Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: UKROBK7 on May 12, 2011, 02:12:25 PM
-
Is it safe to reuse the con rod bolts on these 750 engines or are you supposed to renew them? None of my manuals say to change them but I had to change them on a Ford I worked on as they relied on the known stretch when torqued down to keep everything together.
Hope the answer is that it's OK as I have just seen the price of new ones!!
Rob
-
The Honda manual does not state that they should be . I will have a look in Mark Paris's book and see if there is a mention.
-
I've never replaced them on a first rebuild but also never done another rebuild on same engine
-
I think you might be confusing a CB750 with a Fireblade...
;D
-
I think you might be confusing a CB750 with a Fireblade...
But it might give one peace of mind, knowing that the engine-guts are in good order... ;)
-
These bolts have been tightened once in manufacture and once by me for plastiguaging the bearing clearances so I look forward to what Mark Paris has to say Seamus. The bolts for my early K7 engine are
-
I had a good read of the relevant parts of the book and great detail is covered with respect to the crank and rods, but there is no mention of replacing the bolts.
Apparently, the rods are well over specified for the loads that the engine will impose on them. I suspect that the bolts will be OK.
I do however suggest that you post this question on the US site as Mark does frequent it (Hondaman is his username)
Good luck
Seamus
-
Please replace the rod bolts as they are stretched when fastened. They are intended to do that. The oem for K/series are no longer available but the stronger ones from the F2 are. You do not want to have a loosened rodbolt.
-
I guess your bike will be like most of ours here and never used ' in anger', so personally, I would re-use them, if this is the case with you Rob. Bryan on here has a wealth of knowledge on SOHC engines so does ChrisR (both in the UK) - so interested to hear what they say too. I am just a novice really so I bow to their judgement ! Interested to know what dealers did. No mention in my Haynes manual about con-rod bolt or nut replacement on rebuild
No mention of bolt/nut replacement in original '69 Honda Genuine Service Manual or '77 750 Manual covering the later F models or '88 'Common Service manual" No mention in the Service Bulletins I have seen. However these are really useful info on the CB750 crank shells and con rods in these Service Bulletins which are a god reference.
http://data.sohc4.net/SB750/750_31.pdf (http://data.sohc4.net/SB750/750_31.pdf)
http://data.sohc4.net/SB750/750_14.pdf (http://data.sohc4.net/SB750/750_14.pdf)
Cheers - AshD
-
Hey AshD,
Well mine has and still is used 'in anger'. And I still would strongly recommend to not re-use the bolts. Compared to the cost of a engine rebuild they virtually cost very little. The nuts are the expensive part and they can be re-used. A full set (bolts and nuts) for about 55.00 GBP. I feel it would be saving on the wrong part. But it is just my opinion.
(http://i238.photobucket.com/albums/ff74/voxonda/Croix15Mei2011001-1.jpg)
(http://i238.photobucket.com/albums/ff74/voxonda/Croix15Mei2011-1.jpg)
these pic's, of me and my bike, taken last weekend at circuit of Croix en Ternois in Northern France.
Cheers, Rob
-
I'll add a few things that I believe about the suject but please feel free to make any points if you have more relevant experience. Also if I use the wrong terms, again please add to it.
First is that if a rod lets go then there is usually one hell of a mess of most of the components near it, with that in mind you have to make your own choice as to what you do but I'd guess it's quite a sickening crunchy noise when it happens.
There are many examples on the bike of bolts being torqed and undone again with no material detriment, wheel spindles are a good one and you don't change them. This proves that it's possible given the sizing and specs being of the correct range and you simply don't routinely replace them. In fact to me all of the bike design seems to be of this caliber.
There is a basic difference between "stretch bolts" and bolts that are stretched a little when when torqued to a lower level. A "stretch bolt/stud" as I understand it is specified to be pulled past it's elasticity limit during assembly and remain there during use to give correct assembly and when dismantled would remain at that elongated length i.e. if you were to use them again you would be pulling them even longer the next time and they would probably fail in use.
A normally torqued bolt/stud (non "stretched" application) is specced to be pulled to a point under it's elasticity limit and therefore when removed should return to it's original length, you can use a micrometer to check this theory. I believe the 750 rod bolts to be of this intention.
To this you'd have to consider their history if you know it, possible engine over-revs, previous assembly exceeding the designers torque setting etc.
Another influence is the lubricant/loctite used during torqueing as if you don't conform to spec for example if you put something really slippery onto the threads during assembly then the tourque measured at the spanner can be within range but the load on the bolt material can be at a level above design intent.
