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hi compression screw up #777992 07/05/19 11:00 pm
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Richrd Offline OP
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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?


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Re: hi compression screw up [Re: Richrd] #778024 07/06/19 2:22 pm
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Adam M. Offline
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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
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kevin roberts Offline
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what kind of plugs?


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Re: hi compression screw up [Re: Richrd] #778033 07/06/19 5:14 pm
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koan58 Offline
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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
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Richrd Offline OP
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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
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Tridentman Offline
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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
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kevin roberts Offline
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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.


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Re: hi compression screw up [Re: Richrd] #778057 07/07/19 5:05 am
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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.


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Re: hi compression screw up [Re: koan58] #778108 07/08/19 3:09 am
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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.


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Re: hi compression screw up [Re: Richrd] #778275 07/10/19 2:32 pm
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Richrd Offline OP
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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
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Stein Roger Online Content
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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
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Tigernuts Offline
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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)


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Re: hi compression screw up [Re: Richrd] #779227 07/20/19 3:25 pm
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bodine031 Offline
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Sparkplug indexing washers come in different thickness.

Re: hi compression screw up [Re: Richrd] #779330 07/21/19 10:01 pm
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gavin eisler Online Content
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If the piston crowns have enough meat , machine them down , 9:1 would do fine.


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Re: hi compression screw up [Re: Tigernuts] #779454 07/23/19 6:30 am
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NickL Offline
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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..

Re: hi compression screw up [Re: NickL] #779469 07/23/19 10:17 am
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Tigernuts Offline
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They must have been the plugs. If this link works it shows a cross-section (oddly, the only image I could find is from Russia)
http://www.uazbuka.ru/lib/candle_theory/img/Splug27.gif

I guess there must be a modern equivalent, with nothing projecting into the combustion chamber?


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