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pre unit engine build questions #775050 05/29/19 10:34 am
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Michel1990 Offline OP
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i am getting ready to order some parts for my engine that i am going to use in my Triumph T100 RR replica

the engine is a mix of different parts from different models and years

this is what i have:

5T big bearing casses #05165 1957
1 piece 650 unit crank
3134 in and ex cam
T110 iron head

i am missing a cylinder, but i was thinking about ordering a new cylinder from british bike bits, and i have a 2mm shim to make up the difference in stroke for using 500 casses and a 650 crank.
what pistons could i use with this setup i was thinking about using 9:1 compression pistons, but i cant find these for a pre unit motor. or can i use unit pistons?

i also have a set of new tappet guide blocks these are also for a unit engine ( E4676) could i use these with the pre unit cylinder?

these are all parts i already own so it would be great if i could use these to keep the project affordable. but is this gona work?

Regards Michel


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Re: pre unit engine build questions [Re: Michel1990] #775053 05/29/19 11:28 am
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triton thrasher Offline
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You do know that 650 cylinders have a different base stud spacing from most 500s?

I’ve been told that the last year or two of Pre-unit Speed Twins had the 650 type spacing.

I think a 650 head will have a rather big combustion chamber volume for a slightly over-stroked 500.

The 650 stroke is 2 mm longer. That means the pistons are 1 mm higher at top dead centre (and 1 mm lower at bottom dead centre). How did you arrive at 2 mm spacer thickness?


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Re: pre unit engine build questions [Re: Michel1990] #775057 05/29/19 12:32 pm
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Michel1990 Offline OP
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yes i know that the 500`s have a different stud spacing then the 650`s, i did check with an 650 base gasket and my engine has the wider stud spacing like the 650`s

i do have a spacer i am going to use between the engine and the cylinder and i meassured it and it is 1 mm thick, your right i only need to raise the cylinder 1mm to get the correct stroke.

im not sure what you mean with an over stroked 500, if i use an 650 crank, 650 cylinder and 650 head, with a 1mm spacer with 500cc engine cases doesnt it make it the same as an 650 motor?

thanks for your help!
Michel

Re: pre unit engine build questions [Re: Michel1990] #775060 05/29/19 1:32 pm
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RPM Offline
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If you use a 650 head, cylinder ,crank and the later wide spacing stud cases you are building a 650. No need for that spacer.

Re: pre unit engine build questions [Re: Michel1990] #775077 05/29/19 4:53 pm
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Michel1990 Offline OP
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dont i need that spacer to get the correct compression ratio?

Re: pre unit engine build questions [Re: Michel1990] #775081 05/29/19 6:01 pm
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triton thrasher Offline
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Originally Posted by Michel1990
dont i need that spacer to get the correct compression ratio?


What makes you think you’ll need it?



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Re: pre unit engine build questions [Re: Michel1990] #775093 05/29/19 9:03 pm
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Michel1990 Offline OP
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i always thought the 2mm difference in stroke was in the cases, but its not, its only in the crank? still learning everyday!

now i know what you mean with an over stroked 500, 2mm spacer and a 500 crank with 650 barrel and head will give it a 4mm higher combustion chamber

ok so the only question left wich piston`s should i use?

Regards Michel

Re: pre unit engine build questions [Re: Michel1990] #775095 05/29/19 9:36 pm
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HawaiianTiger Online Content
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If you're going with all 650 top end with iron head, unit pistons will give a lower compression ratio. Maybe not what you want. There is a sacrifice of some power when you do that.
I think Harris is supplying them, or perhaps you can find a NOS set somewhere.
I'm thinking I need to do that myself as I have unit pistons in my bike with T110 iron head and it doesn't make much power.
But be careful. Not many vendors are aware of the difference and they will send you what they have and that will be unit pistons.
Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: pre unit engine build questions [Re: Michel1990] #775097 05/29/19 9:53 pm
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koan58 Offline
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I had assumed that the 500 cases were machined to a 1mm lower deck height as well.

So it is very interesting to hear from RPM (who I'm sure knows his stuff) that the 650 and 500 cases are identical in this regard.

Presumably the barrel heights were 1mm shorter on the 500, or is there a change in gudgeon pin to piston top measurement?

I doubt it will be a problem but I think you will wise to check the height to which the piston rises at TDC to ensure there is no mechanical clash with the head/valves.

Absolutely HT and TT are the ones to inform about the use of available pistons with iron heads.

Re: pre unit engine build questions [Re: Michel1990] #775388 06/01/19 6:38 pm
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Michel1990 Offline OP
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i looked on the lf harris website but i couldnt find the 9-1 pre unit pistons on there,
i was thinking the same thing koan it probably wont be a problem but i was going to check the piston to valve clearance just to be safe when im building this engine.
just have to get all the parts together

Re: pre unit engine build questions [Re: Michel1990] #775390 06/01/19 7:11 pm
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triton thrasher Offline
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Pistons for alloy head 650s are common to Unit and Pre-unit.

