H25-35 connecting rods, which do you preffer

markus

Well-Known Member
#1
Tecumseh updated the design of the small frame, small journal I beam connecting rods from studs and nuts to the threaded bolts. Just wondering what peoples thoughts are as to which would be a preferred design, and what the thoughts are of the aftermarket replacements such as Rotary sells. Those actually look to be cleaner cast but of course possible weaker material.

I have a Ball bearing small journal mid 70's H35 I want to start setting up for rebuild. Its getting some updated things done to it and maybe some old speed parts but nothing crazy. Since a billet rod is NLA I want to utilize the strongest rod I can at least as it will be the weak point. I like the studs and nuts on the early ones, they seem stout and heavy duty, but on the other hand they are a little big and bulky on the dinky rod.



 
#2
I would go with the stud and nut style. From my experience rods break in the I section not on the big end. To strengthen the rod consider having them shot peened. they are small enough to be done economically by a motorcycle or automotive speed shop.

Thinking bigger picture, how about getting some billet rods made. With current scanning technology and cnc machining, short run are more feasible than in the past.
 
#3
Besides the dipper being different, how close in size as those rods to billet OHV rods? Could they be modified to work?



As far the 2 rods pictured, it seems the bottom rod would be stronger. FWIW, most all aftermarket rods (auto and small engines) have the bolt that screw into the housing and not a stud and nut design.

Polishing the beans and removing all the sharp edges really helps, as does shot peening too.
 
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MB165

Active Member
#4
good ol' Dura-lock bolts and machined serrations at the mating surfaces, never had a issue with them once. I prefer the stud & nut setup, especially for reuse. plus theres a little room to play with clearance, sanding the cap surfaces for worn crankpins. be careful with those import pot metal rods....

If anyone can do it, id buy up to half dozen good billet or forged rods for the h35.
 
#5
I wish I still had the capability to do them. I used to always make my own rods when I worked in a CNC job shop, but at my new job, we don't have CNC's - just manual machines for maintenance.

It seems if somebody could make a run of the H35 and H50-H60 rods that they could sell them pretty quick.
 

markus

Well-Known Member
#9
good ol' Dura-lock bolts and machined serrations at the mating surfaces, never had a issue with them once. I prefer the stud & nut setup, especially for reuse. plus theres a little room to play with clearance, sanding the cap surfaces for worn crankpins. be careful with those import pot metal rods....

If anyone can do it, id buy up to half dozen good billet or forged rods for the h35.
Thanks, I'll steer clear of the aftermarket, They just looked like they would be cleaner to stress relieve. I just pulled out my H35 box to see what was in there for spares and have one of each. The studs seems bulkier to look at on its own, but side by side there not any bigger going through the rod and the cut on the rod is the same. Good thought on being able to modify the clearance on them a touch, I didn't think about that :thumbsup:


I have spare stock rods and dippers if anyone with an actual machines/serious to make one wants one for a pattern, They are still pretty cheap to buy/find new though as well. I dont have measurements or detailed drawings of the rod (and you wouldn't want me to try to make any unless you want it wrong :laugh:) I do have Tecumseh detailed crank specs though out of the master parts manual. Heres is a pic of how the the billet ones were made in the 1970's:

This article featuring a small frame H engine shows how they looked when they made them back in the 70's....kinda like a smaller briggs rod is all.





 
#10
the two stroke guys take great pains with rod bolts. They go for $50 a set and are used once, twice in my low RPM applications. With these Tecs, you don't spin enough RPM to warrant concern over the rod bolts, because the crank goes first anyway. And note the .032 safety wire used in the hop up article. Even on those 14,000 RPM 820's, they're still using Loctite.

Remember that exhaust? I made a matching intake for use on Tilly/Tec or maybe a Clinton/Walbro. Not having a stronger connecting rod is of concern. I don't know why Phil doesn't address it above, but he is the guy who used an OHV billet rod in an H35, but had clearance issues and punched a hole in the case. That was his EFI test engine, and no completion was ever documented. I'd like to know just how bad the clearance problem was. I don't mind a little hole and aluminum repair, but it can't look Frankenstein.

Great topic, and like everyone else here, I need at least one of those billet rods. Bonus if we can get it with crank bearings.

Mock up for one of my projects after my current project:

IMG_2950.JPG
 

markus

Well-Known Member
#11
Phils engine is a post 1983, They started to use/offer larger journals (they used the HS40 crankshafts) on some of them. Thats when you see the timing change noted in the manuals to match the HS series engines. The H engines though unfortunately don't have the extra clearance cast into front wall that the HS series blocks do so the aftermarket Rods don't clear.

I remember the exhaust, glad its gone to a good home. I like the tank idea you got going on up front. Looks like it sits high enough for the carb. This Ball bearing H35 I am putting together I want to run a Gem "straight shot" intake which puts me in the same predicament with carb hieght. If I cant come up with a remote tank bike for it I may have to steal that idea from you.
 
#12
Ah, thanks for the explanation on the post 83. I knew of the measurement differences but didn't key on the fact that he used the HS series rod. Duh.

That tank in front/fuel flow question for a float carb is right at the line. In the back of my mind, I'm thinking fuel pump just in case. I used a stainless steel shot glass for that "velocity stack" and took KKK's advice on silver soldering it to an existing Tec flange I had for something. I made a second one from scratch that I may end up using on a Tilly on a Mac I am using on a Kart.

That engine will go on a small frame Cat that I plan to modify into a full suspension bike, keeping the Cat attributes, so I couldn't use an external tank with the look I'm going for. I am enjoying the heck out of this hobby, and enjoy lurking in the various threads like this one.

I wonder if these guys could come up with a rod option.
 
#14
On rods though, the sharp cuts for the upper part of the stud and bigger cuts in the bottom for the larger nuts, along with a bigger hole for the stud vs a threaded hole makes the body of the rod weaker.

If those two rods and fasteners were verified to be made of the same material, the one with bolts would absolutely be stronger.

The stress risers caused by the additional machining has long been proven to be a weaker design.

There's just more meat left with the bolt.

 

delray

Well-Known Member
#15
also interesting if you read more about the lake carb,cam...etc... and what kind of power they think that little guy was putting out. most it was also about the briggs motor they built too.
Photobucket
 

delray

Well-Known Member
#16
looks like that link to that article as been change? they had mention the parts they made for the tecumseh where the same as the briggs? assuming the cams where the same? cam lift for the briggs was 300 lift and had duration info,also called it a 3/4 race cam. i don't here people saying that anymore. only here that from some old timer hot rod guys. nice if they would of specify more info on the tecumseh cam. if it was base on the briggs cam at 300 lift and .040 off the head the valve would of been hitting the head. must of been less lift and ton of duration?
well either way it would of been nice if they specified more info.
here is a rod from another thread that looks to be the same manufacturer without the dipper and has bearings...:thumbsup:

 
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