12ht piston protrusion

[maddog632]

Hi everyone. I am doing a full rebuild of my 12ht at the moment and have just put all the pistons in.
The pistons are protruding .75mm above the block deck. Specs say it should be .53-.68.
My head gasket is 1.5mm thick so if the head gasket crushed down .5 would I have enough clearance or do I need to get a thicker head gasket.
The block is all standard apart from over sized pistons, the block has not been decked.
Thanks again.

 

[Hulsty]

Deck the pistons back to spec

 

[roscoFJ73]

Who supplied the pistons? Ask them and see what they say.

 

[Gengis]

What are the options here?
I have a similar job coming up, my block is on it's way back from the machine shop and they DID deck it.
Toy , as far as I know, did not make an MLS gasket for these engines and the information I have does not show a selection of the standard gaskets in various thicknesses. Again I may be wrong here.

Is decking the pistons an accepted and good practice. Given that the pistons in most cases will be aftermarket.

Is getting the rods resized an accepted and good practice.

In the event of not getting a thicker gasket, which is preferable?

I'll have my protrusion figures in a week or so.

Also, where did you get the figures from?? I searched high and low and eventually got these from Engines Australia, which look very similar to yours.

Gday George Please find attached a quote on Piston assemblies to suit a 12H-T Toyota Pistons come with Rings, Piston pin and Circlips fitted. Piston protrusion is 0.51 - 0.64mm Regards Blake ...

 

[AussieHJCruza]

Pretty certain ADS Injection do an MLS head gasket for the 12HT

 

[relaxedcruiser]

Just realised a blown headgasket today - took the head off - It´s a bit disappointing as I have replaced the gasket only two years and about 40.000 Km ago. When I measured the compression rating a while ago I had to be honest quite shocking high readings of about 38 bar or 550 psi on all cylinders. The manual is just saying that "30 bar or more...". I think that those readings are quite on the high side. I would love to be able to choose from different gaskets to adjust the piston protrusion which in my case are the same as maddog 632´s with 0.75 mm. Never seen any different thickness gaskets for the 12ht - unlike most other engines were there are available.
How is the piston decking done ? I´m a qualified mechanic but have never heard of it.

 

[Gengis]

They wack them into a lathe and skim a bit off the top of the pistons, simple, but it's a solution not liked by all.
The process does change the pistons/mixture swirl characteristics slightly, and could take away a surface hardening if indeed there was any.
Short of finding any gaskets of varying thickness's, the next alternative is to have the rods shortened, not as drasy tic as it seems, they mill the mating surfaces of the big end and then re bore the bigend to suit the bearing again. This also costs a lot more than piston skiming.
.75 sounds a bit much, but I have never seen official Toyota figures, and i am not sure where EA got the 0.51 - 0.64mm from.
Could too high a protruision cause a gasket to go?? dunno, I would be more worries wacking the piston into the head or valves.

Be good to hear some other views, as I start rebuilding in mid January, AND my block was skimmed, so I am expecting similar problems when I measure up.
The local shops here all support skiming the pistons, and only one or two are happy to cut the rods down??? Thinks.
G

 

[dogfishlake]

That is interesting info @Gengis . I am only guessing but I expect that cutting the piston is the way to go. You are only talking about taking .07mm off and as far as the heat treat goes, the entire piston would have been done so it really shouldn't change. The compression would not come down to exactly stock level because you would not be doing anything with the dish area.
Random thoughts.
Hope all goes well OP

 

[relaxedcruiser]

When I woke up this morning I thought about my orbital sander....quite drastic to do that - but I haven´t much time to get it right with taking the pistons out and so on. This time of the year doesnt help and we already booked a ferry for an road trip starting on the 6th of January. Thinking of taping the entire block surface and of cause use the vakuum port on the sander as well as using quite fine grade. quite confident to get it perfectly right - just thinking of the hardened surface. I would have to take off at least 0.1 mm better 0.15 - any thoughts here ?
can´t undersatnd though why there is only one thickness of gasket available.

