I refitted my Z head after blowing a gasket. 20 thou was shaved off it this week, I used a Copper gasket and re-assembled the motor this arvo, but the engine now has a knock..bugger!
I presume the 20 thou has been enough to let the valves or piston tough as it didn't knock prior but I was wondering if there is something i can do to check the cause before I pull it off again, for example thru the spark plug hole is there a test?
The other option is the extra compression has strained a loose bottom end?
I used a long tool as a stethoscope via the top of the head and maybe number 4 is louder, I didn't climb under neath for a listen.
Not sure if the pistons are stock, pic shown.
Any suggestions would be appreciated, Thanks.
Does it knock all the time while running or only once the timing is advanced?
If all the time I'd say you're correct and the pistons are touching the head. Are there any marks on the pistons? Something to indicate they are touching?
As for it being a bottom end knock I would say you would have heard it before the head skimming.. All things being equal.
The 20 thou you skimmed off should not add much to the stress on the bottom end.
I'm leaning toward a piston knock. But come to think of it, if it took 20 thou to get them to touch, there was precious little space there to begin with.
Another thought as I write this. Did you have the motor rotated so 2 pistons were up and protruding when you put the head gasket and head on? If it is cocked just a little you could get the pistons to touch , possibly #4.
Getting the head aligned is possibly compounding the problem.
Hope this helps
A machinist friend introduced me to this product. It sprays on a thin blue film that dries quickly.
Try spraying the underside of the head. The piston contact point will be removed.
Another way would be to spray the top of each piston. If the piston is striking the head gasket or the head, the point of contact will be removed.
It's basically the same as Prussian Blue except it's layer is ultra thin, and it's dry. Available at machinist supply stores, Fastenal, etc. Don't accept a substitute. I tried that.
A increase in compression will stress the bottom end and perhaps would expose a rod that needs a adjusting. Or it could just be the bell on the front of the crank.Check that.
If you suspect a piston hitting, I would loosen the head bolts, remove the plugs and turn the engine over it will lift the head if it is hitting.
Yes. That is always needed when head is shaved, or adding a high compression head. The camber is really snug, and a piston or pistons can hit the chamber head.
Put the head on without a gasket, just loosely fit a few head bolts, turn the engine over to be sure the head doesn't jump.....that is the test.
This is the fix...grind away some of that high compression combustion chamber where contact is found.
Justin - it knocks at all timing settings. No I had all 4 pistons inside the chambers when refitted but I did have two cut-off studs screwed in place to ensure alignment.
Rick I will try this next, I wish I had done the head on without gasket test to see if it lifted.
John looks like a good product to have on hand.
Mack - I'm not familiar with he "bell on the front of the crank", please clarify. Ta
Dan - Good idea.
Today I will remove the head again and see what I can find, hopefully be able to grind some off the head or just align the gasket.
let you know, Thanks for your advice Guys.
I put a Z head on a block that had been decked, and turning the engine off my tractor pto prior to starting it, I heard a knock. I then cranked it very slowly by hand and could feel the interference once each revolution. One piston was just barely touching in the area dyed blue in John's photos. A little "adjustment" with a Dremel tool fixed it.
Well I undone everything and hand cranked it but couldn't hear anything. Took a heap to get the head off. I sprayed VHT copper spray over a copper gasket yesterday when i fitted it and today it was really stuck on, I had to hammer a wedge to get it to split apart, when lifting the head off it destroyed the one day old gasket. Anyway sat the head back on the block and hand cranked and watched the head jump on each stroke! Unreal, 20 thou skim?
Dan I tried your suggestion and ground off where it was marked. Smeared grease on the pistons and refitted, ground some more off etc for about 10 goes. I think I ground more than 20 thou but in the end I gave up before I ground too far. I measured the gap at lift and it was just less than a gasket thickness so I refitted the head using 2 Steel gaskets with Copper spray on and between the two. Cranked her up and it runs quiet as a mouse. Not sure how long it will last but it gets me mobile and I'll look for a stock head. I'll re torque it tomorrow after its cold and again in a week.
I'll post some pics later.
Why look for a std head now that you've got the Z head working? You'll love the power in the hills (not so much with a std head)
Roger I will give a test run on my "test Hill" and then drive it as it is. I'm not convinced it has more power on the hills, well it didn't before but maybe it was a mixture of other faults. I certainly will be leaving it as is and as long as the 2 head gaskets together don't fail but I will source a stock head in the meantime.
