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Dieseljunky2
Dieseljunky2, Mechanic
Category: Small Engine
Satisfied Customers: 3943
Experience:  20+ years in the mechanic industry
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Hi, I have a B&S 11.5 HP motor on an Australian Greenfield

Resolved Question:

Hi, I have a B&S 11.5 HP motor on an AustralXXXXX XXXXXfield ride on lawnmower. A year ago it was pretty sick, & I had the engine fully reconditioned professionally. That included a new piston & rings, which I believe was 30 thou oversize. When the engine was refitted & working it went very well, but as the rings bedded, in the compression became so great, it was hard for the electric starter to turn it over. I can get it started by using 2 x 12V batteries in series, and once it is started, the torque is amazing, and it rarely stalls in even very thick grass. I suspect that the 30 thou oversize rebore has increased the compression, and no compensation has been provided by the engine reconditioner. My question is. Is there head gaskets made by B&S or aftermarket, that are thicker than standard, to compensate for the extra swept volume. Any suggestions welcome.

Many thanks

XXXXX XXXXX Queensland Australia
Submitted: 3 years ago.
Category: Small Engine
Expert:  Dieseljunky2 replied 3 years ago.

Dieseljunky2 :

Hello, can you please provide the engine model and spec numbers? Thanks

Customer :

B&S 11.5HP

Customer :

Model: 28D707 Type: 1126-E1 Code: 971013ZB

Customer :

Is that all the info you need ?

Dieseljunky2 :

OK, for that model, there is not a thicker gasket offered. What I have done in the past with problems like this is to get a piece of copper that is .010" (.25mm) thick then take a marker and trace the gasket onto the copper, cut it out and use it along with the original gasket as a shim. This will lower the compression enough normally to do the job. I have never had to use thicker than that, but if the builder machined the head too much, you may have to use a second shim gasket. Thanks

Customer :

What about grinding & polishing our the combustion chamber in the alloy head to lower the compression ratio slightly. Is this a logical option, or should I stick with putting an extra shim gasket in as you suggest ?

Dieseljunky2 :

You could grind the head out. It would be a gamble as to how much to take out without making it too thin and waiting for it to crack. These heads are no longer available new. An advantage to using a shim, as the engine begins to wear and gets weak, you can remove the shim to boost the compression back up again. You couldn't do that if you grind it out. I would go with the shim, as that way you have not altered anything that cannot be undone. Thanks

Customer :

Good suggestion ! One last question. What would the original design compression ratio be on that particular B&S engine, and what typical PSI reading would you expect on a cranking compression test ? I can probably calculate the shim thickness required, once I know the compression ratio & calculate the difference in original & oversize swept volume. Thanks !

Dieseljunky2 :

Ok, the flat head engines like yours are typicaly 6.5:1 and should yield aroun 120 lbs of compression. Thanks

Customer :

Many thanks ! I'll get to it in the next week or so, and see how I go.

Dieseljunky2, Mechanic
Category: Small Engine
Satisfied Customers: 3943
Experience: 20+ years in the mechanic industry
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