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I have 3 sets of Triumph Twin 750 pistons -- all different top shapes and heights-- is there a diagram somewhere that will detail what compression ratio they are by measuring the height from the wrist pin centers to the top of the pistons? I have a Glenn's Triumph Guide that shows this very well for 650 pistons on page 75---but nothing for 750's------ and-- is the 7.9 compression ratio what I should be building my engine with for normal street use? What other options of low compression pistons are there to choose from? Some of the piston sets are Routts TRW 403-99 pistons. The Routts look to be heavy duty pistons for racing?


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Are these pistons for a 650 big bore kitted bike or for a factory built T140/Tr7?

The reason for asking is that triumph increased the small end size from 11/16" to 3/4" when they went to a 750, clearly any big bore 750 piston would have the 11/16" small end to be able to fit a 650.

Comparing the comp ration for the similar model fitment pistons will always come down to dome height differences, it should be easy to see the difference when next to each other and sliding a gudgeon pin between the two pistons should confirm the difference.


1971 BSA B25T (built as replica of proposed 1972 B25T)
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1972 BSA A65L
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these are the 3/4 in wrist pin 750 twins-- it is easy to see height difference and measure it accurately but the issue is what is the compression ratio of these different height domes? inquiring minds want to accurately know what is going into my engine.


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Hi Bob, sorry I can't be any more help than that, I can definitely understand you wanting to know what's going into your engine, don't think there was anything available lower than the 7.9:1 option.


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Originally Posted by John Harvey
don't think there was anything available lower than the 7.9:1 option.

Also 7.4:1 (71-7085)

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As far as i know there are four comp ratio Hepolite pistons for the T140. Powermax is 10.25:1 (useless for road use); next down is 8.6:1 (highest to safely use with modern petrol); then there's the 7.9:1, and the lowest is 7.4:1.

You'll know it if you've got Powermax pistons - they have very high domes with very large valve cutouts. The other three have much more sensible looking crowns and can be told apart by comparing crown heights and valve cutouts. If I had all three to choose from, I'd go for 8.6:1


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Hi Bob, Can you read part# on top of piston? Going from 8.6 to 7.4 will lower hot cranking compression stock .050 head gasket, not skimmed head will go form 172-175 to 145-148#.

That is just a real life reference. Headgasket thickness, head skimmed or not is all part of equation.
T140 pistons also 8.1, 8.5, 9.5, 10.5. All over the place. LF Harris looks to be made by JCC as does Emgo. Wrist pin clips are slightly different diameter of wire. So clips should match pistons.
Most new pistons have no trade mark or CR markings so really hard to tell without pistons in A line up
Are your pistons out now? Email me some photons.
Don


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don -- pm me a email so I can send some piston pics to you. --- you were going to send me a Triumph Balance worksheet also?


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Hi Bob, I sent the balance sheet to someone...
I'll PM you my email.
Don


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Cam timing is a factor when looking at cylinder pressure.


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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regarding my inquiry on 750 twin pistons--TR7RVMan has helped me out with a measurement on a 8.6 to 1 compression ratio piston--- 1.688 inches from center of wrist pin to top of crown--- can anyone confirm that a 7.9 to 1 compression ration piston is 1.610 inches in the same measurement? please refer to my first post to see what I'm trying to accomplish and about the Routts pistons I have.


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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10:00 at night and 40 degrees, too late and too cold to check now but I should be able to confirm or deny measurement tomorrow,

Steve


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I have lately heard that a .080" head gasket is sometimes used to lower compression-- now you'd have issues with push rod tubes sealing and maybe negatively affecting rocker arm / valve geometry? I also heard of a thicker base gasket doing the same thing? seems like a few thousandths can't really do that much change in compression ratio. Compression Ratio's and how it works in real life will keep you awake at night!

regarding my earlier question here--can anyone confirm that a 7.9 to 1 compression ratio piston is 1.610 inches center of wrist pin to top of crown? I found a part # on the piston top and looked into a old MAP catalogue. MAP says that piston is a 8.5 to 1 -- after I get these piston questions answered I will post my findings on britbike triumph forum.


