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hi compression screw up #777992
07/05/19 11:00 pm
07/05/19 11:00 pm
Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
Richrd Offline OP
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Richrd  Offline OP
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Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
still messing around with this 74 t150. had some wear in the bores and had 3 new pistons on the shelf so had it bored. I knew they were high compression but figured what the hell, use 'em.

just got the top dropped on and found the spark plug tips bashed.

So do I look for a thick base gasket, different plugs, new pistons, or sell the bitch to have more room in the garage?


Rich (last remaining member ThreeMustGetBeers)
"It's not always about going fast. Sometimes it's nice to slow down" (Wendy E.2016)

69 bonney
72 commando
75 commando interstate
06 Suzu..Suzu.. uh appliance
couple of beesas a ducati
and the Snake Bike
and a Honda?
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Re: hi compression screw up [Re: Richrd] #778024
07/06/19 2:22 pm
07/06/19 2:22 pm
Joined: Apr 2005
Posts: 1,758
Mississauga, Ontario.
A
Adam M. Offline
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Adam M.  Offline
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A

Joined: Apr 2005
Posts: 1,758
Mississauga, Ontario.
You have to check a clearance between pistons and a head using some Plasticine first and decide later what to do, I believe thick base gasket will be necessary.

Re: hi compression screw up [Re: Richrd] #778027
07/06/19 3:57 pm
07/06/19 3:57 pm
Joined: Dec 2013
Posts: 4,603
ohio, usa
kevin roberts Offline

DOPE
kevin roberts  Offline

DOPE

Joined: Dec 2013
Posts: 4,603
ohio, usa
what kind of plugs?


every day you do not take a chance is a day of your life that you will never get back.
Re: hi compression screw up [Re: Richrd] #778033
07/06/19 5:14 pm
07/06/19 5:14 pm
Joined: Nov 2012
Posts: 1,087
Isle of Wight, UK
K
koan58 Offline
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Isle of Wight, UK
Assuming the pistons were intended for the T150, I would suspect the head has been milled quite a bit.
If so, then piston to valve clearance may also be worth checking using the plasticine method previously suggested.
If the head has lost enough surface to cause contact issues, a thicker head gasket would be the answer, as the pushrod tube rubber crush is also likely to be too much.
A thicker base gasket will not resolve this issue, it will only assist with the plug (and possible valve) to piston clash.
Personally I've not had much joy with the plasticine test, because you're not left with anything firm enough to measure. It's not good enough to assume that lack of contact during slow rotation means that this will be ok at high rpm.
I have more successfully used fine multicore solder laid across the piston top.
On twins, I have seen advised clearance in the 40+ thou region. With the triple's much less flexy crank, I would expect a lower figure. DMadigan and others may have more of an idea on this.
If it turns out everything is fine apart from the plug clash, another possible route may be adjusting plug height using additional washers under the seat?

Re: hi compression screw up [Re: Richrd] #778037
07/06/19 7:26 pm
07/06/19 7:26 pm
Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
Richrd Offline OP
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Richrd  Offline OP
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Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
what will it be like with these piistons if I get everything to clear?

is this the way to go or should I just look for new lower compression pistons?


Rich (last remaining member ThreeMustGetBeers)
"It's not always about going fast. Sometimes it's nice to slow down" (Wendy E.2016)

69 bonney
72 commando
75 commando interstate
06 Suzu..Suzu.. uh appliance
couple of beesas a ducati
and the Snake Bike
and a Honda?
Re: hi compression screw up [Re: Richrd] #778040
07/06/19 8:20 pm
07/06/19 8:20 pm
Joined: Feb 2008
Posts: 4,380
New Jersey USA
Tridentman Offline

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Tridentman  Offline

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Joined: Feb 2008
Posts: 4,380
New Jersey USA
Do you know the nominal compression ratio of the pistons?
Are there numbers stamped on the piston crowns?
If so-- what are they?
Can you take photos of the piston crowns?
All info useful in trying to help you.

