Rebuilding petrol engine - piston protrusion question

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ExMil109

ExMil109

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Scottish Borders
I'm rebuilding a 2.5 petrol 17H engine for my Series 3 and have got to the point where I'm fitting new pistons.
The block was in bad shape and was rebored +20 thou and the deck was skimmed. Now I'm finding that the new pistons are protruding slightly above the deck. The worst is no 4 which is above the deck by 0.16mm. On no 2 shown below it's 0.13mm.

IMG_0813_web.jpg


I know that on the TDI engines this is normal and you just pick a head gasket thickness based on the protrusion, but on the petrol engine I believe the pistons should not protrude above the deck.
With the head gasket in place, there *might* be just enough clearance, but I doubt this will be true once it's torqued down.
So I'm not sure how to proceed - I could turn 0.16mm off the pistons in a lathe (from the whole set to keep them balanced), but I'm wondering if I can just use a 200tdi head gasket instead?

Has anyone encountered this issue before or got any suggestions for how to solve it? Thanks!
 
I would not machine the pistons, this may not answer your query but it’s a great reference
In this article the block was lightly skimmed
https://www.turnerengineering.co.uk/Article-2.25PetrolEngineRebuild

do you know how much was removed , wa s the original setup ..pistons flush
Turners have a reputation for being helpful as well as quality parts
54592751-72BF-4B9C-956A-B612660F3187.png

This is my 200tdi , I don’t know the petrol the same as this
When the pistons are at tdc the valves are shut , are they recessed in head I would guess plenty of clearance assuming timing correct but
Turners will have the answer , can’t see any info in green bible
 
ExMil109 said:
I'm rebuilding a 2.5 petrol 17H engine for my Series 3 and have got to the point where I'm fitting new pistons.
The block was in bad shape and was rebored +20 thou and the deck was skimmed. Now I'm finding that the new pistons are protruding slightly above the deck. The worst is no 4 which is above the deck by 0.16mm. On no 2 shown below it's 0.13mm.

View attachment 203757

I know that on the TDI engines this is normal and you just pick a head gasket thickness based on the protrusion, but on the petrol engine I believe the pistons should not protrude above the deck.
With the head gasket in place, there *might* be just enough clearance, but I doubt this will be true once it's torqued down.
So I'm not sure how to proceed - I could turn 0.16mm off the pistons in a lathe (from the whole set to keep them balanced), but I'm wondering if I can just use a 200tdi head gasket instead?

Has anyone encountered this issue before or got any suggestions for how to solve it? Thanks!
Click to expand...
youve had far too much skimmed off people should read the data first,there isnt a good answer to this only a bodge
 
Thanks guys.
I was concerned about the block - it was in bad shape after being stored with the head off and I guess the machinist kept going until he got a nice clean surface. Good 17H blocks are hard to find these days.
I will go back to the company that did the work for their views, but I guess the question now is what's the best bodge?
Looking at the 200tdi head gasket (thanks Steve) it looks like it might fit. The headbolt holes are obviously the same, the water jacket holes are a bit different and the area around the pushrods is also different. It would definitely be a bodge...
I recall that people used to fit 2 of the old copper head gaskets to drop the compression a bit after skimming the head too enthusiastically. Maybe an option?
I'm not sure if it's a good idea to machine the pistons. I doubt that taking off 0.20mm would weaken them much, but it's obviously not ideal.
Any other ideas welcome... Thanks.
 
The options are:
(Best/expensiveist order)
Replace the block
Custom gasket (TDI?)
Trim pistons
Offset grind the journals on the crank (nightmare and reduced stroke
Talk to turners and ACR, they may have options on the piston front
 
Thanks for the responses. I've asked Turners for advice, will be interested to see what they think.
Doing some research, what might help is a "decompression plate" like the engine tuners use when they want to add a turbo and need to lower the compression ratio. Of course, these are not off the shelf for LR engines, but the laser cutting firms will make one in 1mm copper if you have the CAD file. Might be an option if all else fails, but I need to calculate what that will do to the compression ratio - I don't want to end up with less BHP than the old 2.25!
I also checked whether a 2.25 piston would work, but it's slightly taller than the 2.5 piston, so that's no help.
I'm thinking of getting a 200tdi head gasket to see how close that is to fitting (similar issue with clearance vs comp ratio) and I'm also thinking about turning the pistons down in a lathe. Thanks Steve and Wildfalcon for the measurements - I think it's safe to turn 0.36mm off these pistons - they're fairly hefty things.
 
ExMil109 said:
Thanks for the responses. I've asked Turners for advice, will be interested to see what they think.
Doing some research, what might help is a "decompression plate" like the engine tuners use when they want to add a turbo and need to lower the compression ratio. Of course, these are not off the shelf for LR engines, but the laser cutting firms will make one in 1mm copper if you have the CAD file. Might be an option if all else fails, but I need to calculate what that will do to the compression ratio - I don't want to end up with less BHP than the old 2.25!
I also checked whether a 2.25 piston would work, but it's slightly taller than the 2.5 piston, so that's no help.
I'm thinking of getting a 200tdi head gasket to see how close that is to fitting (similar issue with clearance vs comp ratio) and I'm also thinking about turning the pistons down in a lathe. Thanks Steve and Wildfalcon for the measurements - I think it's safe to turn 0.36mm off these pistons - they're fairly hefty things.
Click to expand...
Using a copper head gasket always leads to trouble with any Land Rover engine.
Always use composite gasket, petrol or diesel.
 
Update: after spending far too long looking into the options, I don't think the 200TDI head gasket will fit, so I decided to go with machining the pistons. I skimmed 0.36mm off all four in a lathe and re-fitted. All are now below the deck by around 0.20mm which is more or less what Wildfalcon measured on his 17H. It'll be interesting to see what the compression pressures are when it's all back together. At least I can carry on with the build now...
IMG_0052_web.jpg
 
Shame I got to this thread too late to say this but.....For anyone else with the same problem.
Fit the head with the standard gasket and see if there actually is any contact with the piston. If there isn't, no need to worry at all you got yourself a high compression head, by the back door! (But watch for the spark plug fouling the piston head, and/or sparking from plug to piston head, you may want to find a shorter plug.) Also you may need an additive to cope with the extra compression. but the engine should give quite a bit more power for no worse fuel economy.
If there is contact then the technique would be to "pocket" the top of the piston.
See this, very well known technique when souping up an engine! Loads of videos and articles. You will see this is very commonly done!
https://www.google.com/search?q=Poc...me..69i57j0.4677j0j8&sourceid=chrome&ie=UTF-8
 
And just one final, and maybe not quite as pedantic a point as it seems cos you is measuring tiny distances, your DTI wasn't quite vertical.;)
 
Stanleysteamer said:
Fit the head with the standard gasket and see if there actually is any contact with the piston. If there isn't, no need to worry
Click to expand...
I wondered about this - with the composite gasket in place I *thought* there was some clearance, but I didn't know how far the gasket would compress when the head was torqued down and I didn't fancy finding out the hard way once the engine was installed and up to temperature.:eek:
I might have sacrificed a BHP or two by shaving the pistons, but as long as I end up with more BHP than I'm currently getting from my elderly 7:1 2.25 lump then I'll be happy...

Stanleysteamer said:
your DTI wasn't quite vertical
Click to expand...
Aye, it looks a bit off. It might be the camera angle. The end result was ok though - I was aiming for 0.20mm below the deck on the highest piston, which is what I ended up with.
 
As you say, if at the end of the day it runs OK and probably better than it did then you'll be happy. I am sure you will make good use of it and it'll run well for years to come.
All the best
Stan
 
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