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Discussion Starter #1
I am thinking of going with the 904 kit this winter. I have heard that sometimes there are problems with cylinder base gaskets leaking oil. Have any of you with the Big Bore kits on your bike had this problem?

The kits seem to come with 10.5 and 11.5 compression ratios. Is 11.5 to high for pump gas?

What about valve sizes, do I need larger valves?

Thanks, Matt
 

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Just like stock parts they dont leak unless something is wrong.bigger valves will give more hp .You need to have the head ported by someone who knows what he is doing .I have 2mm over intake and exh valves in mine but im not sure 1mm over wouldnt run just as good.I have 10.5 in mine some go with the 11.5 with out trouble.I guess it depends on how good of gas you can get every where you ride .I run 92 or 93 octane in mine with no troubles.To get the motor to run good you will need ether 39mm or 42mm carbs and some good pipes.If you have a 865 motor you might think of getting some better cams to like tpusa 813.The 865 cams are dead after 6500 rpm compared to older 790 cams.Then you might want to get with pieman and get your ing box remaped with more advance and up rev limit to 8500 rpm .
From all the dyno sheets i have seen I cant see much hp gain with the 11.5 to 1 kit over the 10.5 kit .BP runs 13.5 on there race bikes with race fuel and they are like 5 hp faster then there 10.5 kits so who knows .Both the 10.5 and 11.0 kits can make over 80 rear wheel hp if everything is right.
 

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Do not use base gasket, glue the cylinder block to engine case with high quality sealant like Honda bond. That is commonly used with japanese racebikes. Just check that there is enough clearance beetween piston and head without base gasket (at least 0,8 mm, 0,9 mm is clearly on safe side).
It will not leak and gives more compression and squench effect.
 

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Discussion Starter #4
Thanks, I talked to BP and they recommended 1mm Int. and Exh.oversize valves with the addition of an inner valve spring. I will get the heads flowed at a local shop with alot of racing experience. Wow, Mikuni carbs are frickin expensive!!! I will have to wait until tax time to get the carb kit. Until then I will have to "limp" along with the stock carbs. Cams will probably be the next thing . Anybody got a recommendation for jetting with a 904 and stock carbs at sea level? In the meantime I will check out the search function. Thanks
 

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I have not had any base gasket issues at all. Went with 1mm over and no inner spring. The guy that did the port and sleeve pressing was certain the stock spring was good for 1mm over. You can go 1mm over easily with the stock valve seats. Not sure if those who went 2mm required new seats, if so that would up the cost some. Pulls strong all the way to the Procoms redline so I don't expect I am getting any valve float.
 

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Do not use base gasket, glue the cylinder block to engine case with high quality sealant like Honda bond. That is commonly used with japanese racebikes. Just check that there is enough clearance beetween piston and head without base gasket (at least 0,8 mm, 0,9 mm is clearly on safe side).
It will not leak and gives more compression and squench effect.
That is not right you shouldnt be telling people to do that .you have to use the gasket on these motors to get the deck height right its a metal gasket .The 904 kits are set up for these gaskets
 

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I have not had any base gasket issues at all. Went with 1mm over and no inner spring. The guy that did the port and sleeve pressing was certain the stock spring was good for 1mm over. You can go 1mm over easily with the stock valve seats. Not sure if those who went 2mm required new seats, if so that would up the cost some. Pulls strong all the way to the Procoms redline so I don't expect I am getting any valve float.
the single spring will work for awhile but if you do much high rpm running the dual spring will hold up better.
 

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Discussion Starter #8
vertwerks, have put your bike on the dyno? I'm really interested to know what kind of power you are making since I'm basically doing the same thing. Is your cam stock? I'm looking at the TP 813's.
 

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No issues with my engine. Upon assembly I made sure all mating surfaces and stud treads were clean and spotless. Torque per specs.
 

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Quote:
Originally Posted by MONTTU
Do not use base gasket, glue the cylinder block to engine case with high quality sealant like Honda bond. That is commonly used with japanese racebikes. Just check that there is enough clearance beetween piston and head without base gasket (at least 0,8 mm, 0,9 mm is clearly on safe side).
It will not leak and gives more compression and squench effect.
T
hat is not right you shouldnt be telling people to do that .you have to use the gasket on these motors to get the deck height right its a metal gasket .The 904 kits are set up for these gaskets
Mike,

What is the head to piston clearance you are using. I am running pistons over the deck.
 

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The 904 is set up for 0 deck .When you go over on the bore the head hangs over the bore some.You dont want to run these motors with piston out of the hole.pluss with bigger valves and a good cam the valves will hit the piston if you go out of the hole with the piston.No gain to be had there the way the head works on these motors.Even on the 13to1 race pistons the dome only comes above the deck in from the edge of the piston a ways.
 

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The 904 is set up for 0 deck .When you go over on the bore the head hangs over the bore some.You dont want to run these motors with piston out of the hole.pluss with bigger valves and a good cam the valves will hit the piston if you go out of the hole with the piston.No gain to be had there the way the head works on these motors.Even on the 13to1 race pistons the dome only comes above the deck in from the edge of the piston a ways.
You are mostly right but 1mm of head to piston clearance which stock gasket gives is too much. I have 0,8mm on my 347cid V8 Mustang engine with 4,03" bore and as short pistons as Triumph and metal is not meeting metal. I think you know the idea of squench effect and the benefit of running piston as close to head as possible (not talking about compression). I do not suggest this to everybody but if one knows what to do there are many small things one can do if bother. Valve is not hitting piston if the clearance is checked and more room machined if necessary. My Speedmaster cams ( not really performance ones ) are so way off anyway that I don not worry about that. If I had really good ones I would stick with the timing.
 

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Have looked at a 904 piston ? there not the same set up as stock pistons they have to dish them to keep comp. down to 10.5 to 1 you dont want much more then that with pump gas.These motor have very small combustion chambers in them to start with.Wiesco and bp did alot of testing to come up with what would work and last on a street bike.You dont need to change anything on them.
 

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Have looked at a 904 piston ? there not the same set up as stock pistons they have to dish them to keep comp. down to 10.5 to 1 you dont want much more then that with pump gas.These motor have very small combustion chambers in them to start with.Wiesco and bp did alot of testing to come up with what would work and last on a street bike.You dont need to change anything on them.
Sorry, I did not think the pinging problem, In Finland the standard fuel available is 98 OCT ( 94 US ). 11,5:1 with this stuff is no problemo. I had 13,9:1 in my Suzuki GSX1255R without problems, of course combusition chambers were reshaped (and piston to head clearance was minimal to keep the pinging down).

And yes, you do not need to change anything on them, but if that is the target why did you start at all... this is an endless path. Buying parts and putting them to engine is an entry level. Modifying them to best is the following one...you know, you have done it.
 
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