Decking recessed block

[the other tim]

Can any metal be skimmed off a recessed 6cyl 2.5 block without  machining the recesses any deeper, if so how much? I do know you can machine the block to flat but i would end up with my current pistons popping up.

 

 

       thanks Tim

[GT6 Nick]

Not sure. Some people have recommended removing the recesses altogether, but others, such as Las Vegas Steve have re-machined the grooves. That job didn't look difficult.

[motov8id]

if the pistons are .010" above the deck and your head gasket is .050" thick they are still .040 below the head. so how much above ?

[toofast2race]

I have run a 2.5 with pistons 0.005" proud of the block (non-recessed) and it lived for 50,000k's including lots of accidental over revving ... I think I may have got lucky tho'

 

However the most recent engine I built with Mazda pistons and modded standard (shotpeened, ARP bolts and length matched) rods runs 0.020" down the bores for safety

 

Piston tops can be machined by 0.010-0.020" to get desired clearance (at least thats what I do). As standard pistons are quite a way down the bore

 

Why not deck the block and get rid of recesses and then machine pistons to get you desired clearance? Note that crap quality headgaskets can be much thicker (and more prone to fire ring failure)

[the other tim]

Thank. Removing the recesses looks sensible but i need to do some measuring, the top of the piston is above the recess at tdc but below the deck at the moment. I need to check my combustion size (the head has had some chamber work done) What is the thickness of a compressed non recessed gasket.

 

My engine has stood for over 10yrs and has some light rust on the bores (still on standard bore), hopefully this will hone and still be within tolerance, if not it will be rebored. Which Mazda pistons do you use?    

 

       cheers Tim

[toofast2race]

I measured my torqued down gasket as 0.066"

 

Pistons were 77mm Mazda E series - very hard to find now ( I think someone bought them all and is now dribbling them out for a vast profit) and needed a rebushed small end. The oversize gasket required makes you cry too (150 quid)

 

This site is very handy

 

http://www.kb-silvolite.com/calc.php?action=comp

[the other tim]

I've had a cursory measure with feelers and a straight edge, the recess appears to be 30thou and the top of the piston measures around 10thou above the bottom of the recess ie if i had the block decked flat i would have 10thou pop up without any other work. I need to measure the combustion chambers now, unfortunately when i dug out my burette it seems to have morphed into a 3d puzzle I'm presently awaiting delivery on a fresh one.

 

On another note the cam i pulled out was stamped t6 248,  does anyone recognise it? It is a reprofiled job, not that i'll  be using it, just being curious.

 

 

     thanks Tim

[spitNL]

If the compressed gasket is indeed 0.066" thick, like Andy says, you will be fine.

0.066-0.010=0.056"

Borderline for good squish though.

 

I recall the 4cylinder gasket is 0.045"

This would give 0.035",  :-/ Don't think you can run this close.

0.040" is about the minimum you want to go, and even then you will have to accurately measure how high each piston comes above the block.

 

Can you measure the thickness of the old gasket?

And is the new on of the same make?

If not, general rule is multi layer head gaskets compress by about 0.010"

 

Cheers,

 

Frederick

[spitNL]

Just noticed you are measuring with feeler gauges.

That is not nearly accurate enough in this case.

 

first you won't be able to tell when the piston has reached the top, and no you can't rely on the pulley mark.

second it's not accurate enough.

 

Best to use a dial gauge with magnetic base stand

If that's to expensive get a cheap digital caliper, but even with this you can only expect an accuracy of 0.01" in this case.

[the other tim]

Thanks.

            I do actually have a dial gauge and a digital vernier when i can find them!!!! I just needed some ballpark figures to work from. I'll remeasure the fire ring on the old head gasket tomorrow although from memory i think it was 53thou.

 

 

         Tim

[DaveNotSoSideways]

4cyl payen is 0.032 thick compressed after 3-4 torque downs and few hundred miles @ 7200rpm.

 

S H I T gaskets are very think.

 

Do all calcs using a proper payen, as you'll need to fit one anyway after the cheap ones blow every 5miles.

 

Remember they DO compress

[GT6Steve]

I'm sorry,

 

I've been busy and missed that this was a six cylinder thread.  I'm a firm believer in the recessed bore for high compression.  I also accept the alternate argument of it being a fire ring causing preignition if done wrong.  To date I've not had any preignition issues nor blown headgaskets with a 12.5/1 CR.

 

YMMV...

 

 

My dimensions coming from a stock block (ages ago) are 3.235x.025 for the recesses.  I use the Payen gaskets and have zero head gasket issues.  Understanding the manufacturing tolerances of the day, I'd be tickled to hear what others have measured for these recesses.