Re: hi compression screw up [Re: Richrd] #778043
07/06/19 9:43 pm
07/06/19 9:43 pm
Joined: Dec 2013
Posts: 4,603
ohio, usa
kevin roberts Offline

DOPE
kevin roberts  Offline

DOPE

Joined: Dec 2013
Posts: 4,603
ohio, usa
Originally Posted by Richrd
what will it be like with these piistons if I get everything to clear?

is this the way to go or should I just look for new lower compression pistons?



if you're using extended plugs, that may be the entire problem.

by the way, if you hit the spark plugs turning the engine, you need to check your valves as well.


every day you do not take a chance is a day of your life that you will never get back.
Re: hi compression screw up [Re: Richrd] #778057
07/07/19 5:05 am
07/07/19 5:05 am
Joined: Mar 2006
Posts: 1,435
Magnolia, TX
htown Online content
BritBike Forum member
htown  Online Content
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Joined: Mar 2006
Posts: 1,435
Magnolia, TX
In the overall cost of a rebuild, pistons are not particularly expensive. The EMGO ones are perfectly fine. I've used em in Commando's and my Trident with good success as have large numbers of other people. I'd pop for a set of the standard ones for several reasons, solve your contact problems, reduce detonation risks with modern gas, and make it easier to start. IMHO a thicker base or head gasket is just a band aid.


1978 Bonneville T140E
1974 Trident
1970 BSA Thunderbolt
1971 Norton Commando
1972 Norton Commando
1973 Norton Commando
1974 Norton Commando
2018 Kawasaki Z900RS


Everything will be alright in the end. If its not alright, its not the end.
Re: hi compression screw up [Re: koan58] #778108
07/08/19 3:09 am
07/08/19 3:09 am
Joined: Oct 2008
Posts: 5,501
Crossville, TN
DavidP Online content

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DavidP  Online Content

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Joined: Oct 2008
Posts: 5,501
Crossville, TN
Originally Posted by koan58
Assuming the pistons were intended for the T150, I would suspect the head has been milled quite a bit.
If so, then piston to valve clearance may also be worth checking using the plasticine method previously suggested.
If the head has lost enough surface to cause contact issues, a thicker head gasket would be the answer, as the pushrod tube rubber crush is also likely to be too much.
A thicker base gasket will not resolve this issue, it will only assist with the plug (and possible valve) to piston clash.

thumbsup Some idiot once shaved my head. I never had an issue with the plugs (N3), but it turns out the valve clearance was down to nil. This was pre-internet, so I couldn't easily find a thick head gasket, I just doubled up on the base gasket. Never could keep the pushrod tubes from leaking after that.
I think the standard hi-comp pistons are 11:1. Unless you have access to an airport or race gas I would recommend standard pistons.


Stepping on others doesn't make you stand tall.

71 A65L "Zelda"
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72 T150V "Wotan"
Re: hi compression screw up [Re: Richrd] #778275
07/10/19 2:32 pm
07/10/19 2:32 pm
Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
Richrd Offline OP
BritBike Forum member
Richrd  Offline OP
BritBike Forum member

Joined: Aug 2001
Posts: 2,709
Springfield Nebraska
Since i,m planning to sell this bike I decided to go back to standard comp. I did a lot of checking and I had plenty of clearance to the head and valves. Thought about chucking the pistons in the lathe and turning a clearance for the plugs.


