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Piston question

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Piston question
#1
  This topic is about my 1978 Triumph Spitfire 1500
Voda2000 Avatar
Voda2000 Andrew McMillan
Winnipeg, MB, Canada   CAN
I fished a bore scope into the 1500 engine today and took a few photos. The motor number is FM101633UE so i think it was built as a low compression motor. The photos look like flat top pistons to me. Is that what they look like to everyone else?

Is there anyway to check the compression ratio without pulling the head?

The motor was de-smogged before i bought the car 17 years ago and there had been some work like changing the distributor to a 43D4 and a header but i had assumed they never did any internal work as the car only had 70,000 km on it. Now i'm beginning to wonder what was done to it...



1978 Triumph Spitfire

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Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
To me, pic #2 shows what may be the rim before the dished area of a dished pistion.

The flat rim (as I recall) is about 1-6mm

clshore Carter Shore
Beverly Hills, FL, USA   USA
Well they LOOK like flat tops, maybe.

But the borescope is only showing the edge of the crown farthest away from the spark plug hole.

Here is what the dished pistons look like:

https://rimmerbros.com/Item--i-159122SET

The dish is adjacent to the spark plug, the 'flat' portion is farthest away.

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skyking1231 Avatar
skyking1231 Silver Member Frank Strobel
Mt. Sinai, NY, USA   USA
1975 Triumph Spitfire 1500 "Lil' Rose"
the last photo makes me think dished. you can see a line that runs across, that would be the straight part of the dish.

Voda2000 Avatar
Voda2000 Andrew McMillan
Winnipeg, MB, Canada   CAN
Thanks guys. I'm leading towards dished now too. I tried the scope again and this photo looks like there is a ridge around the piston top.

I'm going to budget on replacement as part of this rebuild.



1978 Triumph Spitfire


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clshore Carter Shore
Beverly Hills, FL, USA   USA
I dunno.
Use the bore scope to look CLOSER to the spark plug, the left and right edges of the piston, NOT at the far edge of the piston.
The flat ridge will run straight across the piston, left to right.

skyking1231 Avatar
skyking1231 Silver Member Frank Strobel
Mt. Sinai, NY, USA   USA
1975 Triumph Spitfire 1500 "Lil' Rose"
perhaps a long skinny skewer stick...might be able to get engouh movement to feel any "dish" on suface of piston ? just a thought....

can we start taking bets ???grinning smiley

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tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
AFAIK, Triumph only used domed pistons on six cylinder engines. It was a way of allowing the same cylinder head to be used for both 2L and 2.5L engines, and keep the same CR.
So only late 2L engines got domes.

Dished pistons were to do the opposite on four cylinders - reduce the CR to diminish emissions for an increasingly regulated USA, without changing anything else in the engine
Of course, a CR as low as 7.5 (yes, really!) meant that the power of the Spitfire was now akin to a flabby kitten.
Sorry, guys. It'll need a bit of work to put the tiger back.

Ohn

Tonyfixit Avatar
Tonyfixit Tony M
Duncan, BC, Canada   CAN
Did GT once claim that the dished pistons (and a skimed cyl to compensate) resulted in better results on the dyno than flat pistons with the same comp ratio?

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grumpicus Steve Jackson
Leicester, Leicestershire, UK   GBR
Yes, he did - it allows the use of a cylinder head with shallower chambers.

tapkaJohnD Avatar
tapkaJohnD John Davies
Lancaster, Lancashire, UK   GBR
Bathtub chambers are terrible last century, or the one before that.

"Chambers in the piston" for cars came in well after WW2, but were inveted by Sam Heron, and eminent British aero engineer, who spent much of this laife working in the US.
The "Heron Head" had a flat surface, with depressions in the piston to relieve the valves, which also formed a combustion chamber. Maximal 'squish', like the Triumph but more effective.
Today's direct injection engines rely on specially shaped pistons to shape the charge and stratify it for extreme lean burn.

John

clshore Carter Shore
Beverly Hills, FL, USA   USA
Assuming the motor is sitting upright, carefully introduce some oil into the chamber through the plug hole.
Use the borescope to observe if it collects in the dish, or spreads out across the flat.
I advise using something benign but easy to see, perhaps dark used motor oil.
And use only a small amount, to prevent overflowing the dish, and obscuring the results.
Anyone know what the dish volume is supposed to be?
Somewhat less than that amount.

Voda2000 Avatar
Voda2000 Andrew McMillan
Winnipeg, MB, Canada   CAN
In reply to # 1513032 by clshore Assuming the motor is sitting upright, carefully introduce some oil into the chamber through the plug hole.
Use the borescope to observe if it collects in the dish, or spreads out across the flat.
I advise using something benign but easy to see, perhaps dark used motor oil.
And use only a small amount, to prevent overflowing the dish, and obscuring the results.
Anyone know what the dish volume is supposed to be?
Somewhat less than that amount.

That’s a good idea. Might give it a try when I have a few minutes.

The borescope I have cannot navigate enough to see down more but I’m gong to see if I can borrow a better one.

Ultimately it’s not a big deal l I’ll eventually have the head off but I’m betting they are original.



1978 Triumph Spitfire

clshore Carter Shore
Beverly Hills, FL, USA   USA
In reply to # 1513066 by Voda2000
In reply to # 1513032 by clshore Assuming the motor is sitting upright, carefully introduce some oil into the chamber through the plug hole.
Use the borescope to observe if it collects in the dish, or spreads out across the flat.
I advise using something benign but easy to see, perhaps dark used motor oil.
And use only a small amount, to prevent overflowing the dish, and obscuring the results.
Anyone know what the dish volume is supposed to be?
Somewhat less than that amount.

That’s a good idea. Might give it a try when I have a few minutes.

The borescope I have cannot navigate enough to see down more but I’m gong to see if I can borrow a better one.

Ultimately it’s not a big deal l I’ll eventually have the head off but I’m betting they are original.

Many borescopes come with accessory kits that include a 90 degree mirror attachment that clips onto the end, for looking sideways rather than just straight ahead.

Similar to this:
http://www.triumphexp.com/phorum/read.php?8,1512661,1513066,quote=1#REPLY

carChips Avatar
carChips Victor Harnish
Kelowna, BC, Canada   CAN
1933 MG Magnette
1973 Triumph Spitfire 1500 "Chip"
1989 GMC Sierra 1500 "Bush Truck"
You can check the piston tops with a strong flashlight, with No. 1 at TDC just shine in plug hole. No. 4 will be up too. I think if it has 70,000 kms on it, it might have had a rebuild already.



'S all for now
Vic

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