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Is this a 4K or 5K head?


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Listed as .2 for intake and .3 for exhaust (cold).  I got them set yesterday with no trouble.  I used a little bit different method to compress the lifters:  since getting all 8 pushrods located on the rocker ball was a bit tricky, I dialed out the clearance before the rocker stands were fully seated, and before the head was fully torqued.  My logic was to let the head bolts do the work.  Then I went around one by one and backed them off to the proper clearance.  My recollection of previous hydro engines though, is that there was zero lash adjustment.  Then again, that was an Oldsmobile. 

In the pics you can see #1 with the exhaust valve compressed.  Rocker geometry seems fine to me, but I'd like to get extra eyes on this.  In the other pic you can see my pistons, which were not the dished type I was expecting.  I cleaned most of the gunk off and ran a wire wheel through the top of the bores just checking for cracks and ridges.   Didn't find any.  Block seems to be in good shape, I gotta say Toyota impresses me (this being my first one). 



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Yeah, that looks fine to me.  You should be driving it around for Christmas!

They look like flat-top pistons, the dish in a typical 5K is deeper-


That's why you have an inserted combustion chamber, as those dished pistons are the combustion chamber and the head for them is basically flat.

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Def interesting how Toyota seems to have provided different combinations in different markets.  These pistons have a slight concavity, so I wouldn't call them flat per-se.  But as you say, seems to be designed to work with the dished head.  Now other 5k's in the AU/NZ market seem to have been build with dished pistons and a flat top head I guess.  

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Drove to the exhaust shop this morning with an open exhaust.  Had tried unsuccessfully to get a downpipe made before the dual plane exhaust manifold was installed, but they had to use a good bit of guess work, resulting in the pipe pointing out to the side about 30 degrees haha.  The chop jobs done here on exhausts are frightening to look at, so this particular setup is just temporary.  Now I'm onto drawing up a flange to get it laser cut in stainless.  Gonna order the proper sized stainless pipe sections and maybe weld it up myself for better results. 

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  • 3 weeks later...

We have taken a few trips that were 2-3 hours duration, and the van has been flawless.  Plenty of passing power, and incredibly smooth pull at low RPM in 3rd or even 4th. Now I'm excited to move ahead with the dual electric fans and hopefully ditch the clutch fan.  Combined with the future exhaust system I have in mind, I'm looking forward to actually being able to hear the engine instead of the fan.  

Today I re-torqued all the critical bolts, the manifolds had all loosened a bit.  Reset the rocker clearance, which had all tightened a bit, wasn't expecting that.  Plugs look really good except for #4 had oil on it (which has been ever since I've owned the van).  Have enjoyed the cooler running temps, and have not added a drop of water after the initial run-in fill.  I'm going to cleanse the system now and go back to coolant, and reinstall the thermostat.  Before the new head, it was losing so much water there was no point wasting money on coolant, and the thermostat would get too much scale to function properly.

Incidentally, the VLT ceramic paint is holding up really nicely on the manifolds.  First time I have used the stuff.  Wife wasn't thrilled about baking parts in the oven, think I may need to get my own oven in the shop....


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Electric fans are great!  The lack of noise is very noticeable.  We always just use one, never a problem, although all the cars have the new alloy radiators in.

The rockers tighten up as the valves settle in, and the manifold bolts often need a little wind-up after running for a bit. All sounds good!

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  • 2 years later...

Follow up report, 2+ years on.  All the work discussed in this thread was pre-Covid mania.  Van has mostly been perfect but during the lengthy lockdowns it often sat for a month or two at a time.  Ran into either a batch of bad gas, or just sediment from the tank clogging up the works.  There is a lesson to be learned there about making sure you run your equipment.  I ended up tearing down the carb a couple of times to get it running right again. 

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But the purpose of this post is to once again reach into the collected wisdom here, to see if I can sort out a pinging problem that has developed.  I always run 95 octane, and after all the head work in this thread the van did run perfectly.  At first I was blaming the bad gas, but that should be thoroughly run out by now.  Symptoms are typical under-load rattling, even a touch on gear change.

To recap the work done prior on this 5K: New valves, springs, and head machine work.  Dual chamber exhaust mani, all new bolts.  Cleaned the pistons and bores at that time.  New (chinese) carb, new (chinese) electronic distributor with DUAL vacuum lines (more on that).  No choke (tropical weather), no EGR, no vacuum leaks.  Doesn't use oil, runs cool, very happy running overall, but has lost the torque it had before.  