 

[REDREEFER]

Well, this hits close to home..... I have, in fact recently been down this road and will try convey what I found out.....

In my case I used pistons from EA (Diesel Power Parts brand) which seam to be fairly common choice in the aftermarket and on the box it clearly states that piston protrusion is not to be trifled with! I did take notice to this but really didn’t think much of it as I was getting the machine shop to do the short block including skimming the deck and boring to first oversized. I just assumed that with the factory 12HT FSM and warning stickers if there was an issue I would have heard about it. As it turned out they called me to come get it which I did and back in my shop I realize that there was what appeared to be an awful lot of piston hanging out of the bores and written on the deck was .036” which is .91mm.
DPP and EA spec sheets have there protrusion parameters a little different from each other but both less than the .91 that they made it.

I asked my machine shop about this and and I got this response:
I am trying to wrap my head around why the engine had a protrusion spec of .034” with the original pistons. How was this engine running being out of spec this much? You confirmed that the original gasket is a standard thickness? Most pistons can be machined down to accommodate a correct piston protrusion spec, however this is very uncommon as the aftermarket pistons are usually a direct match. I have never heard of a piston manufacturer making pistons taller in order to be cut down to size.

At that point I was not sure what to do as I was now trying to figure out if the protrusion was pretty much the same with the new pistons as the old how is it that DPP and EA want it to be less, what to do??!

I asked EA about this and got this response:

Hi rob

These pistons are made to the highest protrusion specs so to cover all engines

The specs above are to suit our pistons

Sorry not stocking any thicker style head gaskets

So what the machine shops do in Australia is do a dry fitment as you have done to get a correct measurement

And then machine the top of the pistons to get the correct clearances

So at that point after trying to find a factory spec in the FSM or anywhere else for the 12HT piston protrusion and not having any luck I made the decision to take the whole 9 yards back to the machine shop and have them machine the pistons as @Hulsty and others had suggested and they were able to get the PP at the top end of the DPP spec.
And in the end all is good, the engine purrs along quite nicely!

My own hypothesis on this is that DPP wants a lower specification for the protrusion due to possible greater piston expansion at higher temps with the Alfin pistons? Not really sure but my 2c is that it’s a good idea to follow the manufacturers specification, that is if they include it in the box!

I’m not so sure about @relaxedcruiser’s method of achieving this reduced PP but I think he’s on the right tack!

Cheers guys, I hope the loooooong explanation of my situation was worth the read!

All the best in the next Decade! The last 10’s been a wild ride!


Piston set I got from EA, this is a pic I found on mud, mine were first oversized at .50mm

The don’t screw it up sticker......

The EA spec from 1999

The DPP spec sheet that EA sent me as the latest spec Feb 2005

The finish product! ❤

 

[relaxedcruiser]

Thanks a lot Redreefer for sharing this information - after hunting for the headgasket yesterday I found a company here who is doing headgaskets to order. They are closed this time of the year, but after their break I´ll contact them and see wether they can supply me with an gasket thicker to get the PP back to spec. In the meantime I´ll put the head back on with 0.75 protrusion without trying my orbitol method - it has run smooth for the last 40.000km like this. I really need the HJ back on the road now - otherwise I would get it done right now.
I wonder how many pistons get fitted without respecting the PP - and how they go....
As I mentioned before I get 38 bar or 550 psi even on all cylinders....I think that this is too much but then while the combustion takes place that pressure is again a lot higher and I don´t know if that is very much effected by the initial compression.
By the way...your engine bay looks really stunning !

 

[REDREEFER]

No worries RelaxedCruiser and thanks for the compliment! Thats great that you found a company that might be able to help with a thicker gasket. I was meaning to touch on that as well in my first post, I do know that there is some manufacturer somewhere in Oz that does make thicker MLS gaskets. John Barron Of Radd Cruisers had some made up a number of years ago and apparently worked out well. I think that if I had access to that option I probably would have done that as well in the long run though I think milling the piston tops is a better option.

