Author Topic: CB450 Top end Rebuild (Read 4558 times)

holyspokes · « **on:** December 14, 2014, 11:35:12 PM »

I know this is SOHC/4 but, I'm sure you guys have a lot of good info and experience with twins as well.

I have a 74 CB450 that blew a piston. I'm in the process of rebuilding the top end and I found some damage to the head. There are small chips out of the head around the valves.

Can I continue with the head in this condition or what would my next move be?

Right cylinder with blown piston:

Left cylinder head

calj737 · « **Reply #1 on:** December 15, 2014, 05:29:36 AM »

Hard for my eyes to see from the picture (too old I reckon) but, if the chips are around the seating area of the valve, then a re-cut is mandatory. If the chips and scratches are in the combustion chamber, having them ground out and polished professionally would be my advice.

There's a member here, MRIECK (Mike Rieck) that is especially renowned for his over the top quality of head work. Lives in the Boston area and super knowledgeable. Ship him some pics via his PM and he'll tell you the facts.

holyspokes · « **Reply #2 on:** December 15, 2014, 08:24:55 AM »

Thanks man will do!

grcamna2 · « **Reply #3 on:** December 15, 2014, 08:42:06 AM »

Do you access to a valve spring compressor ? if so I would remove the valves & springs(tag each bag for left and right & keep the springs to each valve) and then clean up the valve seat area w/ a small wire brush,scotch brite,etc. so we can all see the condition of the valve seats.I see the chipping around the head gasket area(which doesn't seem very critical to me)and if we could all see the combustion chambers w/o carbon it would make it easy to tell if there's any problem.

holyspokes · « **Reply #4 on:** December 15, 2014, 09:26:06 AM »

No valve springs on the 450 has a torsion bar setup. The chipping is around the valve ring as well as the combustion chamber. Looks like I'll be looking into getting it machined..

strynboen · « **Reply #5 on:** December 15, 2014, 10:20:36 AM »

put a plug inn and fill fuel in the komp champer..and see if it leakes down...if not..it can be ok....small marks can be polished down..in the aleminium parts

is there not a cast iron dome in 450 heads...or is it a 500 t head..have it run lean..and melted hole in the pistom dome??

grcamna2 · « **Reply #6 on:** December 15, 2014, 10:53:13 AM »

Quote from: holyspokes on December 15, 2014, 09:26:06 AM

No valve springs on the 450 has a torsion bar setup. The chipping is around the valve ring as well as the combustion chamber. Looks like I'll be looking into getting it machined..

Sorry,forgot.OK no valve springs makes it easier to remove the valves and inspect the valve seats

holyspokes · « **Reply #7 on:** December 16, 2014, 06:25:38 PM »

Thanks, I'll try the leak down test once I get it back together. I've checked around with some auto machine shops to get the heads re machined and prices are unreal! I have a NOS set of pistons coming shortly. I'm going to button everything up and see what happens while I search for a donor engine. Thanks for the replies everyone!

Any advice on reassembly? mainly concerning lining the cams up, cam chain, etc. I have a manual, but first hand experience is always appreciated.

Thanks!

grcamna2 · « **Reply #8 on:** December 16, 2014, 08:18:42 PM »

holyspokes, Make sure to take it to a machine shop who likes to do Japanese motorcycles & is already set up to do them;I know of a good one who's in the W. MA. area and his prices are Right

calj737 · « **Reply #9 on:** December 17, 2014, 03:24:23 AM »

Reach out to CB750CafeRacer (he's in Long Beach). He will have a qualified and reasonable referral for you.

I'd machine it before wasting time and energy buttoning it up, but that's me.

grcamna2 · « **Reply #10 on:** December 17, 2014, 05:45:33 AM »

Quote from: calj737 on December 17, 2014, 03:24:23 AM

Reach out to CB750CafeRacer (he's in Long Beach). He will have a qualified and reasonable referral for you.

I'd machine it before wasting time and energy buttoning it up, but that's me.

+1

strynboen · « **Reply #11 on:** December 17, 2014, 07:26:02 AM »

the leak down test..are done upside dovn..on you tabel..just teight the tensins speing..and fill gas in the head

klik on photo to get it bigger

Bill/BentON Racing · « **Reply #12 on:** December 18, 2014, 05:29:02 AM »

May have some parts if needed, Bill
Here's a couple pics of the last one I dd, dem heads and valve train mucho different!

holyspokes · « **Reply #13 on:** December 18, 2014, 02:09:48 PM »

Wow that engine looks incredible! Wish mine looked like that.

So after looking at the spark plug, I believe the piston failed due to overheating on that side. From what I read this could have been from incorrect spark plug heat range and improper ignition timing.

The top end was last done by a shotty local mechanic (my fault for not recognizing the red flags that he wasn't skilled enough for the job). Is it possible that this happened because he set the advance wrong? Got the bike back with multiple leaks, and excuse after excuse. Even had to take it back to redo the valves as they were clattering like crazy. Would have done it myself but felt I had to get the work I paid for

I'd love to get the heads machined, cylinders bored, and larger piston set, however funds prevent me from doing all this right now. I figured with another set of new CB450 pistons I already have, I could button it up the way it is and run it. If the piston damage wasn't due to the scoring at the top of the head, any reason to believe this wouldn't work for a while anyways?

I plan to tear down and re do in the future, but while I'm waiting to rebuild or find a donor engine I might as well run it anyways right? A rebuild is due for the future regardless anyways.

calj737 · « **Reply #14 on:** December 18, 2014, 02:51:53 PM »

Overheating on these air-cooled engines can also be attributed to a very lean fuel condition. Too hot a range of plugs, poor timing can also contribute.

Understanding your situation, clean it up well, Polish up the chambers, and assemble it. Set the valves, then time it, check the valves, and confirm your carbs are right. Perform a plug chop shortly after getting the bike running to confirm your fuel mixture and plug choice. You should be fine.

If you run into problems, there's quite a few members here with great knowledge and experience with that engine.

grcamna2 · « **Reply #15 on:** December 18, 2014, 03:03:46 PM »

A leak test of the valves while the engine is apart can sure save you lots of time and aggravation later;see what strynboen said in his earlier post.I hope you have the new pistons 'Fit' correctly to the cylinders and glaze break/hone for new rings.

holyspokes · « **Reply #16 on:** December 18, 2014, 04:02:21 PM »

thanks everyone. Ive not done a leak down test before. How would I do this with the head off the bike? Thanks!

calj737 · « **Reply #17 on:** December 18, 2014, 04:40:41 PM »

Scroll up and read Reply 11 written by strynboen

holyspokes · « **Reply #18 on:** December 18, 2014, 04:44:46 PM »

so with the head upside down, rotate the cams until the valves seat, fill with gas, and see if any leaks through and out the bottom?

Thanks all

Drew_ATL · « **Reply #19 on:** October 04, 2018, 08:40:39 PM »

Quote from: Bill/BentON Racing on December 18, 2014, 05:29:02 AM

May have some parts if needed, Bill
Here's a couple pics of the last one I dd, dem heads and valve train mucho different!

Can't wait for Bill to work on my '71 CL450 Scrambler!

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Author Topic: CB450 Top end Rebuild (Read 4558 times)