I've been doing daily hill tests in my speedster (Stock head running NH) as you might remember using a GPS speedo and making improvements and tweeking etc. Its running really nice now and climbs the hill about 10k's faster than the Coupe did so I'm looking forward to trying the Coupe now. I got it running today without the knocking but not running exactly right (vaporizer). I sprayed CRC around the inlet manifold when running and it increases in rpm so I guess I have a leak. I mucked around with the inlet, off/on a few times but can't improve it even using new copper rings so I've put some permatex blue around the copper and will let it sit over night. If its running right tomorrow I'll do a hill test.
There are other important variables in an engine than compression - maybe the camshaft is worn or has a bad regrind - some regrinds are known to give bad low rpm performance (can sometimes be helped by advancing the cam 7.5 degrees with new dovel pin holes in the timing gear.)
Yep, had to grind some alum. off the inside of each chamber for mine to fit, testing with Prussian blue each time, took several test fittings to get it done.
Glad you found that 'knock'. I found out mine the same way, bolted up the new to me 'Z' head (it was really a used one I found) and installed it straight on. Then when firing up the runabout for the first time, rumpt, rumpt, rumpt sounds from the motor. Shut it down immediately.
Pulled head and saw marks on the tops of the pistons, so figured that out fast.
The pistons will past above the top of the block, and depending on the type of replacement piston, and if the block was skimmed, or other dimensions, the new high compression heads will allow the piston to strike. Or sometimes the gasket is off center and piston will hit the edge of the gasket. Noticed your pistons stick above the block, that will do it!
Advise from the 'Z' mfg. to check that before bolting down the head. See Step 4
Again, nice you corrected it, and that new head should be fine.
On the last Z head I installed, there was some interference between the pistons and the head, but it wasn't in the squish areas of the head, it was at the perimeters of the combustion chambers. The block had been bored 60 thou, and the larger diameter pistons were hitting the edges of the combustion chambers. A little work with a die grinder took care of it in short order.
I had the same problem when I swapped the Z head for a 90-year-old Haibe Giant Power head on the same engine. I had the Haibe milled .125", so the chambers were smaller due to their curved shape.
Fortunately I discovered this both times while doing the test as outlined above, not after everything was buttoned up. As Dan says, you should ALWAYS test to see whether everything will fit before bolting the head down tight. Bigger pistons could cause interference, even with a stock head.
By the way, if you apply the Prussian blue to the piston, rather than the head, the contact areas will show up on the head in exactly the places you need to do the grinding.
(Message edited by coupelet on September 13, 2015)
Roger that could be relevant as I have no history on the cam.
Guys the interesting thing is clearance was fine with the Z head until i had it shaved last week. You wouldn't think 20 thou would make this much difference!
Here is a video of the lift with NO gasket on and before I started grinding.
Not an uncommon problem with a Z head and the same holds true with other reproduction high compression heads. Piston strikes without a head gasket and most are not flat and will rock on a fresh deck block. I am glad that someone is making them but know that I will end up working them over.
I like to end up with about .050” of squish (clearance between the piston and the combustion chamber) which is about the thickness of a head gasket after it has been compressed. Once piston strikes are ground out, a gasket is installed, head tighten, your good to go.
Yes you milling the head .020” could have cause you to have piston strikes!
Dan, great feedback and pics really help understand plus the Z head instruction really helps.
I didn't have any blue on hand and used grease on the piston and it gave me the same info but I wasn't sure how much I could grind off the head without risk.
Mike that's really good info and explains the situation really well, thanks.
This is a great resource thread ...
The first Z head that I installed 15 years ago caused me to have 2 broken valves and 2 bent valves. I had to grind the top of the combustion chamber for clearance after replacing all those bad valves.
The instructions would have saved me the trouble had I followed them closely.......
Royce did you notice any real improvement after you fitted the Z?
I'm not convinced of the advantages yet and here is why -
I have two T's and both are as I purchased them.
27 Coupe has Z head running a vaporizer
26 Speedster stock head running a NH
I have a test hill that has a right angle turn to start so I can't get a variable run up. I use a portable GPS speedo, same unit in both cars.
The Coupe is heavier than the speedster and the best it has slowed to prior to blowing a head gasket on the climb was 20 mph, sometimes slower but 20 was the best it will do.
The speedster was performing about the same when I first tried it which I was disappointed with this considering it is lighter however the motor sometimes had a slight miss under load.
I found a few bits of junk floating in the cowl tank and due to the way it was built I can't get the tank out for cleaning so I have fitted a few fuel filters in the line and then cleaned out the carby. I also fitted a 6v fuel pump & pressure regulator to ensure correct flow through the filters. The speedster now runs smooth all the time & climbs really well and usually drops to only 26 mph. That's 6 mph better than the Coupe.
