engine rebuild

[chopper750]

ok guys so itested my compression on my motor cylinder #1 didnt even reach 60psi but #4 got to about 150 so im guesing i need to rebuild.

my question is do i just need to replace the rings with .25 over or do i have to replace the pistons themselves
and i know i need to replace all corrosponding seals and gaskets

also whats a good place to get quality sets ive seen some on ebay but they are really cheap too cheap haha but i did see a kit that comes from australia i think brand name is FORSETI

any info would be great never gone this deep into a motor

 

[dirtdigger]

Put a little oil in cylinder 1 and try compression test again. If it comes up substantially then it is rings if it stays the same or increases slightly then you have a valve sealing issue or you may have a rocker arm adjusted to tight holding the valve open but that could indicate a sunken valve also. You wont know what you need until you get it apart. Oversize rings are to be used with the corresponding oversize pistons. Std pistons get standard rings. You need to see how things look before ordering parts, if the bores and pistons are scuffed then you need to bore to the size it takes to clean up the cylinders with the corresponding piston and ring size. Honda offered 4 oversizes but some are getting hard to find. For stock size 750 you cant beat the OEM Honda parts for quality and reliability, I buy all my oem stuff from Service Honda...fast shipping and very good prices and good priced shipping and online microfisch.

 

[chopper750]

well then should i just go bigger size to get more ccs out of it or just go .25 or whatever so the engine will have many more rebuilds in it.
also if the piston walla are within spec can you just use stnd size rings ?

 

[dirtdigger]

If the piston walls are still within spec then you can use standard rings but you still at a minimum need to hone the cylinder walls with new rings or you run the risk of the new rings not seating which will cause lower compression, excessive blowby and possible oil consumption. Size of the bore all depends on how deep your pockets are and what you want to do with the bike. You can over bore with the stock sleeves to 850cc. The cylinder sleeves are replaceable so worrying about future rebuilds isn't a problem plus you will never wear out the stock sleeves unless you ride everyday for years, they last a long time if you build it right.

 

[chopper750]

ok thamks digger ima take off my jugs asap what is the specs for using stock rings ? maybe i can get away with just new rings
and it has to do something with the ring gaps too right ?

 

[dirtdigger]

stock Honda ring end gap spec is .0079 to .016" You will need to hone the cylinders with the new rings. See what it looks like when you take it apart and post some pictures of the piston skirts if you have any doubt as to the condition.

 

[chopper750]

what are the specs for the cylinders for using stnd rings like how big can they be to fit stnd rings or how big do they need to be to fit first over size pston and rings im guesing its just plus .25 to the original standrd size

 

[dirtdigger]

std cylinder bore size is 61.01 to 61.02mm or 2.402 to 2.4024" max std bore service limit is 61.1mm or 2.4055in std piston size is 60.965 to 60.985mm or 2.4002 to 2.4009in piston service limit is 60.85mm or 2.3957in. Oversizes are std piston size plus what ever oversize you are using.

 

[chopper750]

thanks man.. where do you get your honda sets ? also i took off the oil pan and found some metal flakes in there not alot but they were definetly metal pieces is that bad or normal ?

 

[dirtdigger]

Yes it is normal to have metal in the oil pan, because engine and transmission share the same oil you will get metal from transmission and clutch. The tranmission and clutch do generate some metal particles. I get all my factory Honda parts from Servicehonda.com they have always treated me very well and have good prices and fast shipping.

 

[chopper750]

Alright so finally got around to the engine put some oil in the bad cylinder went up to 150 so definitely bad rings in that one.. can the compression be so bad from sitting so long ?

 

[dirtdigger]

If it has sat for years then yes you can have low compression. Normally you wont double your compression with the oil test, more like 20 or maybe more. If you put way to much oil in it can give false high compression. A miniscule oil layer between the cylinder and rings form the actual seal and if the cylinder was bone dry from sitting for years it can have reduced compression. If possible try and get it running and put some miles on it and see how well the compression comes back.

 

[chopper750]

yea it went up to 150 with oil in there but it had alot.. i kept cranking it most came out and i tested again today only went up to 70 so exactly twenty haha so i tore her down and i also found the pucks were bad, nothing holding them down and also the head gasket was leaking hopefully the cylinder are still within spec for standard rings

hey thanks for your help so far digger and what do you think of those cheap ebay big bore kits ?

 

[chopper750]

oh and digger if i leak test my valves and they dont leak do i just leave them alone ? and if they do leak what i do ? cut the valves ive heard mc valves dont get cut or do i just do a 3 angle valve guide job 60 45 and 30 and them lap the valves once the seats have been cut ?

 

[chopper750]

so ive been reading around and i found that stripping the 2 bolts that hold each cam tower down strip all the time with removal cause it happened to me to haha.. my question is should i just go up a size in bolt just rethread it or use helicoils and use the same size bolt as stock

 

[dirtdigger]

I am not a fan of the cheap ebay piston kits...they are cheap and cheaper quality and I have heard of sizing variances and having to do some machining to get compression ratio to a usable level. By the time you are said and done and if you plan on adding more go fast goodies later you would be better using a quality piston kit that will be able to handle higher rpm and not as prone to detonation damage. The cheap kits are normally cast pistons and the good ones are forged and much, much stronger and machined to closer quality control standards. If you want to do a good rebuild you dont have to worry about for many miles I would have the head freshened up. Most of the time if the valves are leaking then the valves are shot and they really are not that expensive but you should have the seats machined so you get a good seal. I really dont like having automotive machine shops do head work on motorcycles. Automotive tolerances can be looser and still "get by" but motorcycle parts are physically smaller and need closer machining tolerances. I use APE or Cycle Xchange personally but you typically have a 1 month wait for turn around but you will get top notch quality machine work.

The threads in the heads are weak. Larger bolts as well as helicoils can be used. The high performance shops back in the 70's made specific larger studs for the 750 heads and some shops also used helicoils. I helicoil all the threads in my heads because I know I wont pull the threads after several removals. My opinion is helicoils are stronger than the next size larger thread because you are using steel threads with the helicoil instead of using larger but still crappy aluminum threads.

 

[chopper750]

ok the helicoils totally make sense steel on steel is much strongr then steel on aluminum and actually saw the 6mm 1.0 helicoils at myu njob today totally by chance so its meant to be plus ill get a good discount..
well the motor is torn down the pston skirts look really good no scratches on them and same with the cylinder the scoring do maybe a hone will do the job and thats more money i can spend on some good valves and gaskets
as far as the valves go ive read that alot of people just regrind the seats and then lap the valves cause the valves are noramlly good for a while but do you know where to get a set of good valves ?

 

[chopper750]

digger do you know where to get a maual that has torque specs ? ive been thinking about selling the bike cause im running out of time before school but im hoping to finish it i have big plans for the bike but hwo know what time will allow if i do sell it i want the engine running at least

 

[dirtdigger]

Cycle Xchange has good valves and at a fair price. They are better than stock....lighter and flow better. Honda discontinued new factory valves but you never know what local dealers still have some on the shelves.

All manuals have torque specs but I only use the factory manuals because some aftermarket ones have had a couple of conflicting info problems. You should be able to find torque specs online but you really should have a manual when you go to reassemble so you have all the clearance and assembly sequences. Any manual is better than no manual.

 

[chopper750]

so your saying stock valves cant be recut ? and i will look into the cycle x valves
any chance you could email me a copy of the manual you got or at least the engine rebuild part