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Hi Bob, I used an .080 thick head gasket for over 20k miles to very good results. Head had been skimmed .009”. I put 5k miles on bike using normal .050” head gasket.

Carefully tested hot compression before & after it lowered compression 20#. 172 to 152#. Improved running & reduced ping greatly. A old Triumph/Harley mechanic recommend it until overhaul was needed. He installed hundreds of low compression pistons in old Harleys. Their combustion chamber is nearly identical to Triumphs so similar issues with California fuel.

In real life with stock valves & head never been ground, just light lapping, Tip of stems ground .003”. Valve adjuster tips refinished. About .002-.003”.

So with head skim .009”. We’ll call it +.020” head gasket, rocker geometry didn’t change very much. Stock original push rods & covseal rocker box gaskets.

Grinding, valves, seats on overhaul effects geometry of course.


Regarding PRT crush in practice I found even .015 crush is enough. They later T140 eliminated the white square seal, same orings. Zero crush. They don’t seem to leak any worse.
You could always put a shim under white seal with sealant under it on tappet block.
Don


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I have a set of standard pistons (assuming 7.9:1) that I bought for the jubilee. I told Bob I’d measure them, but had to leave town the other day and won’t be back near my garage until Monday.

Steve


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Bob ,I measured a standard bore piston removed from my 79T140D.. It should be an original 7.9 part.. I got 1.550 using a caliper and machinist square...


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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thanks and yes that makes total sense--- for 750 twin piston--- since a 8.5 comp ratio piston is 1.610" taken from data in a old MAP catalogue.


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Hi Bob -

I have STD piston set for my '77 Jubilee. I can confirm Tony's figures.

Steve


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‘69 BSA Rocket 3 (patiently awaiting it's turn)
"Vintage Bike". What's in your garage?

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I ordered 8.6:1 pistons. Seeing this I just measured with a caliper and a square and obtained somewhat similar to that 1.688 that Don mentioned. Is difficult to obtain a precise measurement but if is not that number is a bit less but never 1.550 like the 7.9:1 mentioned by Tony.
JCC seems the only new ones in STD condition.

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I just checked my previous reply....made a written mistake with my measurement of the 1979 7.9 piston.. It's 1.600...


79 T140D, 89 Honda 650NT ,61 A10 .On a bike you can out run the demons
"I don't know what the world may need
But a V8 engine is a good start for me
Think I'll drive to find a place, to be surly"
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Well, either my tool is defective, my measurement is defective or I have 7.4:1 piston….


'77 T140J Silver Jubilee
'82 T140ES TMA Royal
‘69 BSA Rocket 3 (patiently awaiting it's turn)
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
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Right now I am reasonably confident I have correct numbers for 650 pistons. Mostly because of page 75 in Glenn's Triumph guide-- for 750 twin pistons the data is coming in not quite clearly enough for me to be confident I have those numbers correct yet. anybody know of a manual that has written info I can reference for Triumph 750 twin piston Compression Ratio to height measurements? There may be some confusion caused because it's not just heights that determine piston CR's?== when I am confident I have good info I will post it on Britbike.


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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my previous post somehow included a ebay link on the work "manuals" -- I didn't do that or it was my mistake


59 Bonne (in high school!)--67 TR6c (building)--68 Bonne(building)--69 Bonne (sold!)- 70 TR6r (sold!)-79 TR7v custom (building) - CRF 250x & XR 400 dual sport w/ SM wheels (super fun!) & just bought two Honda CT-90 trail bikes in rough shape!
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Originally Posted by Bob Fletcher
my previous post somehow included a ebay link on the work "Manuals" -- I didn't do that or it was my mistake
the webmaster can choose to automatically hyperlink certain words at the website's discretion
the only way around the hyperlinking , if it bothers you
is to misspell the word like M-anual or L-ucas

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Hi JubeePrince, If Possible could you email me photos of your pistons. Top view & side view straight on.
Would no surprise me if factory installed 7.4 as even with 7.9 ping was already an issue in California according to period motorcycle magazine.

I didn’t think to measure my new 7.4 pistons.
Don


1973 Tiger 750
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