Rich (last remaining member ThreeMustGetBeers)
"It's not always about going fast. Sometimes it's nice to slow down" (Wendy E.2016)

69 bonney
72 commando
75 commando interstate
06 Suzu..Suzu.. uh appliance
couple of beesas a ducati
and the Snake Bike
and a Honda?
Re: hi compression screw up [Re: DavidP] #779068
07/18/19 9:01 pm
07/18/19 9:01 pm
Joined: Sep 2008
Posts: 1,096
Skudeneshavn Norway
S
Stein Roger Offline
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Stein Roger  Offline
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S

Joined: Sep 2008
Posts: 1,096
Skudeneshavn Norway
Originally Posted by DavidP
Originally Posted by koan58
Assuming the pistons were intended for the T150, I would suspect the head has been milled quite a bit.
If so, then piston to valve clearance may also be worth checking using the plasticine method previously suggested.
If the head has lost enough surface to cause contact issues, a thicker head gasket would be the answer, as the pushrod tube rubber crush is also likely to be too much.
A thicker base gasket will not resolve this issue, it will only assist with the plug (and possible valve) to piston clash.

thumbsup Some idiot once shaved my head. I never had an issue with the plugs (N3), but it turns out the valve clearance was down to nil. This was pre-internet, so I couldn't easily find a thick head gasket, I just doubled up on the base gasket. Never could keep the pushrod tubes from leaking after that.
I think the standard hi-comp pistons are 11:1. Unless you have access to an airport or race gas I would recommend standard pistons.

The base gasket has no influence on the PRT spacing. Just think about it... whistle
The head and rocker box gaskets however, does.

Re: hi compression screw up [Re: Richrd] #779215
07/20/19 9:31 am
07/20/19 9:31 am
Joined: Sep 2014
Posts: 988
Naarfuk, UK
T
Tigernuts Offline
BritBike Forum member
Tigernuts  Offline
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T

Joined: Sep 2014
Posts: 988
Naarfuk, UK
Seems like you've got the Powermax 11.25:1 pistons. I used them in my T160 and first time I turned it over closed up one earth electrode completely and touched another. This was back in the early 1980s, so the plugs I got to solve the problem might not be available now, but they were Champion special plugs, N3 heat range, with the earth electrodes in the form of small dowels fitted at right angles to the live electrode at the correct gap. Can't remember what they were called, maybe N3R or something like that? Anyway, it solved the problem. There was no issue with valve / piston clearance and the engine was very sweet (but, 5 star petrol was readily available back then - don't know how well it'd do on modern fuel)


If anything other than a blank space is visible here, something's wrong.
Re: hi compression screw up [Re: Richrd] #779227
07/20/19 3:25 pm
07/20/19 3:25 pm
Joined: Aug 2005
Posts: 1,549
melbourne florida
B
bodine031 Offline
BritBike Forum member
bodine031  Offline
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B

Joined: Aug 2005
Posts: 1,549
melbourne florida
Sparkplug indexing washers come in different thickness.

Re: hi compression screw up [Re: Richrd] #779330
07/21/19 10:01 pm
07/21/19 10:01 pm
Joined: Aug 2001
Posts: 4,529
argyll. scotland, uk
gavin eisler Offline
BritBike Forum member
gavin eisler  Offline
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Joined: Aug 2001
Posts: 4,529
argyll. scotland, uk
If the piston crowns have enough meat , machine them down , 9:1 would do fine.


71 Devimead A65 750
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Re: hi compression screw up [Re: Tigernuts] #779454
58 minutes ago
58 minutes ago
Joined: Jan 2014
Posts: 178
Oceania
N
NickL Online content
BritBike Forum member
NickL  Online Content
BritBike Forum member
N

Joined: Jan 2014
Posts: 178
Oceania
Originally Posted by Tigernuts
Seems like you've got the Powermax 11.25:1 pistons. I used them in my T160 and first time I turned it over closed up one earth electrode completely and touched another. This was back in the early 1980s, so the plugs I got to solve the problem might not be available now, but they were Champion special plugs, N3 heat range, with the earth electrodes in the form of small dowels fitted at right angles to the live electrode at the correct gap. Can't remember what they were called, maybe N3R or something like that? Anyway, it solved the problem. There was no issue with valve / piston clearance and the engine was very sweet (but, 5 star petrol was readily available back then - don't know how well it'd do on modern fuel)



The old racing plugs N54R etc were that type. Lodge RL49 were similar but they worked, no need for plug changes cold to hot..


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