I finally got the Coupe running properly after the head gasket process (plus I found an air leak in the inlet manifold) so I took it for a hill test. Was very disappointed to find it was really slow & had to change into low to get up! I will do some more tweeking before I panic too much but on the way home I was buzzing along on the flat and I noticed I was doing 42 mph and thought wow so I opened it up and it reached 47 mph and held that for a couple of miles? It has never gone that fast before? I don't really want speed I would rather have pull but what a pleasant surprise considering the failed hill climb test.
When I got home I checked the compression's, Coupe with Z head 88, 88, 89, 88 psi. Speedster 55, 55, 57, 55 psi.
Diff ratio check, jacked up rear wheel etc both cars have the same ration (3.64)
Hence I was wondering if the Z gives top end at the risk of hill power?
I know Roger doesn't think this is the norm.
No, it's common knowledge that higher compression gives more power over the whole rpm band. You have other differences between the cars that matters - the vaporizer is known to give less power than a NH and the camshafts are unknown.
I suspect you have a reground camshaft in the coupe that has a modern grind that isn't suited for the low rpm characteristics of a model T engine - thus it gives even less power than an original camshaft when the rpms goes down in hill climbing.
I would suggest some reading, like at the Tulsa model T club's Technical pages about heads and camshafts. Google Tulsa Model T club and you'll find it.
A badly reground cam with low power at low rpms but more at the high end can be helped by advancing the timing gear 7.5 degrees. It moves the power band to lower rpms, lose some at the top but gain where you need it more. I'm very happy with my camshaft reground by Antique Auto Ranch in Spokane, Wa. In combination with an advanced timing gear, 3:1 gears and a Prus head it runs up hills like it was flat (ok, it's a light car)
Roger that sounds likely as I've always noticed the Coupe struggles a bit every time I change into top gear. I try to use the camber of the road to assist.
I'll have a read of that site, thanks.
I have an after market inlet manifold and plan to test a different Carby. I picked up a Solex from the local vintage car club parts shed last time in town. I'm looking forward to trying it out.
As always thanks for your input Roger.
The performance difference has been stunning on every car that I have installed a Z head on. You likely have other issues that are hurting performance so much that you can't see any improvement.
Adding all those fuel filters is something that you need to reverse in order to get the car up to its performance potential. You will need to clean all the gunk out of the gas tank too. It is a ritual that I have to do with every Model T that enters my sphere of influence.
Fix the basic stuff, and the car will likely go faster than it needs to with no modifications of any kind. The vaporizer carburetor also works fantastic if in good shape.
A short story about my 24 Fordor. I bought the car from the second owner and while it would run it had a very tired engine. No problem I had a engine that was rebuilt with new, rods, pistons, deck block, reground cam and a milled high head that I installed into the car.
The car had standard gears with a Ruckstell and no matter what tuning I did, it wouldn't make any power at low end to the point I always use Ruckstell at intersections just to get through the traffic light before it would turn red, from a standing start.
Along came the new Z head and one was installed, made a difference but still lack low end torque, got through intersections a bit better but still always used Ruckstell.
This was about the same time my own club members were involved with getting new cams made. Larry Young's work in that undertaking was and has been a blessing in that today we have a couple of choices of new cam manufactures.
Long and the short, it took me a while to get on board with the idea and I finally installed and new 250. I really can't offer hard data on the improvement, X horse power to XX but I didn't need to use Ruckstell anymore, had 55 mph plus top end but more important would pull my test hill in Ford high gaining speed all the way to the top. Before changing the cam to pull my test hill I would end up in Ruckstell Ford low pedal near the top.
All that said higher compression is really a good thing but even more important is a good cam. Another note, all regrind cams are not created equal some are really good and others are a waste of time.
If you haven't already, do look at our clubs website and read the information Larry Young has about Cams.
Weight has a real impact on our performance. My friend Nick Nicholas built a hill climb car that John Steel and myself get to abuse at events. Our first outing I beat up Nick and had a lot of fun teasing him about my superior skills as a racer. Then John took a run and beat my best time, stealing my thunder. John, is not a better driver than Nick or myself, he just weight less than us. I am about 25 lbs lighter than Nick and John about 25 maybe 30 lbs lighter than me. At Lincoln this year John won our class and was asked how he did better. His answer "two fat guys and a skinny guy".
The hill climbs are 500 to 550' in length and running in the high 9's to low 10 seconds.
Ok not so short of story.
Royce the filters are on the Speedster which is driving great! Besides they have no effect on fuel flow to the carby as I have a fuel pump and pressure regulator. In fact the fuel pressure to the carby is much more constant than a gravity system like the Coupe is running.
I agree cleaning out the tank is vital and I removed the Coupes and washed it clear when first purchased.
I'll source a stock head at some stage and will be interested to run a trial if the Z needs to come off again for some reason.
Mike that's interesting feedback which really supports Rogers thoughts, if I ever have the cam out I will try this.