably some oddball JDM edition. I think the additional holes for the injector manifold is bigger than the 2 carb intake studs tho.

Can you guys help me ID what head I got? ..at the usual spot where you'd find the code, it's mysteriously blank. It's got notches for injectors, but square exhaust ports. Inlet ports are 40mm diameter. ...I'm thinking....N42??

Mine is the same...maybe 1.5mm in it....you can see the bridge washer is not sitting flat behind the nut in the pic above but the pic below is a bit clearer