Even saying all this it seems that these rods and bolts have a pretty good capacity/reliability (tough) for normal non-tuned engines and there seems to be no real reports of failures in these circumstances i.e. re-using them.
Hope that makes sense.
-
Hello K2-K6,
+1 on you. But.......The rodbolts can not be compared to wathever bolt in the bike. The loads on the rodbolts are of another dimension then on a axle for that matter. Like AshD stated if you're not using the bike you can consider leaving them on the bike. But IMHO it still is a bad saving. All the SOHC engine builders I know, myself included, opt to replace the bolts.
Cheers, Rob
-
I agree with your making up your own mind and would never want to hear a motor being wrecked like that.
I feel I need to clarfy my reasoning of stressing in that I was only intending to compare an example of tightening a bolt to below it's eleasticity limit and not the direction of the loadpath (the axle being essentially in shear as opposed to the rod bolts in pure tension for their duty cycle) so hopefully that makes sense.
I think people that use them as you do can bring a very qualified experience to this discussion and a very nice bike as well that you have Rob.
Thanks.
-
Good discussion ! I have been in touch with Mark Paris and he says:
" you can reuse them so long as they are still straight enough to not have to be driven out or into the rod holes, and have not been overtorqued. For example, in engines that are run at high RPM quite a bit (lots of 6500+ RPM use on a continuous basis, like track racing), the bolts take a 'set' and show a slight bend to them. This often requires they be tapped out of their holes to disassemble the rod cap: if they tap out (or slip out) without much force, they are not "bent" and can be reused.
Almost all of them have a tiny bit of bend from normal use, which you will notice as this: they go into the rod hole in one orientation easier than the other. When you go to install them in the rod, test it by inserting it one way, then turned 180 degrees and inserted again. You may well find that one orientation is easier to insert: use that orientation. This prevents the bolt from being stressed back in the other direction, which causes them to stretch a bit. If you're not racing, they will be fine. The occasional trip to redline we all enjoy won't hurt them, either."
Started the rebuild - long way to go yet so I have time to make my mind up!!
Rob
-
Update! Just spoken with a chap I knew from a Honda dealership. He says "Always replace the bolts - they are stretch bolts. Use the old bolts for plastigauging etc but then through them away".
Prices for the K7 bolts from Honda up to engine number 2733086 (mine) =
-
As you've said a good discussion, I like to find out about something with real reasoning behind it.
I suppose that logic dictates that cost although a consideration cannot control what has to happen in a technical sense as if they break then the cost saving will become irrelevant.
With that in mind, if you ran a dealership and could replace the bolts at the customers cost and not run the risk of the customer coming back to you with a broken motor then I guess you'd always advise that path as a default but it doesn't answer the question of re-use from a specification point of view.
Mark Paris would be hard to argue against though as he seems to be right in the thick of these motors.
Assume you can get original Honda bolts or verified better spec. Safest option is to change them as nobody can definitively answer the original question.
-
Personally I would go with what Hondaman says if its the first strip down. I thought 'Stretch' bolts were the type where you tighten and keep measuring the length to a stated length, thus tightening to the correct torque. Don't forget that Bryan worked at a Honda dealer too!
Cheers
Ash
-
Always follow your own instinct, wether good or bad. Let keep it at that.
-
The "Old" Hondas were not designated as "Stretch" bolts, just done up to a torque which, in theory at least, is well within the elastic limit of the metal.
The only motorcycle bolts that were done up to a stretch were the old (pre-unit) Triumphs and believe it or not that was because micrometers were more common than torque wrenches(accurate ones that is) when the bottom end was designed.
ALSO BEWARE that a lot of torque wrenches are aonly +or- 15% of the indicated torque,
Snap on are +or- 10% for a torque wrench and +or- 5% for a torque meter
-
My gut feeling was to change the bolts as at least all you have done is possibly waste a relatively small amount of money compared to trying to salvage an engine! The information from Mark Paris is what I have always done on early British bikes when spares were difficult to get (bit different now there are specialists) and makes sense. The article in a 1976 Motorcycle Mechanics magazine that I have found also says "No need to change the bolts".
The decision has been made for me as today a surprise parcel arrived with 8 bolts. This was courtesy of my wife! She says "stop messing about and get them changed, you know you will be happier". Women !!
Rob
-
Again proof that we have to listen to women more! ;)
-
Did I tell you about the time I got home and found my wife hammering in a wall nail with a CB750 con rod she found next to my computer! luckily it was from a scrap engine ;D