My first idea was maybe the vacuum secondary stopped functioning.  Difficult to determine while running, but not sure why it would cause pinging.  Second thought was maybe I have got carbon buildup, just from sitting or whatever, making this pinging a case of detonation.  Plan to get some SeaFoam or equivalent to run through the engine.  Will do a closer visual inspection down the #1 spark plug hole, but lacking an actual borescope.   

My third idea centers around the vacuum advance.  Never seen one with dual setup, and very little reference info.  So far do not have difinitive info on which connection is for Port and which is for Manifold vacuum.  Conflicting info on whether this is dual-advance, or possibly one advance and one retard.  I did some experimenting on removing one vac. source, switching the sources, and tweaking the timing just a bit in both directions, but all of those ideas made it worse.  Of course, it ran perfectly before and I didn't touch it until now, so its a bit of a mystery what changed.  Next step is to verify if the vacuum is actually advancing (retarding?) the timing, to eliminate a leaky diaphragm.  

My third idea is about this van having a stock coil, which is maybe tired.  Any advice welcome here, this is my first ever Toyota.  Other ideas, a weak fuel pump?  Although by visuals it seems to squirt fine, not exactly using the scientific method here.    

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Take both vac hoses off the dizzy and block them, and set the timing without them to see how it runs. With a timing light on it you will see the ignition advance as you rev it. It should go from 10deg to 30deg or so.

Then drive it and see if it pings. Usually you have to retard it to stop it pinging, but that does kill performance. Here's what I found-


But if yours was running fine, the curve should be OK. That leaves timing that has shifted, or a vac hose problem, crappy fuel.. The coil dying gives a misfire rather than pinging, and the fuel pump shouldn't affect the firing.

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Thanks for pointing me in the right direction.  I popped off the dizzy cap this morning, and by sucking through the vac hose determined that the 'inner' diaphragm has a leak.  According to what I have read, this should be the manifold vac attachment, possibly a high altitude or emissions advance on some Toyotas, good for an extra few degrees.  The 'outer' diaphragm is working fine, and pulls the plunger through a reasonable degree arc (by eyeball).  Just did a test run with both of them capped off, and the pinging was gone, so next I'm going to reattach port vacuum to the outer vac nipple and see how she does.  

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Confirmed that with neither vac line attached, the pinging goes away.  Driveability was decent too, so this is a fallback cure if I can't sort out the vacuum advance.  I would like to get it dialed in as it was before, if possible.  I ran a fresh line from port vacuum to the outer diaphragm on the distributor, and capped off manifold vac.  This condition still gives a *slight* pinging, mainly on the tip-in transition.  My gut says it has just a few degrees too much total advance, but backing off the base timing results in poor idle.  I'm reminded that the only thing which seems to have changed is that secondary diaphragm....but if it were working, I'd have even more advance right....this had me stumped.  

Then it hit me.  With a working secondary diaphragm, the manifold vacuum at idle would keep it advanced a few degrees until the throttle opened....meaning my base setting could be backed off a few degrees from where it sits now....meaning the total advance would be a few degrees less on the port vacuum side while driving.  I'm going to dig around and see if I can find the old distributor and the single advance it had, to compare the total 'pull' with this replacement distributor. 

It seems there are a couple of solutions: 1) replace or fix the vacuum can, 2) mechanically limit the total advance of the contact plate, 3) reduce the amount of port vacuum signal somehow, 4) run with no vac advance and just be happy with the springs & weights.

There are also tuning / maintenance steps that could improve the situation: 1) getting an actual timing light and vacuum gauge to tune with, 2) adjust any valves that may need it, since there are not many miles on this valvetrain still something could need a tweak, 3) drain the gas tank to be sure of what is in the bottom, and eliminate any possibility of water or bad gas. 

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OK, that's definitely better.  Without going and looking this up, something I haven't had to think about for years, the vacs work like this-

Manifold vac givers maximum advance at idle, that's fine because the engine gets very little air in to burn and it does struggle. The moment you press the accelerator the inlet vac falls off, the advance goes backwards to whatever you set it to for idle and the car drives away. As you accelerate the weights in the dizzy give you more advance and fire the rich mixture just before it pings.  When you're up to speed and throttle back, inlet vacuum climbs a bit, so vac advance comes on and advances the ignition to minimise pollution while you have, once again, a small amount of air to burn. About 10cc of air goes into a cylinder at 100kph and about 0.7cc of fuel.


Ported vac depends on the flow of air past the port, so at low revs it gives no advance, and slowly adds advance as you go faster.  What that gives you is an advance control linked to how much throttle you have open, rather than just the rpm that gives weight advance.

Generally cars used one or the other I think, the twin vac dizzys were only introduced in the dying days before ECU.


Ah- here ya go...


The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

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