It would be interesting to know what others have done in this situation, also it would be nice to know what the factory PP spec is and what others are measuring before disassembly? I haven’t yet fully broken the motor in yet and thus not taken any compression readings but I will in the future once she’s broken in.

Regarding your compression numbers....did you confirm the 550Psi with another tester to rule out possible gauge anomaly’s?
Also did you use the DPP Alfin pistons as I did?

Cheers!

 

[relaxedcruiser]

No - actually I haven´t proofed that compression reading with annother tester. It´s a bit odd that the PP is not mentioned at all in the workshop manual. I wonder wether there is annother manual or a supplement for the reconditioning of this engines.
I would love to know which pistons have been used when the PO partly recon´d the motor.
May be they can be indentified by the stamping pattern on the piston.
Note that the PO sleeved this cylinder.
I

 

[REDREEFER]

Yes, not sure why the factory PP spec is not listed anywhere, if anybody has this info please chime in

Here is a pic of mine with the EA/DPP pistons, looks like yours are different. They may be OEM in second oversized at 1.00 mm, mine were first oversized at .50mm. This was shot before the machining took place to correct the PP. .036”= .9144mm

Ya, she must of had a bad happening at some point in its life having to be sleeved like that.

 

[relaxedcruiser]

searched a bit and found this: They look like mine - Taiwan but at least Alfin type. Could be coincidence but they have the same "4" stamping.. and they are advertised in New Zealand.
They are in a clearance sale right now - Toyota 12H-T diesel turbo pistons +1.00mm

 

[REDREEFER]

Well that’s good, I can’t imagine anybody going that far on one of these, sleeve and all, without using Alfin pistons.

Good luck relaxedcruiser, if you find anything on that MLS gasket maybe post it up if you don’t mind

To the OP, I hope all this chatter has helped you to figure you next move.

 

[youtubeknowledge]

Just realised a blown headgasket today - took the head off - It´s a bit disappointing as I have replaced the gasket only two years and about 40.000 Km ago. When I measured the compression rating a while ago I had to be honest quite shocking high readings of about 38 bar or 550 psi on all cylinders. The manual is just saying that "30 bar or more...". I think that those readings are quite on the high side. I would love to be able to choose from different gaskets to adjust the piston protrusion which in my case are the same as maddog 632´s with 0.75 mm. Never seen any different thickness gaskets for the 12ht - unlike most other engines were there are available.
How is the piston decking done ? I´m a qualified mechanic but have never heard of it.

Hey mate, I'm new to this site. I've also discovered a blown headgasket on a old 60 series (12HT) I've bought recently. My plans are to put it back on the road, i've bought it as project. I'm trying to tackle all the problems alone and but I'm no mechanic by any means so all the advice is appreciated. Im in the process of the taking off the head. Im at the point where I have to take off the manifolds. Im struggling to take off all the bolts on the intake manifold, there are some bolts that are near impossible to take off without taking off the injection pump (?????). Im also trying to take the head out without lifting the engine out, because i dont have the tools to lift it out. Any advice on taking out the intake manifold will be much appreciated.
Cheers

 

[duncanrm]

Im struggling to take off all the bolts on the intake manifold, there are some bolts that are near impossible to take off without taking off the injection pump (?????).
Cheers

It kinda sounds like you're trying to take off the Oil Cooler cover.. not the intake manifold. ?? Photo?

 

[youtubeknowledge]

It kinda sounds like you're trying to take off the Oil Cooler cover.. not the intake manifold. ?? Photo?

Hey, thanks for the quick response. yep i can put a photo. The red arrow points to nuts that i cant get to

 

[duncanrm]

Surely you can sneak a ratcheting spanner in there for that one nut?