I Had to cut my timing chain! What should I do now?

[Kylelm]

Many moons ago I purchased a 1978 Honda cb750 super sport. After Many years of sitting and rusting outside I decided to sell it and take the loss. I was asking 300 but no one would take it because the motor was locked up. So I then decided to keep it and turn it into a cafe racer. With the pistons locked the way they were I had to cut the timing chain. It took me beating on the frozen piston with a 2lb hammer to break it loose. Well all is apart now and I wanted to rebuild this bike, but I didn't want to have to break open the engine. Does anyone know what my options are? My plans were to replace pistons and rings, rebore the jugs and start building the bike.

 

[dan1951]

Never beat the pistons out with a hammer that force is transfered to the wrist pin and crank journals, I would split the cases and do a total inspection of the lower including plastigaging the journals, oil pump inspection, gear sector, Replacement of all oil seals, In other words do a complete rebuild. But thats entirely up to You. I do not like volunteering a persons input without permission but Dirtdigger is from what I have read is a Master mech. on these machines. Maybe He will give You more guidance than I can. Take Care.

 

[Kylelm]

I will check on that. I did this a few years back before I had any knowledge of motors. I guess I will need to do a complete rebuild but I sure hate it.

 

[dan1951]

I will check on that. I did this a few years back before I had any knowledge of motors. I guess I will need to do a complete rebuild but I sure hate it.

You will thank Yourself in the end if Your going to keep it, Nothing worse than something else going wrong after You get it together.Just an insurance policy that it will run for years to come troublefree

 

[dirtdigger]

You will need to split the cases to replace the cam chain anyway. I do not and will not use or suggest to use a master link cam chain. More than likely beating on the pistons you will have made flat spots on the rod bearings which will need to be replaced. You may also want to have the connecting rods themselves checked now to make sure you didn't bend them, yes it can happen. As far as the cylinders and pistons go, if you have to have them bored anyway you may want to consider an 836 kit. Stock cylinders bored and the big bore pistons. If you want a stock size motor then you can have your cylinders bored to a factory over size or you can have them re sleeved for stock size. Not really sure what your intentions are for the motor. If you want more HP it gets expensive fast because you will need to start looking into upgrading other parts.

Other option would be to find another running motor and just swap it in, but if your bike is indeed a Super Sport then finding another good running SS motor will be hard to come by. But you can swap any SOHC motor into the frame you will just loose a couple of HP but nothing major.

Either put a different used motor in or go through the motor you have but do it right and check the stuff out in the bottom end. You could always part out the ss motor and put it toward a different one.

 

[Kylelm]

Where can I find the 836 kit? So once I open the case I just need to check the rods? Will a machine shop need to do this or is this something I can do? How bad will it be to change those bearings? I am pretty new to this type of work so sorry for all the specifics but you seem to know a great deal on cb's.

Thanks dirtdigger

 

[dirtdigger]

First thing is buy a good factory Honda shop manual for the cb750. They can be pricey but are the most accurate for specs and pictures. The book will tell you everything on how to check bearings and what clearances and how to select bearings. Once you have the bottom end apart take a look at everything...these motors have bearings like in a car not ball or roller bearings. I would read up more on the motor before tearing into it if you have never been in a motor before. They are fairly easy to work on but the potential is there to do a lot of damage and cost a lot of money.

836 kits can be found at APE aperaceparts.com, dynoman.net, and also cyclexchange.net. I use APE exclusively for machine work and most of the parts I use. All of these sites have a good amount of useful information pertaining to the 750 motor also.

 

[Kylelm]

I just looked up the kits and they are pretty high. I will definitely do what you say. I have had the bike since 98 and I took it apart over a year ago so I'm in no hurry. I have a jeep cj I spend a lot of time and money on too but I don't know much about bikes. I want to build it to just drive around on the weekends. Something real simple maybe just kick start even. I would like to make it handle better than stock too. More like today's bikes.

 

[losttravlyr]

Red n Kate the couple that have this shop in okla.are very good. i have never heard anything bad about any of his stuff and they go the extra mile in cust service. some of the best frames and hardtails today are coming from him. he also does machine work and has a very good selection of gaskets and internal honda motor stuff. the prices are reasonable as well some top notch machine work. he has a 836 flat top piston kit{non wiseco} for 199.00, ring, sets are 23.00 for one piston,timing chains 35.00 and a lot of other stuff we need for these motors. check them out,,

http://www.cycleonemanufacturing.com/id280.html

 

[dirtdigger]

Nice link. Just want to note, The SS motor will need specific pistons as the cylinder head chamber volume is larger, or you have to mill the head a bunch and possibly the cylinder to head the compression to acceptable range. I cant imagine how low compression would get with flat top pistons. Standard K style pistons will drop compression ratio of the SS F2 motor by at least one compression point. You would be running under 9 to 1 which would seriously hurt power. Just food for thought and something to remember for F2 owners.

 

[losttravlyr]

Nice link. Just want to note, The SS motor will need specific pistons as the cylinder head chamber volume is larger, or you have to mill the head a bunch and possibly the cylinder to head the compression to acceptable range. I cant imagine how low compression would get with flat top pistons. Standard K style pistons will drop compression ratio of the SS F2 motor by at least one compression point. You would be running under 9 to 1 which would seriously hurt power. Just food for thought and something to remember for F2 owners.

good point i didnt think of that. in my parts stash i have a 75 ss that the motor has the head off {way it was when i got it}that i was thinking of using in building a motor with a 836 kit and dual carbs,but perhaps i should use one of my k motors instead since i would use the flat tops? i have never run anything but k motors so not really aware of the power band difference. i am looking more for reliability,longevity and bulletproof as well something to embarrass the stock harley fools. .

 

[dirtdigger]

75 would not be the F2 motor with the bigger head. The 75 SS motor is nothing more than a K motor. This head started in 77. Casting on the head will be a 410. If it is not a 410 head then it doesn't have the bigger chamber. The concern I have with that cheap 836 kit is for that price they are a cast piston not forged. Maybe good for a stock type rebuild but not strong enough for higher rpm or a lot stronger build. If you look at his wiseco kit it is actually higher priced then other places I use. APE gives discount on boring with a piston purchase from them as well. Not sure if you are shopping around or if you use these guys exclusively but its good to have options available.

9 to 1 is perfectly fine for todays gas and you dont need premium with it. I run 10.5 or more with premium fuel and have no issues.

 

[dirtdigger]

A good condition stock 750 with a good accurate tuneup will embarrass all but the big CI and v rod oil leakers. That in fact is one of my favorite past times...Sturgis is a lot of fun for me in that respect. lol

 

[HYPNOSHAUN]

oh dear

Many moons ago I purchased a 1978 Honda cb750 super sport. After Many years of sitting and rusting outside I decided to sell it and take the loss. I was asking 300 but no one would take it because the motor was locked up. So I then decided to keep it and turn it into a cafe racer. With the pistons locked the way they were I had to cut the timing chain. It took me beating on the frozen piston with a 2lb hammer to break it loose. Well all is apart now and I wanted to rebuild this bike, but I didn't want to have to break open the engine. Does anyone know what my options are? My plans were to replace pistons and rings, rebore the jugs and start building the bike.

mate just strip it down mate, you cannot do a good job by half.