 

HTH, Steve

[toofast2race]

TBH I was quoting headgasket from memory (I was at the gold mine) so I may have been wrong  :-/ sorry .

 

I have checked my CR spreadsheet and found I used 1.4mm / 0.055" as the uncompressed thickness and 0.039" as the measured head to deck clearance after a second torque down (Payen gasket)

[the other tim]

I measured the recesses as 3.5mm wide and 0.76mm (30 thou) deep.

 

After much measuring, remeasuring and double checking with a dti, digital vernier and in the case of the old h/g a micrometer too i came up with 10 thou pop up from the recess (20 thou below the deck). That's on no1 piston, no doubt there will be some variance on other pistons. The old h/g fire ring is 63 thou (effective h/g thickness) and the rest of the h/g is 32-33 thou. The bore is still standard at 74.7.

 

I'll measure the combustion chamber at the weekend.

[spitNL]

After much measuring, remeasuring and double checking with a dti, digital vernier and in the case of the old h/g a micrometer too i came up with 10 thou pop up from the recess (20 thou below the deck).

Wait till you do that on a v8. :

Had height differences between the pistons of 0.030".

The pistons on my 1500 Spitfire on the other hand were all exactly at deck height.

 

What pistons are you using? AE, County?

 

Frederick

[the other tim]

Stamped on the piston crown is N46, other than that i can't tell you any more untill i get them out at the weekend. The block should be measured on Monday, hopefully it's within tolerance.

[GT6 Mike]

Had to machine 35 thou of my block to remove the recess, variation between pistons hieghts was around 8 to 10 thou from memory, once the pistons were machined the all protruded 8 thou.

[the other tim]

Pistons are Hepolite, they look to be in fair order, the rings are all stuck though!!

[tim_trinda]

I measured the recesses as 3.5mm wide and 0.76mm (30 thou) deep.

 

After much measuring, remeasuring and double checking with a dti, digital vernier and in the case of the old h/g a micrometer too i came up with 10 thou pop up from the recess (20 thou below the deck). That's on no1 piston, no doubt there will be some variance on other pistons. The old h/g fire ring is 63 thou (effective h/g thickness) and the rest of the h/g is 32-33 thou. The bore is still standard at 74.7.

 

I'll measure the combustion chamber at the weekend.

 

Tim

 

30 thou is what I had skimmed off the block on the engine I'm putting together. 30 thou removed the recesses (which were approx 28 thou deep) and left a pop-up on the lowest piston of 10 thou. I had to have 5 pistons skimmed by 2-7 thou to get the pop-up on all pistons to 10thou. Interestingly the variation in pop-up was not due to the County pistons (which were all identical compression height) but by the crank and/or rods.

 

Cheers

 

 

[spitNL]

Interestingly the variation in pop-up was not due to the County pistons (which were all identical compression height) but by the crank and/or rods.

If it is a combination of the crank and rods, swapping them around between the cylinders may get you closer to deck height.

 

Pistons are Hepolite, they look to be in fair order, the rings are all stuck though!!

Good thing you got them out then.

[tim_trinda]

If it is a combination of the crank and rods, swapping them around between the cylinders may get you closer to deck height.

 

Too hard!! And increased potential for cock-ups ... for me anyway. I'd still have to have some pistons machined so it was easier to stick with the existing numbers on the rods so I knew which bore they belonged to.

[flatter4]

 

I understood 10 thou. pop-up on a six was safe (unless doing crazy / race rpm)

Proper (Payen) gasket compressed thickness should be circa 40 thou. (non-recessed block)

 

Anyone got different to this?

 

[tim_trinda]

I understood 10 thou. pop-up on a six was safe (unless doing crazy / race rpm)

Proper (Payen) gasket compressed thickness should be circa 40 thou. (non-recessed block)

 

Anyone got different to this?

 

I hope so! The engine I'm building is as described by Chris Witor in this article http://www.t2000register.org.uk/original_site/SIXappeal/SIXappealsnippets/chriswitortech1.htm

[the other tim]

I think i would have serious doubts about running 30thou squish (10 thou pop up)  on a 2.5 as the crank appears have  very little if any main bearing/big end journal overlap, the 2000 crank looks far stronger to me but then i'm new to Triumphs and would be more than happy to be corrected.

 

      Thanks Tim

 

    

[gt5r]

Your going round in circles - your talking less than 2 hp increase on a good day - your not consentrating on the right areas.

 

2.0 ltr - revs - high compression and long cam duration - consentrate on making it rev!!!

 

2.5 ltr - torque - high lift cam - short duration - consentrate on max torque @ 2800 rpm

 

Dummy build - zero the block/pistons - relabilty - piss around and it will become unrelaible