Pistons for iron headed 650s are different.

The very common 9:1 late pistons do work with the iron head, but the compression ratio won’t be 9:1. It measured about 7.9:1 on mine.

The highest compression pistons I’ve heard of as standard for road 650s with iron heads are 8:5:1 1954-55 T110 pistons. I don’t know of any source for them now.

Last edited by triton thrasher; 06/01/19 7:11 pm.

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Re: pre unit engine build questions [Re: Michel1990] #775401 06/01/19 10:05 pm
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I found a set +.030 NOS 8.5:1cr for 328$ !!!

So, with a little searching you might find them, but bend over...

Cheers,.
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: pre unit engine build questions [Re: Michel1990] #775451 06/02/19 6:33 pm
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Michel1990 Offline OP
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ok i will keep searching and keep my wallet ready,
if i would use the 7:1 cr iron head pistons would this be noticeable in the loss of power compared to a 8,5:1 piston
i would like an engine with some power

Michel

Re: pre unit engine build questions [Re: Michel1990] #775454 06/02/19 6:55 pm
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My personal opinion is that 8.5:1 is ideal for these motors. You get good low end response with some power on tap at the higher revs.
9:1 is standard on unit motors and with that you get some sensitivity to fuel quality which results in a potentially hot running motors and consequent damage. It is more critical to get the timing and mixture as close as possible.
With 7:1 the power seems flat with no appreciable power band. Not bad, but not exciting at all, and not as sensitive to tuning accuracy.
I would like to look into getting the compression up with the available pistons but I probably don't have the patience to do the math on this....
Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: pre unit engine build questions [Re: Michel1990] #775455 06/02/19 7:08 pm
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Michel1990 Offline OP
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or like triton trasher says 9:1 unit pistons and you get about 7,9:1 cr if i understand him correctly, not as good as 8,5:1 cr but better then the 7:1 pistons

Michel

Re: pre unit engine build questions [Re: Michel1990] #775456 06/02/19 7:17 pm
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koan58 Offline
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Michel,
I think what TT and HT are saying is that pistons specifically for iron head 650's are hard to find and hence (very) expensive.
The commonly available standard 9:1 unit pistons are reasonably priced. Of course they are only 9:1 with an alloy head.
TT has supplied his own compression ratio measurement of those pistons with an iron head (7.9:1). I wouldn't doubt this figure +/- much.
I believe pre-unit 6T's were 7:1, pre-unit T120's were 8.5:1. 9:1 became standard with the unit 650.
I think 7.9:1 would make for a well balanced motor that copes with ordinary fuel, while giving decent performance.

I think you will be happy with the 9:1 unit pistons in your iron engine.

Re: pre unit engine build questions [Re: HawaiianTiger] #775464 06/02/19 8:32 pm
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triton thrasher Offline
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Originally Posted by HawaiianTiger
I found a set +.030 NOS 8.5:1cr for 328$ !!!

So, with a little searching you might find them, but bend over...

Cheers,.
Bill


And you need barrels that can take +30 pistons.


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Re: pre unit engine build questions [Re: Michel1990] #775467 06/02/19 9:04 pm
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I should probably do CC check on my bike. It certainly doesn't feel like 8:1, more like 7:1 seat of the pants estimate for what it's worth.
When I found those +30's there were also some standard ones available which is often the case. Why would you buy new standard pistons?
Only if you were replacing the cylinder at the same time. That hasn't happen nearly as much as over-bores so now there's some standards floating around.
I've only ever used these E3000's once on a bike and it went like stink. Fond memories of a 55 T110.

Cheers,
Bill


Bikes
1974 Commando
1985 Honda Nighthawk 650
1957 Thunderbird/T110 "Black Tiger"
Antique Fans: Loads of Emersons (Two six wingers) plus gyros and orbiters.
Re: pre unit engine build questions [Re: Michel1990] #775471 06/02/19 9:11 pm
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koan58 Offline
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I think Michel needs barrels full stop (8-tud 650) doesn't he?
Considering his intentions in his 1st post, I would think $300 for a pair of pistons doesn't fit with the plan.
What are you currently using in your iron head engine TT? Are they the ones you measured at 7.9:1?

Re: pre unit engine build questions [Re: Michel1990] #775472 06/02/19 9:35 pm
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triton thrasher Offline
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Originally Posted by Michel1990
ok i will keep searching and keep my wallet ready,
if i would use the 7:1 cr iron head pistons would this be noticeable in the loss of power compared to a 8,5:1 piston
i would like an engine with some power

Michel


I have used 9:1 late Unit pistons and am now using 7:1 6T iron head pistons. I think it runs better with the latter.


Amateur Loctite enthusiast.

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